diff --git a/CHANGELOG.md b/CHANGELOG.md
index 33e6bb6..8c60474 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -58,6 +58,97 @@
   truncating cast. The facade, `clawhdf5-io` and `clawhdf5-ann` are not
   covered.
 
+### Range reads, milestone M1: the `Storage` trait (2026-09-26)
+- **Breaking: `clawhdf5_format::error::FormatError` and `clawhdf5::Error`
+  are now `#[non_exhaustive]`.** An exhaustive `match` on either needs a
+  wildcard arm. `FormatError` has two new variants: `Storage(String)` (a
+  storage backend failed to serve a read) and
+  `ContiguousStorageRequired(&'static str)` (an operation not yet converted
+  to range reads was asked of a backend without the whole file in memory).
+- New `clawhdf5_format::storage::Storage`, the synchronous, `no_std` read
+  interface of `docs/design/range-reads.md` option (a): `read_at(offset:
+  u64, len) -> Cow<[u8]>`, `read_ranges`, `len()` and an `as_contiguous()`
+  fast path; implemented for `[u8]`, `Vec<u8>`, and references, `Box`es
+  and (with `std`) `Arc`s of a `Storage`. Slices and `Vec`s serve borrowed
+  bytes, so parsing an in-memory file costs no copy.
+- **The metadata parsers read through `Storage`.** Each converted parser has
+  an `*_in<S: Storage + ?Sized>(&S, ..)` core (a `&dyn Storage` works too),
+  and its `&[u8]` function is now a thin wrapper over it, so no caller
+  changes. The wrappers compile to a `[u8]` instance of the same code, so a
+  structure read in memory is a bounds check and a borrowed slice, with no
+  indirect call and no copy. Converted: the superblock
+  (`Superblock::parse_in`), its extension and cache image
+  (`read_superblock_extension_in`, `cache_image_state_in`), object headers
+  with their continuation chunks (`ObjectHeader::parse_in`), local and
+  global heaps, symbol-table nodes and the group B-tree (v1), fractal heaps,
+  fixed and extensible array chunk indexes, shared messages and the SOHM
+  table (`message_data_in`, `message_data_with_sohm_in`,
+  `load_sohm_table_in`, …), attributes (`extract_attributes_full_in`,
+  `extract_attributes_tolerant_in`, `AttributeMessage::parse_in_storage`),
+  fill values (`dataset_fill_value_from_storage`) and virtual-dataset
+  mappings (`DataLayout::resolve_vds_mappings_in`); also
+  `signature::find_signature_in`. Each structure is read with bounded reads
+  (a prefix, then the structure) instead of slicing the whole file; the
+  open-ended `&file_data[addr..]` slices in these modules are gone. Bounds
+  errors keep their values (absolute position, file length).
+- Reads sized by untrusted fields are bounded by what the parser uses, so
+  a crafted size cannot turn one structure into a read of the rest of the
+  file on a range backend: local-heap names are read in growing pieces
+  (64 bytes first) rather than to the end of the data segment; a fractal
+  heap indirect block is read up to the entry covering the object (the
+  whole block only when that entry is unallocated); paged fixed and
+  extensible array data blocks over 1 MiB are read page by page, only the
+  pages in use; and a block under one checksum whose claimed size runs
+  past the end of the file fails its bounds check before any read (with
+  the `checksum` feature). An object header's prefix is one read (was
+  two).
+- Structures still indexed by a v2 B-tree — dense attribute storage, a SOHM
+  B-tree index and huge fractal-heap objects found through their B-tree —
+  are not converted yet (the
+  v2 B-tree and dense groups come with milestone M3); over a backend without
+  the whole file in memory they are the clean `ContiguousStorageRequired`
+  error, never a partial result. Raw data, chunk B-tree (v1) indexes and VL
+  data are milestone M2.
+- **No behaviour change**, checked three ways (2026-09-26, tank): every
+  existing test passes unchanged; the conformance sweep
+  (`conformance/run.sh --no-fetch`) gives a byte-identical `results.json`
+  at `f2ff2c4` and on this branch, and identical per-file probe output for
+  696 of the 697 files — the exception, `cve-2025-2310.h5`, reports one of
+  two errors depending on which parallel chunk decode fails first, at
+  `f2ff2c4` as on this branch; and a transcript of every converted `&[u8]`
+  function's result over the fixtures, the conformance corpus and the
+  h5py-written files below (748 files, 7 603 object headers) is
+  byte-identical between the two builds.
+- **Speed on local files** (provisional: tank was shared with other jobs;
+  both builds linked into one binary and timed alternately, 200 rounds;
+  new Criterion bench `clawhdf5/benches/local_metadata_bench.rs` over a
+  400-group version-1 file, `clawhdf5-format/tests/fixtures/v1_groups_400.h5`):
+  against `f2ff2c4`, listing the file through the facade is 2.7% faster,
+  `ObjectHeader::parse` is within ±1%, symbol-table nodes and the group
+  B-tree walk are about 19% faster (their entry loops were tightened),
+  local-heap names and `resolve_group_children` 1.5–3% faster. The same
+  harness run on two copies of the old code differs by up to 2%. The
+  Criterion bench itself, old and new as separate binaries run alternately
+  (3 rounds), agrees except for `ObjectHeader::parse`, which it puts about
+  7% slower (25.9 vs 24.1 µs for 401 headers) while the listing that
+  parses those headers is 5–8% faster; that one remains unexplained and is
+  to be rechecked on an idle machine.
+- New equivalence harness `clawhdf5-format/tests/storage_equivalence.rs`:
+  every converted parser runs over the file as a slice and over
+  `storage::CountingStorage` — a `Storage` that serves an in-memory buffer
+  through `read_at` only (`as_contiguous()` is `None`), copying what it
+  serves and counting reads — and must give identical results. It walks
+  the fixtures, files h5py writes for it (extensible arrays with super
+  blocks and paged data blocks, paged fixed arrays, large v1 and dense
+  groups, a user block, SOHM list and B-tree indexes, dense, shared and
+  committed-type attributes, fixed and extensible array data blocks over
+  1 MiB, whole and truncated), and with `CLAWHDF5_STORAGE_CORPUS=<dir>` a
+  corpus (all 653 HDF5 files of the conformance corpus pass). Only the
+  structures listed above as not converted may answer
+  `ContiguousStorageRequired`, and only in the checks that reach them; a
+  converted parser falling back to the whole file fails it. Milestones M2
+  and M3 extend it.
+
 ### Chunked full reads (2026-09-26)
 - **Chunks are decoded straight into the output, into reused buffers.** A
   full read of a chunked dataset faulted in about three times its size in
diff --git a/crates/clawhdf5-format/src/attribute.rs b/crates/clawhdf5-format/src/attribute.rs
index d22a552..78aaf12 100644
--- a/crates/clawhdf5-format/src/attribute.rs
+++ b/crates/clawhdf5-format/src/attribute.rs
@@ -17,6 +17,7 @@ use crate::fractal_heap::FractalHeapHeader;
 use crate::message_type::MessageType;
 use crate::object_header::ObjectHeader;
 use crate::shared_message;
+use crate::storage::{Storage, require_contiguous};
 use crate::vl_data;
 
 /// A parsed HDF5 attribute message.
@@ -52,7 +53,7 @@ impl AttributeMessage {
     ///
     /// `length_size` is needed for dataspace dimension parsing.
     pub fn parse(data: &[u8], length_size: u8) -> Result<AttributeMessage, FormatError> {
-        Self::parse_impl(data, length_size, None)
+        Self::parse_impl(data, length_size, None::<(&[u8], u8)>)
     }
 
     /// [`AttributeMessage::parse`] with access to the rest of the file, which
@@ -67,13 +68,24 @@ impl AttributeMessage {
         offset_size: u8,
         length_size: u8,
     ) -> Result<AttributeMessage, FormatError> {
-        Self::parse_impl(data, length_size, Some((file_data, offset_size)))
+        Self::parse_in_storage(data, file_data, offset_size, length_size)
     }
 
-    fn parse_impl(
+    /// [`AttributeMessage::parse_in_file`] with the file behind any
+    /// [`Storage`].
+    pub fn parse_in_storage<S: Storage + ?Sized>(
+        data: &[u8],
+        file: &S,
+        offset_size: u8,
+        length_size: u8,
+    ) -> Result<AttributeMessage, FormatError> {
+        Self::parse_impl(data, length_size, Some((file, offset_size)))
+    }
+
+    fn parse_impl<S: Storage + ?Sized>(
         data: &[u8],
         length_size: u8,
-        file: Option<(&[u8], u8)>,
+        file: Option<(&S, u8)>,
     ) -> Result<AttributeMessage, FormatError> {
         ensure_len(data, 0, 2)?;
         let version = data[0];
@@ -88,19 +100,19 @@ impl AttributeMessage {
 
     /// The bytes of an embedded datatype/dataspace message, following the
     /// shared-message reference when `shared` is set.
-    fn embedded_message<'a>(
+    fn embedded_message<'a, S: Storage + ?Sized>(
         bytes: &'a [u8],
         shared: bool,
         msg_type: MessageType,
         length_size: u8,
-        file: Option<(&[u8], u8)>,
+        file: Option<(&S, u8)>,
     ) -> Result<Cow<'a, [u8]>, FormatError> {
         if !shared {
             return Ok(Cow::Borrowed(bytes));
         }
         let (file_data, offset_size) = file.ok_or(FormatError::UnresolvedSharedMessage)?;
         let shared_ref = shared_message::parse_shared_ref_sized(bytes, offset_size, length_size)?;
-        shared_message::resolve_shared_message(
+        shared_message::resolve_shared_message_in(
             file_data,
             &shared_ref,
             msg_type,
@@ -145,10 +157,10 @@ impl AttributeMessage {
         })
     }
 
-    fn parse_v2(
+    fn parse_v2<S: Storage + ?Sized>(
         data: &[u8],
         length_size: u8,
-        file: Option<(&[u8], u8)>,
+        file: Option<(&S, u8)>,
     ) -> Result<AttributeMessage, FormatError> {
         // Flags: bit 0 = datatype is shared, bit 1 = dataspace is shared.
         let flags = data.get(1).copied().unwrap_or(0);
@@ -199,10 +211,10 @@ impl AttributeMessage {
         })
     }
 
-    fn parse_v3(
+    fn parse_v3<S: Storage + ?Sized>(
         data: &[u8],
         length_size: u8,
-        file: Option<(&[u8], u8)>,
+        file: Option<(&S, u8)>,
     ) -> Result<AttributeMessage, FormatError> {
         // Flags: bit 0 = datatype is shared, bit 1 = dataspace is shared.
         let flags = data.get(1).copied().unwrap_or(0);
@@ -418,7 +430,20 @@ pub fn extract_attributes_full(
     offset_size: u8,
     length_size: u8,
 ) -> Result<Vec<AttributeMessage>, FormatError> {
-    extract_attributes_with(file_data, header, offset_size, length_size, &mut Err)
+    extract_attributes_full_in(file_data, header, offset_size, length_size)
+}
+
+/// [`extract_attributes_full`] over any [`Storage`]. Dense attribute
+/// storage is indexed by a v2 B-tree, which is not read over [`Storage`]
+/// yet: on a backend without the whole file in memory an object with dense
+/// attributes is [`FormatError::ContiguousStorageRequired`].
+pub fn extract_attributes_full_in<S: Storage + ?Sized>(
+    file: &S,
+    header: &ObjectHeader,
+    offset_size: u8,
+    length_size: u8,
+) -> Result<Vec<AttributeMessage>, FormatError> {
+    extract_attributes_with(file, header, offset_size, length_size, &mut Err)
 }
 
 /// Like [`extract_attributes_full`], but an attribute that cannot be read
@@ -434,6 +459,17 @@ pub fn extract_attributes_tolerant(
     header: &ObjectHeader,
     offset_size: u8,
     length_size: u8,
+) -> Result<(Vec<AttributeMessage>, Vec<FormatError>), FormatError> {
+    extract_attributes_tolerant_in(file_data, header, offset_size, length_size)
+}
+
+/// [`extract_attributes_tolerant`] over any [`Storage`] (see
+/// [`extract_attributes_full_in`] for dense storage).
+pub fn extract_attributes_tolerant_in<S: Storage + ?Sized>(
+    file_data: &S,
+    header: &ObjectHeader,
+    offset_size: u8,
+    length_size: u8,
 ) -> Result<(Vec<AttributeMessage>, Vec<FormatError>), FormatError> {
     let mut errors = Vec::new();
     let attrs = extract_attributes_with(file_data, header, offset_size, length_size, &mut |e| {
@@ -445,8 +481,8 @@ pub fn extract_attributes_tolerant(
 
 /// Read every attribute; each one that fails goes to `on_error`, which
 /// either stops the read (returns the error) or skips that attribute.
-fn extract_attributes_with(
-    file_data: &[u8],
+fn extract_attributes_with<S: Storage + ?Sized>(
+    file_data: &S,
     header: &ObjectHeader,
     offset_size: u8,
     length_size: u8,
@@ -514,6 +550,19 @@ pub fn find_attribute_in_file(
     name: &str,
     offset_size: u8,
     length_size: u8,
+) -> Result<Option<AttributeMessage>, FormatError> {
+    find_attribute_in(file_data, header, name, offset_size, length_size)
+}
+
+/// [`find_attribute_in_file`] over any [`Storage`] (see
+/// [`extract_attributes_full_in`] for dense storage, whose name index still
+/// needs the whole file in memory).
+pub fn find_attribute_in<S: Storage + ?Sized>(
+    file_data: &S,
+    header: &ObjectHeader,
+    name: &str,
+    offset_size: u8,
+    length_size: u8,
 ) -> Result<Option<AttributeMessage>, FormatError> {
     let attr_info = find_attribute_info(header, offset_size)?;
     let dense = attr_info
@@ -523,18 +572,19 @@ pub fn find_attribute_in_file(
         // Compact only (or dense storage without a name index, which a
         // listing reports): as a listing finds it.
         return Ok(
-            extract_attributes_tolerant(file_data, header, offset_size, length_size)?
+            extract_attributes_tolerant_in(file_data, header, offset_size, length_size)?
                 .0
                 .into_iter()
                 .find(|a| a.name == name),
         );
     };
+    let contiguous = require_contiguous(file_data, "dense attribute storage (a v2 B-tree)")?;
     let btree_hdr =
-        BTreeV2Header::parse(file_data, to_usize(btree_addr)?, offset_size, length_size)?;
-    let fh = FractalHeapHeader::parse(file_data, to_usize(fh_addr)?, offset_size, length_size)?;
+        BTreeV2Header::parse(contiguous, to_usize(btree_addr)?, offset_size, length_size)?;
+    let fh = FractalHeapHeader::parse_in(file_data, fh_addr, offset_size, length_size)?;
     if btree_hdr.tree_type != ATTRIBUTE_NAME_INDEX || btree_hdr.record_size < 4 {
         return Ok(
-            extract_attributes_tolerant(file_data, header, offset_size, length_size)?
+            extract_attributes_tolerant_in(file_data, header, offset_size, length_size)?
                 .0
                 .into_iter()
                 .find(|a| a.name == name),
@@ -560,7 +610,7 @@ pub fn find_attribute_in_file(
     // hash is the last field.
     let hash = jenkins_lookup3(name.as_bytes());
     let hash_at = usize::from(btree_hdr.record_size) - 4;
-    let records = find_btree_v2_records(file_data, &btree_hdr, offset_size, &mut |r| match r
+    let records = find_btree_v2_records(contiguous, &btree_hdr, offset_size, &mut |r| match r
         .get(hash_at..hash_at + 4)
     {
         Some(h) => u32::from_le_bytes([h[0], h[1], h[2], h[3]]).cmp(&hash),
@@ -572,8 +622,10 @@ pub fn find_attribute_in_file(
             continue;
         };
         let attr = fh
-            .read_managed_object(file_data, id_bytes, offset_size)
-            .and_then(|d| AttributeMessage::parse_in_file(&d, file_data, offset_size, length_size));
+            .read_managed_object_in(file_data, id_bytes, offset_size)
+            .and_then(|d| {
+                AttributeMessage::parse_in_storage(&d, file_data, offset_size, length_size)
+            });
         // One that cannot be read is left out, as from a listing.
         if let Ok(attr) = attr
             && attr.name == name
@@ -586,8 +638,8 @@ pub fn find_attribute_in_file(
 
 /// The attributes stored in the object header itself (compact storage), and
 /// each one's creation order into `orders`.
-fn extract_compact_attributes(
-    file_data: &[u8],
+fn extract_compact_attributes<S: Storage + ?Sized>(
+    file_data: &S,
     header: &ObjectHeader,
     offset_size: u8,
     length_size: u8,
@@ -601,7 +653,7 @@ fn extract_compact_attributes(
                 // Shared attribute: resolve the reference to get actual attribute data
                 shared_message::parse_shared_ref_sized(&msg.data, offset_size, length_size)
                     .and_then(|shared_ref| {
-                        shared_message::resolve_shared_message(
+                        shared_message::resolve_shared_message_in(
                             file_data,
                             &shared_ref,
                             MessageType::Attribute,
@@ -610,7 +662,7 @@ fn extract_compact_attributes(
                         )
                     })
                     .and_then(|resolved| {
-                        AttributeMessage::parse_in_file(
+                        AttributeMessage::parse_in_storage(
                             &resolved,
                             file_data,
                             offset_size,
@@ -618,7 +670,7 @@ fn extract_compact_attributes(
                         )
                     })
             } else {
-                AttributeMessage::parse_in_file(&msg.data, file_data, offset_size, length_size)
+                AttributeMessage::parse_in_storage(&msg.data, file_data, offset_size, length_size)
             };
             let attr = attr.and_then(|a| check_in_header(a, header));
             match attr {
@@ -650,8 +702,8 @@ fn find_attribute_info(
 /// Extract attributes from dense storage (fractal heap + B-tree v2), and
 /// each one's creation order into `orders`.
 #[allow(clippy::too_many_arguments)]
-fn extract_dense_attributes(
-    file_data: &[u8],
+fn extract_dense_attributes<S: Storage + ?Sized>(
+    file_data: &S,
     attr_info: &AttributeInfoMessage,
     fh_addr: u64,
     offset_size: u8,
@@ -661,7 +713,7 @@ fn extract_dense_attributes(
     on_error: &mut dyn FnMut(FormatError) -> Result<(), FormatError>,
 ) -> Result<(), FormatError> {
     // Parse fractal heap
-    let fh = FractalHeapHeader::parse(file_data, to_usize(fh_addr)?, offset_size, length_size)?;
+    let fh = FractalHeapHeader::parse_in(file_data, fh_addr, offset_size, length_size)?;
 
     // Parse B-tree v2 for name index (type 8)
     let btree_addr = attr_info
@@ -670,9 +722,10 @@ fn extract_dense_attributes(
             expected: 1,
             available: 0,
         })?;
+    let contiguous = require_contiguous(file_data, "dense attribute storage (a v2 B-tree)")?;
     let btree_hdr =
-        BTreeV2Header::parse(file_data, to_usize(btree_addr)?, offset_size, length_size)?;
-    let records = collect_btree_v2_records(file_data, &btree_hdr, offset_size, length_size)?;
+        BTreeV2Header::parse(contiguous, to_usize(btree_addr)?, offset_size, length_size)?;
+    let records = collect_btree_v2_records(contiguous, &btree_hdr, offset_size, length_size)?;
 
     for record in &records {
         // Per HDF5 spec, both type 8 and type 9 records start with heap_id:
@@ -689,9 +742,9 @@ fn extract_dense_attributes(
 
         // The data in the heap is a complete attribute message
         let attr = fh
-            .read_managed_object(file_data, id_bytes, offset_size)
+            .read_managed_object_in(file_data, id_bytes, offset_size)
             .and_then(|attr_data| {
-                AttributeMessage::parse_in_file(&attr_data, file_data, offset_size, length_size)
+                AttributeMessage::parse_in_storage(&attr_data, file_data, offset_size, length_size)
             });
         match attr {
             Ok(attr) => {
@@ -1101,4 +1154,76 @@ mod tests {
         let strs = attr.read_as_strings().unwrap();
         assert_eq!(strs, vec!["abcd", "EFGH"]);
     }
+
+    /// Every object's attributes in h5py-written files read identically
+    /// through a read_at-only CountingStorage — compact ones, shared ones
+    /// and those behind an Attribute Info message — except dense storage,
+    /// whose v2 B-tree index is not read over Storage yet: that is the clean
+    /// ContiguousStorageRequired error, never a partial list. Through a
+    /// slice as Storage every object matches.
+    #[test]
+    fn storage_reads_match_slice_reads() {
+        use crate::storage::CountingStorage;
+        let files: [(&str, &[u8]); 5] = [
+            ("attrs", include_bytes!("../tests/fixtures/attrs.h5")),
+            (
+                "mixed_attrs",
+                include_bytes!("../tests/fixtures/mixed_attrs.h5"),
+            ),
+            (
+                "dense_attrs",
+                include_bytes!("../tests/fixtures/dense_attrs.h5"),
+            ),
+            (
+                "dense_attrs_root",
+                include_bytes!("../tests/fixtures/dense_attrs_root.h5"),
+            ),
+            (
+                "shared_fill_value",
+                include_bytes!("../tests/fixtures/shared_fill_value.h5"),
+            ),
+        ];
+        let (mut same, mut dense, mut attrs) = (0, 0, 0);
+        for (name, file) in files {
+            let sb = crate::superblock::Superblock::parse(file, 0).unwrap();
+            let (os, ls) = (sb.offset_size, sb.length_size);
+            let mut addrs = vec![sb.root_group_address];
+            addrs.extend(
+                crate::group_v2::resolve_group_children(file, &sb, sb.root_group_address)
+                    .unwrap()
+                    .iter()
+                    .map(|e| e.object_header_address),
+            );
+            let storage = CountingStorage::new(file.to_vec());
+            for addr in addrs {
+                let header = ObjectHeader::parse(file, addr as usize, os, ls).unwrap();
+                let want = extract_attributes_full(file, &header, os, ls);
+                let slice_storage = extract_attributes_full_in(&file, &header, os, ls);
+                assert_eq!(format!("{slice_storage:?}"), format!("{want:?}"));
+                let got = extract_attributes_full_in(&storage, &header, os, ls);
+                let got_t = extract_attributes_tolerant_in(&storage, &header, os, ls);
+                let is_dense = find_attribute_info(&header, os)
+                    .unwrap()
+                    .is_some_and(|i| i.fractal_heap_address.is_some());
+                if is_dense {
+                    let e = FormatError::ContiguousStorageRequired(
+                        "dense attribute storage (a v2 B-tree)",
+                    );
+                    assert_eq!(got.unwrap_err(), e, "{name}");
+                    assert_eq!(got_t.unwrap_err(), e, "{name}");
+                    dense += 1;
+                } else {
+                    attrs += want.as_ref().map_or(0, Vec::len);
+                    assert_eq!(format!("{got:?}"), format!("{want:?}"), "{name}");
+                    let want_t = extract_attributes_tolerant(file, &header, os, ls);
+                    assert_eq!(format!("{got_t:?}"), format!("{want_t:?}"), "{name}");
+                    same += 1;
+                }
+            }
+        }
+        assert!(
+            same >= 5 && dense >= 2 && attrs >= 5,
+            "{same} {dense} {attrs}"
+        );
+    }
 }
diff --git a/crates/clawhdf5-format/src/btree_v1.rs b/crates/clawhdf5-format/src/btree_v1.rs
index 0e9b7fe..ca4d9d6 100644
--- a/crates/clawhdf5-format/src/btree_v1.rs
+++ b/crates/clawhdf5-format/src/btree_v1.rs
@@ -3,8 +3,8 @@
 #[cfg(not(feature = "std"))]
 use alloc::vec::Vec;
 
-use crate::addr::to_usize;
 use crate::error::FormatError;
+use crate::storage::{Storage, read_exact_at};
 
 /// A parsed B-tree v1 node.
 #[derive(Debug, Clone)]
@@ -75,13 +75,28 @@ impl BTreeV1Node {
         file_data: &[u8],
         offset: usize,
         offset_size: u8,
+        length_size: u8,
+    ) -> Result<BTreeV1Node, FormatError> {
+        Self::parse_in(file_data, offset as u64, offset_size, length_size)
+    }
+
+    /// [`Self::parse`] over any [`Storage`]: one read of the node's header,
+    /// one of its keys and children.
+    pub fn parse_in<S: Storage + ?Sized>(
+        file: &S,
+        offset: u64,
+        offset_size: u8,
         _length_size: u8,
     ) -> Result<BTreeV1Node, FormatError> {
         // signature(4) + node_type(1) + node_level(1) + entries_used(2) = 8
         // + left_sibling(offset_size) + right_sibling(offset_size)
         let os = offset_size as usize;
         let header_size = 8 + os * 2;
-        ensure_len(file_data, offset, header_size)?;
+        let header = read_exact_at(file, offset, header_size)?;
+        let file_data: &[u8] = &header;
+        // The header's read checked that `offset + header_size` fits.
+        let body_start = offset + header_size as u64;
+        let offset = 0usize;
 
         if &file_data[offset..offset + 4] != b"TREE" {
             return Err(FormatError::InvalidBTreeSignature);
@@ -103,31 +118,30 @@ impl BTreeV1Node {
         } else {
             Some(read_offset(file_data, pos, offset_size)?)
         };
-        pos += os;
 
         // For type 0: keys are offset_size bytes, children are offset_size bytes
         // Layout: key[0], child[0], key[1], child[1], ..., key[N-1], child[N-1], key[N]
         let eu = entries_used as usize;
         let key_size = os; // For type 0, key = offset_size
         let needed = eu * (key_size + os) + key_size; // eu children + (eu+1) keys
-        ensure_len(file_data, pos, needed)?;
+        let body = read_exact_at(file, body_start, needed)?;
+        let file_data: &[u8] = &body;
 
         let mut keys = Vec::with_capacity(eu + 1);
         let mut children = Vec::with_capacity(eu);
 
-        for _i in 0..eu {
-            // key[i]
-            let key = read_offset(file_data, pos, offset_size)?;
-            keys.push(key);
-            pos += key_size;
-            // child[i]
-            let child = read_offset(file_data, pos, offset_size)?;
-            children.push(child);
-            pos += os;
+        if os == 0 {
+            // What reading the first key reports (and keeps `chunks_exact`
+            // below from being given a zero size).
+            return Err(FormatError::InvalidOffsetSize(offset_size));
         }
-        // final key
-        let key = read_offset(file_data, pos, offset_size)?;
-        keys.push(key);
+        // `needed` bytes: key[0], child[0], ..., child[eu - 1], key[eu].
+        let (pairs, last) = file_data.split_at(eu * (key_size + os));
+        for pair in pairs.chunks_exact(key_size + os) {
+            keys.push(read_offset(pair, 0, offset_size)?);
+            children.push(read_offset(pair, key_size, offset_size)?);
+        }
+        keys.push(read_offset(last, 0, offset_size)?);
 
         Ok(BTreeV1Node {
             node_type,
@@ -151,11 +165,21 @@ pub fn collect_symbol_table_nodes(
     offset_size: u8,
     length_size: u8,
 ) -> Result<Vec<u64>, FormatError> {
-    collect_symbol_table_nodes_inner(file_data, btree_address, offset_size, length_size, 0)
+    collect_symbol_table_nodes_in(file_data, btree_address, offset_size, length_size)
 }
 
-fn collect_symbol_table_nodes_inner(
-    file_data: &[u8],
+/// [`collect_symbol_table_nodes`] over any [`Storage`]: two reads per node.
+pub fn collect_symbol_table_nodes_in<S: Storage + ?Sized>(
+    file: &S,
+    btree_address: u64,
+    offset_size: u8,
+    length_size: u8,
+) -> Result<Vec<u64>, FormatError> {
+    collect_symbol_table_nodes_inner(file, btree_address, offset_size, length_size, 0)
+}
+
+fn collect_symbol_table_nodes_inner<S: Storage + ?Sized>(
+    file: &S,
     btree_address: u64,
     offset_size: u8,
     length_size: u8,
@@ -165,12 +189,7 @@ fn collect_symbol_table_nodes_inner(
         return Err(FormatError::NestingDepthExceeded);
     }
 
-    let node = BTreeV1Node::parse(
-        file_data,
-        to_usize(btree_address)?,
-        offset_size,
-        length_size,
-    )?;
+    let node = BTreeV1Node::parse_in(file, btree_address, offset_size, length_size)?;
 
     if node.node_type != 0 {
         return Err(FormatError::InvalidBTreeNodeType(node.node_type));
@@ -184,7 +203,7 @@ fn collect_symbol_table_nodes_inner(
         let mut result = Vec::new();
         for &child_addr in &node.children {
             let child_snods = collect_symbol_table_nodes_inner(
-                file_data,
+                file,
                 child_addr,
                 offset_size,
                 length_size,
@@ -323,4 +342,48 @@ mod tests {
         assert_eq!(node.entries_used, 1);
         assert_eq!(node.children, vec![0x50]);
     }
+
+    /// Nodes and trees, cut at every length, parse identically through a
+    /// `read_at`-only storage.
+    #[test]
+    fn storage_parse_matches_slice_parse() {
+        use crate::storage::CountingStorage;
+        let nodes = [
+            build_btree_node(0, 0, &[0, 5, 10], &[0x100, 0x200], None, None, 8),
+            build_btree_node(0, 0, &[0, 5], &[0x100], Some(0x40), Some(0x80), 4),
+            build_btree_node(1, 2, &[0, 5], &[0x100], None, Some(0x80), 8),
+        ];
+        for (n, node) in nodes.iter().enumerate() {
+            let os = if n == 1 { 4 } else { 8 };
+            for cut in 0..=node.len() {
+                let f = &node[..cut];
+                let storage = CountingStorage::new(f.to_vec());
+                let want = BTreeV1Node::parse(f, 0, os, 8);
+                let got = BTreeV1Node::parse_in(&storage, 0, os, 8);
+                assert_eq!(format!("{got:?}"), format!("{want:?}"));
+            }
+        }
+        let leaf1 = build_btree_node(0, 0, &[0, 5], &[0xA00], None, None, 8);
+        let leaf2 = build_btree_node(0, 0, &[5, 10], &[0xB00], None, None, 8);
+        let internal = build_btree_node(0, 1, &[0, 5, 10], &[0, 256], None, None, 8);
+        let mut file = vec![0u8; 512 + internal.len()];
+        file[..leaf1.len()].copy_from_slice(&leaf1);
+        file[256..256 + leaf2.len()].copy_from_slice(&leaf2);
+        file[512..].copy_from_slice(&internal);
+        for cut in [file.len(), 300, 260, 100, 10] {
+            let mut f = file.clone();
+            if cut < 512 {
+                // Truncate the leaves, keep the root.
+                f[cut..512].fill(0);
+            }
+            let storage = CountingStorage::new(f.clone());
+            assert_eq!(
+                collect_symbol_table_nodes_in(&storage, 512, 8, 8),
+                collect_symbol_table_nodes(&f, 512, 8, 8)
+            );
+        }
+        let storage = CountingStorage::new(file);
+        collect_symbol_table_nodes_in(&storage, 512, 8, 8).unwrap();
+        assert_eq!(storage.reads(), 6);
+    }
 }
diff --git a/crates/clawhdf5-format/src/data_layout.rs b/crates/clawhdf5-format/src/data_layout.rs
index c9b93c7..cf439d6 100644
--- a/crates/clawhdf5-format/src/data_layout.rs
+++ b/crates/clawhdf5-format/src/data_layout.rs
@@ -8,6 +8,7 @@ use std::string::String;
 
 use crate::addr::to_usize;
 use crate::error::FormatError;
+use crate::storage::Storage;
 
 /// A single VDS (Virtual Dataset) source mapping.
 ///
@@ -310,6 +311,16 @@ impl DataLayout {
         &mut self,
         file_data: &[u8],
         length_size: u8,
+    ) -> Result<(), FormatError> {
+        self.resolve_vds_mappings_in(file_data, length_size)
+    }
+
+    /// [`Self::resolve_vds_mappings`] over any [`Storage`]: one read of the
+    /// global heap collection holding the mappings.
+    pub fn resolve_vds_mappings_in<S: Storage + ?Sized>(
+        &mut self,
+        file_data: &S,
+        length_size: u8,
     ) -> Result<(), FormatError> {
         if let DataLayout::Virtual {
             global_heap_address,
@@ -319,11 +330,8 @@ impl DataLayout {
         } = self
             && let Some(addr) = *global_heap_address
         {
-            let coll = crate::global_heap::GlobalHeapCollection::parse(
-                file_data,
-                to_usize(addr)?,
-                length_size,
-            )?;
+            let coll =
+                crate::global_heap::GlobalHeapCollection::parse_in(file_data, addr, length_size)?;
             let obj = coll.get_object(*global_heap_index as u16).ok_or(
                 FormatError::GlobalHeapObjectNotFound {
                     collection_address: addr,
@@ -1306,4 +1314,44 @@ mod tests {
         let blob = [0x01u8, 0, 0, 0, 0, 0, 0, 0, 0];
         assert!(parse_vds_mappings(&blob, 8).unwrap().is_empty());
     }
+
+    /// A virtual dataset's mappings resolve identically through a
+    /// read_at-only CountingStorage, in two reads of the global heap.
+    #[test]
+    fn vds_mappings_through_storage_match_slice() {
+        use crate::message_type::MessageType;
+        use crate::object_header::ObjectHeader;
+        use crate::storage::CountingStorage;
+        let file: &[u8] = include_bytes!("../tests/fixtures/vds_same_file.h5");
+        let sb = crate::superblock::Superblock::parse(file, 0).unwrap();
+        let (os, ls) = (sb.offset_size, sb.length_size);
+        let storage = CountingStorage::new(file.to_vec());
+        let mut virtuals = 0;
+        for child in
+            crate::group_v2::resolve_group_children(file, &sb, sb.root_group_address).unwrap()
+        {
+            let h =
+                ObjectHeader::parse(file, child.object_header_address as usize, os, ls).unwrap();
+            let Some(msg) = h
+                .messages
+                .iter()
+                .find(|m| m.msg_type == MessageType::DataLayout)
+            else {
+                continue;
+            };
+            let mut want = DataLayout::parse(&msg.data, os, ls).unwrap();
+            if !matches!(want, DataLayout::Virtual { .. }) {
+                continue;
+            }
+            let mut got = want.clone();
+            want.resolve_vds_mappings(file, ls).unwrap();
+            storage.reset();
+            got.resolve_vds_mappings_in(&storage, ls).unwrap();
+            assert_eq!(format!("{got:?}"), format!("{want:?}"));
+            assert!(matches!(&got, DataLayout::Virtual { mappings, .. } if !mappings.is_empty()));
+            assert_eq!(storage.reads(), 2);
+            virtuals += 1;
+        }
+        assert!(virtuals >= 1);
+    }
 }
diff --git a/crates/clawhdf5-format/src/error.rs b/crates/clawhdf5-format/src/error.rs
index 7f9e3d2..449ff13 100644
--- a/crates/clawhdf5-format/src/error.rs
+++ b/crates/clawhdf5-format/src/error.rs
@@ -12,7 +12,11 @@ use std::string::String;
 use core::fmt;
 
 /// Errors that can occur when parsing HDF5 binary format structures.
+///
+/// Non-exhaustive: new failure modes (new storage backends, new file
+/// features) add variants, so a `match` needs a wildcard arm.
 #[derive(Debug, Clone, PartialEq, Eq)]
+#[non_exhaustive]
 pub enum FormatError {
     /// The HDF5 magic signature was not found at any valid offset.
     SignatureNotFound,
@@ -243,6 +247,13 @@ pub enum FormatError {
     /// A metadata cache image block libhdf5 refuses to load (the reason is
     /// libhdf5's own error text).
     InvalidCacheImage(&'static str),
+    /// The [`Storage`](crate::storage::Storage) backend failed to serve a
+    /// read (an I/O or network error, or a short read inside the file).
+    Storage(String),
+    /// The operation still needs the whole file as one slice and the
+    /// [`Storage`](crate::storage::Storage) backend has no contiguous view
+    /// (`as_contiguous()` is `None`); the text names the operation.
+    ContiguousStorageRequired(&'static str),
 }
 
 impl fmt::Display for FormatError {
@@ -529,6 +540,16 @@ impl fmt::Display for FormatError {
             FormatError::InvalidCacheImage(why) => {
                 write!(f, "invalid metadata cache image: {why}")
             }
+            FormatError::Storage(why) => {
+                write!(f, "storage read failed: {why}")
+            }
+            FormatError::ContiguousStorageRequired(what) => {
+                write!(
+                    f,
+                    "{what} needs the whole file in memory, which this storage backend does \
+                     not provide"
+                )
+            }
         }
     }
 }
diff --git a/crates/clawhdf5-format/src/extensible_array.rs b/crates/clawhdf5-format/src/extensible_array.rs
index 9256326..42cf595 100644
--- a/crates/clawhdf5-format/src/extensible_array.rs
+++ b/crates/clawhdf5-format/src/extensible_array.rs
@@ -13,16 +13,19 @@ use crate::addr::to_usize;
 use crate::chunk_grid::ChunkGrid;
 use crate::chunked_read::ChunkInfo;
 use crate::error::FormatError;
+use crate::storage::{PAGED_BLOCK_ONE_READ_MAX, Storage, Window, read_exact_at};
 
 /// Verify the Jenkins lookup3 checksum stored immediately after
-/// `data[start..end]`, as every Extensible Array structure carries one.
+/// `data[start..end]`, as every Extensible Array structure carries one. `w`
+/// is a window of the file and `start`/`end` are relative to it.
 ///
 /// A corrupt chunk index yields addresses pointing at the wrong bytes, so a
 /// mismatch is an error: otherwise the damage surfaces as plausible data read
 /// from the wrong chunk.
 #[cfg(feature = "checksum")]
-fn verify_checksum(data: &[u8], start: usize, end: usize) -> Result<(), FormatError> {
-    ensure_len(data, end, 4)?;
+fn verify_checksum(w: &Window<'_>, start: usize, end: usize) -> Result<(), FormatError> {
+    w.ensure(end, 4)?;
+    let data: &[u8] = &w.bytes;
     let stored = u32::from_le_bytes([data[end], data[end + 1], data[end + 2], data[end + 3]]);
     let computed = crate::checksum::jenkins_lookup3(&data[start..end]);
     if computed != stored {
@@ -35,7 +38,7 @@ fn verify_checksum(data: &[u8], start: usize, end: usize) -> Result<(), FormatEr
 }
 
 #[cfg(not(feature = "checksum"))]
-fn verify_checksum(_data: &[u8], _start: usize, _end: usize) -> Result<(), FormatError> {
+fn verify_checksum(_w: &Window<'_>, _start: usize, _end: usize) -> Result<(), FormatError> {
     Ok(())
 }
 
@@ -81,19 +84,6 @@ fn read_offset(data: &[u8], pos: usize, size: u8) -> Result<u64, FormatError> {
     })
 }
 
-fn ensure_len(data: &[u8], offset: usize, needed: usize) -> Result<(), FormatError> {
-    if offset
-        .checked_add(needed)
-        .is_none_or(|end| end > data.len())
-    {
-        return Err(FormatError::UnexpectedEof {
-            expected: offset.saturating_add(needed),
-            available: data.len(),
-        });
-    }
-    Ok(())
-}
-
 fn is_undefined_addr(addr: u64, offset_size: u8) -> bool {
     match offset_size {
         2 => addr == 0xFFFF,
@@ -131,6 +121,16 @@ impl ExtensibleArrayHeader {
         offset: usize,
         offset_size: u8,
         length_size: u8,
+    ) -> Result<Self, FormatError> {
+        Self::parse_in(file_data, offset as u64, offset_size, length_size)
+    }
+
+    /// [`Self::parse`] over any [`Storage`]: one read of the header.
+    pub fn parse_in<S: Storage + ?Sized>(
+        file: &S,
+        offset: u64,
+        offset_size: u8,
+        length_size: u8,
     ) -> Result<Self, FormatError> {
         // EAHD: signature(4) + version(1) + client_id(1) + element_size(1) +
         //   max_nelmts_bits(1) + idx_blk_elmts(1) + min_dblk_nelmts(1) +
@@ -138,9 +138,10 @@ impl ExtensibleArrayHeader {
         //   6 stats fields (each length_size) + index_block_address(offset_size) + checksum(4)
         let min_size =
             4 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 6 * length_size as usize + offset_size as usize + 4;
-        ensure_len(file_data, offset, min_size)?;
+        let w = Window::read(file, offset, min_size)?;
+        w.ensure(0, min_size)?;
 
-        let d = &file_data[offset..];
+        let d: &[u8] = &w.bytes;
         if &d[0..4] != b"EAHD" {
             return Err(FormatError::ChunkedReadError(
                 "invalid Extensible Array header signature".into(),
@@ -173,7 +174,7 @@ impl ExtensibleArrayHeader {
         pos += ls; // skip max_idx_set (6th stats field)
         let index_block_address = read_offset(d, pos, offset_size)?;
         pos += offset_size as usize;
-        verify_checksum(file_data, offset, offset + pos)?;
+        verify_checksum(&w, 0, pos)?;
 
         Ok(ExtensibleArrayHeader {
             client_id,
@@ -194,11 +195,11 @@ impl ExtensibleArrayHeader {
     }
 }
 
-/// Read a single element from the extensible array element data.
+/// Read a single element at offset `pos` of the window `w`.
 /// Returns (chunk_info, bytes_consumed) or None if unallocated.
 #[allow(clippy::too_many_arguments)]
 fn read_element(
-    data: &[u8],
+    w: &Window<'_>,
     pos: usize,
     client_id: u8,
     element_size: u8,
@@ -208,15 +209,11 @@ fn read_element(
     grid: &ChunkGrid,
 ) -> Result<(Option<ChunkInfo>, usize), FormatError> {
     let os = offset_size as usize;
+    let data: &[u8] = &w.bytes;
 
     if client_id == 0 {
         // Non-filtered: just address
-        if pos + os > data.len() {
-            return Err(FormatError::UnexpectedEof {
-                expected: pos + os,
-                available: data.len(),
-            });
-        }
+        w.ensure(pos, os)?;
         if is_undefined(data, pos, offset_size) {
             return Ok((None, os));
         }
@@ -244,15 +241,7 @@ fn read_element(
         }
         let chunk_size_bytes = es - os - 4;
         let elem_total = os + chunk_size_bytes + 4;
-        if pos
-            .checked_add(elem_total)
-            .is_none_or(|end| end > data.len())
-        {
-            return Err(FormatError::UnexpectedEof {
-                expected: pos.saturating_add(elem_total),
-                available: data.len(),
-            });
-        }
+        w.ensure(pos, elem_total)?;
         if is_undefined(data, pos, offset_size) {
             return Ok((None, elem_total));
         }
@@ -316,9 +305,9 @@ fn page_nelmts(header: &ExtensibleArrayHeader) -> Option<usize> {
 /// paged. The bitmap lives in the super block, not here — a paged data block
 /// stores only its prefix, then one slot per page.
 #[allow(clippy::too_many_arguments)]
-fn read_data_block_elements(
-    file_data: &[u8],
-    db_offset: usize,
+fn read_data_block_elements<S: Storage + ?Sized>(
+    file: &S,
+    db_offset: u64,
     nelmts: usize,
     header: &ExtensibleArrayHeader,
     offset_size: u8,
@@ -331,21 +320,28 @@ fn read_data_block_elements(
     // EADB: signature(4) + version(1) + client_id(1) + header_address(offset_size)
     //       + block offset(arr_off_size)
     let db_header_size = 4 + 1 + 1 + offset_size as usize + arr_off_size(header);
-    ensure_len(file_data, db_offset, db_header_size)?;
+    let prefix = read_exact_at(file, db_offset, db_header_size)?;
 
-    if &file_data[db_offset..db_offset + 4] != b"EADB" {
+    if &prefix[0..4] != b"EADB" {
         return Err(FormatError::ChunkedReadError(
             "invalid Extensible Array data block signature".into(),
         ));
     }
 
-    let mut pos = db_offset + db_header_size;
+    // Positions below are relative to the data block.
+    let mut pos = db_header_size;
     let page = page_nelmts(header).ok_or_else(|| {
         FormatError::Overflow("Extensible Array page element count overflows usize".into())
     })?;
+    let elem_bytes = if header.client_id == 0 {
+        offset_size as usize
+    } else {
+        header.element_size as usize
+    };
 
     let mut chunks = Vec::new();
-    let read_run = |from: usize,
+    let read_run = |w: &Window<'_>,
+                    from: usize,
                     count: usize,
                     first_index: usize,
                     chunks: &mut Vec<ChunkInfo>|
@@ -353,7 +349,7 @@ fn read_data_block_elements(
         let mut p = from;
         for i in 0..count {
             let (info, consumed) = read_element(
-                file_data,
+                w,
                 p,
                 header.client_id,
                 header.element_size,
@@ -371,18 +367,19 @@ fn read_data_block_elements(
     };
 
     if nelmts <= page {
-        // Prefix and elements are covered by one checksum.
-        let elem_bytes = if header.client_id == 0 {
-            offset_size as usize
-        } else {
-            header.element_size as usize
-        };
+        // Prefix and elements are covered by one checksum. One window holds
+        // all of it (or ends at the end of the file), so its bounds checks
+        // are the whole-file ones.
         let end = nelmts
             .checked_mul(elem_bytes)
             .and_then(|b| pos.checked_add(b))
             .ok_or_else(|| FormatError::Overflow("Extensible Array data block span".into()))?;
-        verify_checksum(file_data, db_offset, end)?;
-        read_run(pos, nelmts, start_index, &mut chunks)?;
+        // The checksum's bounds check comes first: make it before reading.
+        #[cfg(feature = "checksum")]
+        Window::check_extent(file, db_offset, end, 4)?;
+        let w = Window::read(file, db_offset, end.saturating_add(4))?;
+        verify_checksum(&w, 0, end)?;
+        read_run(&w, pos, nelmts, start_index, &mut chunks)?;
         return Ok(chunks);
     }
 
@@ -390,18 +387,32 @@ fn read_data_block_elements(
     // each holding `page` elements followed by a checksum. Pages whose bit is
     // clear were never written; their slot still occupies the file, so stride
     // over it rather than reading zeros as addresses.
-    verify_checksum(file_data, db_offset, pos)?;
-    pos += 4;
-    let elem_bytes = if header.client_id == 0 {
-        offset_size as usize
+    let npages = nelmts.div_ceil(page);
+    // The whole data block in one window when it is small: every position
+    // checked below lies inside it (or past the end of the file). A larger
+    // block is read as its prefix, then each page in use on its own.
+    let block_len = pos
+        .saturating_add(4)
+        .saturating_add(npages.saturating_mul(page.saturating_mul(elem_bytes).saturating_add(4)));
+    let whole = if block_len <= PAGED_BLOCK_ONE_READ_MAX {
+        Some(Window::read(file, db_offset, block_len)?)
     } else {
-        header.element_size as usize
+        None
     };
+    let head_w;
+    let head = match &whole {
+        Some(w) => w,
+        None => {
+            head_w = Window::read(file, db_offset, pos + 4)?;
+            &head_w
+        }
+    };
+    verify_checksum(head, 0, pos)?;
+    pos += 4;
     let page_stride = page
         .checked_mul(elem_bytes)
         .and_then(|b| b.checked_add(4))
         .ok_or_else(|| FormatError::Overflow("Extensible Array page stride".into()))?;
-    let npages = nelmts.div_ceil(page);
     for p in 0..npages {
         // One bit per page across the whole super block, packed contiguously
         // and MSB-first within each byte, as H5VM_bit_get reads it.
@@ -411,10 +422,20 @@ fn read_data_block_elements(
             .is_some_and(|byte| byte & (0x80 >> (bit % 8)) != 0);
         if initialised {
             let count = core::cmp::min(page, nelmts - p * page);
+            // `w` holds the page from `base` on (positions below are
+            // relative to it, and `pos` to the data block).
+            let page_w;
+            let (w, base) = match &whole {
+                Some(w) => (w, 0),
+                None => {
+                    page_w = Window::read(file, db_offset.saturating_add(pos as u64), page_stride)?;
+                    (&page_w, pos)
+                }
+            };
             // Each page carries its own checksum, over a full page's worth of
             // slots even when the last one holds fewer live elements.
-            verify_checksum(file_data, pos, pos + page * elem_bytes)?;
-            read_run(pos, count, start_index + p * page, &mut chunks)?;
+            verify_checksum(w, pos - base, pos - base + page * elem_bytes)?;
+            read_run(w, pos - base, count, start_index + p * page, &mut chunks)?;
         }
         pos = pos
             .checked_add(page_stride)
@@ -436,6 +457,32 @@ pub fn read_extensible_array_chunks(
     chunk_dimensions: &[u32],
     element_size: u32,
     offset_size: u8,
+    length_size: u8,
+) -> Result<Vec<ChunkInfo>, FormatError> {
+    read_extensible_array_chunks_in(
+        &file_data,
+        header,
+        dataset_dims,
+        max_dims,
+        chunk_dimensions,
+        element_size,
+        offset_size,
+        length_size,
+    )
+}
+
+/// [`read_extensible_array_chunks`] over any [`Storage`]: one read of the
+/// index block's prefix, one of the whole index block, and the same for
+/// every super block and data block it references.
+#[allow(clippy::too_many_arguments)]
+pub fn read_extensible_array_chunks_in<S: Storage + ?Sized>(
+    file: &S,
+    header: &ExtensibleArrayHeader,
+    dataset_dims: &[u64],
+    max_dims: Option<&[u64]>,
+    chunk_dimensions: &[u32],
+    element_size: u32,
+    offset_size: u8,
     _length_size: u8,
 ) -> Result<Vec<ChunkInfo>, FormatError> {
     let os = offset_size as usize;
@@ -452,16 +499,17 @@ pub fn read_extensible_array_chunks(
     // Parse index block (EAIB): signature(4) + version(1) + client_id(1)
     // + header address(offset_size), then the inline elements, then the
     // direct data block addresses, then the super block addresses.
-    let ib_offset = to_usize(header.index_block_address)?;
+    // Positions below are relative to the index block.
+    let ib_offset = header.index_block_address;
     let ib_header_size = 4 + 1 + 1 + os;
-    ensure_len(file_data, ib_offset, ib_header_size)?;
+    let prefix = read_exact_at(file, ib_offset, ib_header_size)?;
 
-    if &file_data[ib_offset..ib_offset + 4] != b"EAIB" {
+    if &prefix[0..4] != b"EAIB" {
         return Err(FormatError::ChunkedReadError(
             "invalid Extensible Array index block signature".into(),
         ));
     }
-    let mut pos = ib_offset + ib_header_size;
+    let mut pos = ib_header_size;
 
     let mut chunks = Vec::new();
     let total_elements = to_usize(header.num_elements)?;
@@ -521,13 +569,19 @@ pub fn read_extensible_array_chunks(
                 .and_then(|n| n.checked_mul(os).and_then(|b| p.checked_add(b)))
         })
         .ok_or_else(|| FormatError::Overflow("Extensible Array index block span".into()))?;
-    verify_checksum(file_data, ib_offset, ib_end)?;
+    // The whole index block in one window: every position read below is
+    // before `ib_end`.
+    // The checksum's bounds check comes first: make it before reading.
+    #[cfg(feature = "checksum")]
+    Window::check_extent(file, ib_offset, ib_end, 4)?;
+    let w = Window::read(file, ib_offset, ib_end.saturating_add(4))?;
+    verify_checksum(&w, 0, ib_end)?;
 
     // 1. Elements stored inline in the index block.
     let n_inline = (header.idx_blk_elmts as usize).min(total_elements);
     for i in 0..n_inline {
         let (info, consumed) = read_element(
-            file_data,
+            &w,
             pos,
             header.client_id,
             header.element_size,
@@ -551,8 +605,8 @@ pub fn read_extensible_array_chunks(
         if global_index >= total_elements {
             return Ok(chunks);
         }
-        ensure_len(file_data, pos, os)?;
-        let addr = read_offset(file_data, pos, offset_size)?;
+        w.ensure(pos, os)?;
+        let addr = read_offset(&w.bytes, pos, offset_size)?;
         pos += os;
         if !is_undefined_addr(addr, offset_size) {
             if dblk_nelmts > page_nelmts(header).unwrap_or(usize::MAX) {
@@ -563,8 +617,8 @@ pub fn read_extensible_array_chunks(
                 ));
             }
             chunks.extend(read_data_block_elements(
-                file_data,
-                to_usize(addr)?,
+                file,
+                addr,
                 dblk_nelmts,
                 header,
                 offset_size,
@@ -584,16 +638,16 @@ pub fn read_extensible_array_chunks(
         if global_index >= total_elements {
             break;
         }
-        ensure_len(file_data, pos, os)?;
-        let sb_addr = read_offset(file_data, pos, offset_size)?;
+        w.ensure(pos, os)?;
+        let sb_addr = read_offset(&w.bytes, pos, offset_size)?;
         pos += os;
         let (ndblks, dblk_nelmts) = sblk_info(u, dmin).ok_or_else(|| {
             FormatError::Overflow("Extensible Array super block layout overflows usize".into())
         })?;
         if !is_undefined_addr(sb_addr, offset_size) {
             chunks.extend(read_super_block(
-                file_data,
-                to_usize(sb_addr)?,
+                file,
+                sb_addr,
                 ndblks,
                 dblk_nelmts,
                 header,
@@ -618,9 +672,9 @@ pub fn read_extensible_array_chunks(
 /// + block offset + the page-init bitmap for every data block it owns
 /// + one address per data block + checksum.
 #[allow(clippy::too_many_arguments)]
-fn read_super_block(
-    file_data: &[u8],
-    sb_offset: usize,
+fn read_super_block<S: Storage + ?Sized>(
+    file: &S,
+    sb_offset: u64,
     ndblks: usize,
     dblk_nelmts: usize,
     header: &ExtensibleArrayHeader,
@@ -631,9 +685,9 @@ fn read_super_block(
 ) -> Result<Vec<ChunkInfo>, FormatError> {
     let os = offset_size as usize;
     let sb_header_size = 4 + 1 + 1 + os + arr_off_size(header);
-    ensure_len(file_data, sb_offset, sb_header_size)?;
+    let prefix = read_exact_at(file, sb_offset, sb_header_size)?;
 
-    if &file_data[sb_offset..sb_offset + 4] != b"EASB" {
+    if &prefix[0..4] != b"EASB" {
         return Err(FormatError::ChunkedReadError(
             "invalid Extensible Array super block signature".into(),
         ));
@@ -655,29 +709,38 @@ fn read_super_block(
     let bitmap_bytes = per_dblk_bitmap
         .checked_mul(ndblks)
         .ok_or_else(|| FormatError::Overflow("Extensible Array page bitmap size".into()))?;
-    let bitmap_start = sb_offset + sb_header_size;
-    ensure_len(file_data, bitmap_start, bitmap_bytes)?;
-    let bitmap = &file_data[bitmap_start..bitmap_start + bitmap_bytes];
-
+    // Positions below are relative to the super block, whose bytes (up to
+    // its checksum) are all in one window.
+    let bitmap_start = sb_header_size;
+    // The bitmap's bounds check, then (with checksums) the checksum's, come
+    // before anything else is read from the block: make them before reading
+    // it, so size fields stretching it past the end of the file cost no read.
+    Window::check_extent(file, sb_offset, bitmap_start, bitmap_bytes)?;
     let mut pos = bitmap_start + bitmap_bytes;
-    let mut chunks = Vec::new();
-    let mut global_idx = start_index;
 
     // One checksum covers the prefix, the bitmap and every data block address.
     let sb_end = ndblks
         .checked_mul(os)
         .and_then(|b| pos.checked_add(b))
         .ok_or_else(|| FormatError::Overflow("Extensible Array super block span".into()))?;
-    verify_checksum(file_data, sb_offset, sb_end)?;
+    #[cfg(feature = "checksum")]
+    Window::check_extent(file, sb_offset, sb_end, 4)?;
+    let w = Window::read(file, sb_offset, sb_end.saturating_add(4))?;
+    w.ensure(bitmap_start, bitmap_bytes)?;
+    let bitmap = &w.bytes[bitmap_start..bitmap_start + bitmap_bytes];
+
+    let mut chunks = Vec::new();
+    let mut global_idx = start_index;
+    verify_checksum(&w, 0, sb_end)?;
 
     for i in 0..ndblks {
-        ensure_len(file_data, pos, os)?;
-        let addr = read_offset(file_data, pos, offset_size)?;
+        w.ensure(pos, os)?;
+        let addr = read_offset(&w.bytes, pos, offset_size)?;
         pos += os;
         if !is_undefined_addr(addr, offset_size) {
             chunks.extend(read_data_block_elements(
-                file_data,
-                to_usize(addr)?,
+                file,
+                addr,
                 dblk_nelmts,
                 header,
                 offset_size,
@@ -888,11 +951,11 @@ mod tests {
         assert_eq!(chunks[1].offsets, vec![20]);
     }
 
-    /// Build a synthetic EA with inline elements + one direct data block.
-    #[test]
-    fn read_inline_plus_data_blocks() {
+    /// A synthetic EA with inline elements + one direct data block: the
+    /// file, with the header at 0x100 (8-byte offsets and lengths, 4 chunks
+    /// of 10 elements from 0x1000 on).
+    fn build_inline_plus_data_blocks() -> Vec<u8> {
         let os: u8 = 8;
-        let ls: u8 = 8;
         let osv = os as usize;
         let chunk_byte_size = 10u64 * 8; // 10 elements × 8 bytes
         let idx_blk_elmts = 2u8;
@@ -982,8 +1045,17 @@ mod tests {
             dbpos += osv;
         }
         stamp_checksum(&mut file_data, aedb_offset, dbpos);
+        file_data
+    }
 
-        let header = ExtensibleArrayHeader::parse(&file_data, aehd_offset, os, ls).unwrap();
+    /// Build a synthetic EA with inline elements + one direct data block.
+    #[test]
+    fn read_inline_plus_data_blocks() {
+        let (os, ls) = (8u8, 8u8);
+        let chunk_byte_size = 10u64 * 8;
+        let base_addr = 0x1000u64;
+        let file_data = build_inline_plus_data_blocks();
+        let header = ExtensibleArrayHeader::parse(&file_data, 0x100, os, ls).unwrap();
         let ds_dims = vec![40u64];
         let chunk_dims = vec![10u32];
         let chunks = read_extensible_array_chunks(
@@ -1019,7 +1091,8 @@ mod tests {
     fn read_element_unallocated() {
         let data = vec![0xFFu8; 16];
         let grid = ChunkGrid::fixed_array(&[50], None, &[10]).unwrap();
-        let (info, consumed) = read_element(&data, 0, 0, 8, 8, 80, 0, &grid).unwrap();
+        let (info, consumed) =
+            read_element(&Window::whole(&data), 0, 0, 8, 8, 80, 0, &grid).unwrap();
         assert!(info.is_none());
         assert_eq!(consumed, 8);
     }
@@ -1039,8 +1112,17 @@ mod tests {
         data[12..16].copy_from_slice(&0u32.to_le_bytes());
 
         let grid = ChunkGrid::fixed_array(&[50], None, &[10]).unwrap();
-        let (info, consumed) =
-            read_element(&data, 0, 1, elem_size as u8, os, 80, 2, &grid).unwrap();
+        let (info, consumed) = read_element(
+            &Window::whole(&data),
+            0,
+            1,
+            elem_size as u8,
+            os,
+            80,
+            2,
+            &grid,
+        )
+        .unwrap();
         let ci = info.unwrap();
         assert_eq!(ci.address, 0x2000);
         assert_eq!(ci.chunk_size, 120);
@@ -1048,4 +1130,38 @@ mod tests {
         assert_eq!(ci.offsets, vec![20]);
         assert_eq!(consumed, elem_size);
     }
+
+    /// The Storage path reads exactly what the slice path reads: the array
+    /// whole, cut at every length through its structures, and with a byte
+    /// damaged in each of them, through a read_at-only CountingStorage.
+    #[test]
+    fn storage_reads_match_slice_reads() {
+        use crate::storage::CountingStorage;
+        let full = build_inline_plus_data_blocks();
+        let mut files = Vec::new();
+        for cut in 0x100..0x340 {
+            files.push(full[..cut].to_vec());
+        }
+        for at in [0x104, 0x150, 0x204, 0x216, 0x230, 0x304, 0x318] {
+            let mut damaged = full.clone();
+            damaged[at] ^= 1;
+            files.push(damaged);
+        }
+        files.push(full);
+        let mut compared = 0;
+        for f in files {
+            let storage = CountingStorage::new(f.clone());
+            let want = ExtensibleArrayHeader::parse(&f, 0x100, 8, 8);
+            let got = ExtensibleArrayHeader::parse_in(&storage, 0x100, 8, 8);
+            assert_eq!(format!("{got:?}"), format!("{want:?}"));
+            let Ok(h) = want else { continue };
+            for dims in [&[40u64][..], &[25]] {
+                let want = read_extensible_array_chunks(&f, &h, dims, None, &[10], 8, 8, 8);
+                let got = read_extensible_array_chunks_in(&storage, &h, dims, None, &[10], 8, 8, 8);
+                assert_eq!(format!("{got:?}"), format!("{want:?}"), "{} bytes", f.len());
+                compared += 1;
+            }
+        }
+        assert!(compared > 100);
+    }
 }
diff --git a/crates/clawhdf5-format/src/fill_value.rs b/crates/clawhdf5-format/src/fill_value.rs
index d8c34f3..f0f442e 100644
--- a/crates/clawhdf5-format/src/fill_value.rs
+++ b/crates/clawhdf5-format/src/fill_value.rs
@@ -117,9 +117,21 @@ pub fn dataset_fill_value_in(
     messages: &[HeaderMessage],
     offset_size: u8,
     length_size: u8,
+) -> Result<Option<Vec<u8>>, FormatError> {
+    dataset_fill_value_from_storage(&file_data, messages, offset_size, length_size)
+}
+
+/// [`dataset_fill_value_in`] with the file behind any
+/// [`Storage`](crate::storage::Storage). (The trait is not imported here:
+/// its `len` would shadow the slice method in this module.)
+pub fn dataset_fill_value_from_storage(
+    file: &dyn crate::storage::Storage,
+    messages: &[HeaderMessage],
+    offset_size: u8,
+    length_size: u8,
 ) -> Result<Option<Vec<u8>>, FormatError> {
     fill_value_from(messages, |msg| {
-        crate::shared_message::message_data_with_sohm(file_data, msg, offset_size, length_size)
+        crate::shared_message::message_data_with_sohm_in(file, msg, offset_size, length_size)
             .map(|data| data.into_owned())
     })
 }
@@ -444,4 +456,37 @@ mod tests {
             .collect();
         assert_eq!(filled, [2, 3, 7, 8]);
     }
+
+    /// Fill values, shared ones in the SOHM heap included, resolve
+    /// identically through a read_at-only CountingStorage.
+    #[test]
+    fn storage_reads_match_slice_reads() {
+        use crate::object_header::ObjectHeader;
+        use crate::storage::CountingStorage;
+        let file: &[u8] = include_bytes!("../tests/fixtures/shared_fill_value.h5");
+        let sb = crate::superblock::Superblock::parse(file, 0).unwrap();
+        let (os, ls) = (sb.offset_size, sb.length_size);
+        let storage = CountingStorage::new(file.to_vec());
+        let mut shared = 0;
+        let children =
+            crate::group_v2::resolve_group_children(file, &sb, sb.root_group_address).unwrap();
+        assert!(children.len() >= 3);
+        for child in children {
+            let h =
+                ObjectHeader::parse(file, child.object_header_address as usize, os, ls).unwrap();
+            shared += h
+                .messages
+                .iter()
+                .filter(|m| {
+                    m.msg_type == MessageType::FillValue
+                        && crate::shared_message::is_shared(m.flags)
+                })
+                .count();
+            let want = dataset_fill_value_in(file, &h.messages, os, ls);
+            assert_eq!(want, Ok(Some((-7i32).to_le_bytes().to_vec())));
+            let got = dataset_fill_value_from_storage(&storage, &h.messages, os, ls);
+            assert_eq!(got, want, "{}", child.name);
+        }
+        assert!(shared >= 2);
+    }
 }
diff --git a/crates/clawhdf5-format/src/fixed_array.rs b/crates/clawhdf5-format/src/fixed_array.rs
index 5afc996..de80b7a 100644
--- a/crates/clawhdf5-format/src/fixed_array.rs
+++ b/crates/clawhdf5-format/src/fixed_array.rs
@@ -10,16 +10,19 @@ use crate::addr::to_usize;
 use crate::chunk_grid::ChunkGrid;
 use crate::chunked_read::ChunkInfo;
 use crate::error::FormatError;
+use crate::storage::{PAGED_BLOCK_ONE_READ_MAX, Storage, Window, len_usize, read_exact_at};
 
 /// Verify the Jenkins lookup3 checksum stored immediately after
-/// `data[start..end]`, as every Fixed Array structure carries one.
+/// `data[start..end]`, as every Fixed Array structure carries one. `w` is
+/// a window of the file and `start`/`end` are relative to it.
 ///
 /// A corrupt chunk index silently yields addresses pointing at the wrong
 /// bytes, so a mismatch has to be an error rather than a shrug: without this
 /// the damage surfaces as plausible-looking data from the wrong chunk.
 #[cfg(feature = "checksum")]
-fn verify_checksum(data: &[u8], start: usize, end: usize) -> Result<(), FormatError> {
-    ensure_len(data, end, 4)?;
+fn verify_checksum(w: &Window<'_>, start: usize, end: usize) -> Result<(), FormatError> {
+    w.ensure(end, 4)?;
+    let data = &w.bytes;
     let stored = u32::from_le_bytes([data[end], data[end + 1], data[end + 2], data[end + 3]]);
     let computed = crate::checksum::jenkins_lookup3(&data[start..end]);
     if computed != stored {
@@ -32,7 +35,7 @@ fn verify_checksum(data: &[u8], start: usize, end: usize) -> Result<(), FormatEr
 }
 
 #[cfg(not(feature = "checksum"))]
-fn verify_checksum(_data: &[u8], _start: usize, _end: usize) -> Result<(), FormatError> {
+fn verify_checksum(_w: &Window<'_>, _start: usize, _end: usize) -> Result<(), FormatError> {
     Ok(())
 }
 
@@ -74,19 +77,6 @@ fn read_length(data: &[u8], pos: usize, size: u8) -> Result<u64, FormatError> {
     read_offset(data, pos, size)
 }
 
-fn ensure_len(data: &[u8], offset: usize, needed: usize) -> Result<(), FormatError> {
-    if offset
-        .checked_add(needed)
-        .is_none_or(|end| end > data.len())
-    {
-        return Err(FormatError::UnexpectedEof {
-            expected: offset.saturating_add(needed),
-            available: data.len(),
-        });
-    }
-    Ok(())
-}
-
 fn is_undefined(data: &[u8], pos: usize, size: u8) -> bool {
     let s = size as usize;
     if pos + s > data.len() {
@@ -102,13 +92,24 @@ impl FixedArrayHeader {
         offset: usize,
         offset_size: u8,
         length_size: u8,
+    ) -> Result<Self, FormatError> {
+        Self::parse_in(file_data, offset as u64, offset_size, length_size)
+    }
+
+    /// [`Self::parse`] over any [`Storage`]: one read of the header.
+    pub fn parse_in<S: Storage + ?Sized>(
+        file: &S,
+        offset: u64,
+        offset_size: u8,
+        length_size: u8,
     ) -> Result<Self, FormatError> {
         // FAHD signature(4) + version(1) + client_id(1) + element_size(1) +
         // max_nelmts_bits(1) + num_elements(length_size) + data_block_addr(offset_size) + checksum(4)
         let min_size = 4 + 1 + 1 + 1 + 1 + length_size as usize + offset_size as usize + 4;
-        ensure_len(file_data, offset, min_size)?;
+        let w = Window::read(file, offset, min_size)?;
+        w.ensure(0, min_size)?;
 
-        let d = &file_data[offset..];
+        let d: &[u8] = &w.bytes;
         if &d[0..4] != b"FAHD" {
             return Err(FormatError::ChunkedReadError(
                 "invalid Fixed Array header signature".into(),
@@ -131,7 +132,7 @@ impl FixedArrayHeader {
         pos += length_size as usize;
         let data_block_address = read_offset(d, pos, offset_size)?;
         pos += offset_size as usize;
-        verify_checksum(file_data, offset, offset + pos)?;
+        verify_checksum(&w, 0, pos)?;
 
         Ok(FixedArrayHeader {
             client_id,
@@ -157,15 +158,39 @@ pub fn read_fixed_array_chunks(
     chunk_dimensions: &[u32],
     element_size: u32,
     offset_size: u8,
+    length_size: u8,
+) -> Result<Vec<ChunkInfo>, FormatError> {
+    read_fixed_array_chunks_in(
+        &file_data,
+        header,
+        dataset_dims,
+        max_dims,
+        chunk_dimensions,
+        element_size,
+        offset_size,
+        length_size,
+    )
+}
+
+/// [`read_fixed_array_chunks`] over any [`Storage`]: one read of the data
+/// block's prefix, one of the whole data block (pages included).
+#[allow(clippy::too_many_arguments)]
+pub fn read_fixed_array_chunks_in<S: Storage + ?Sized>(
+    file: &S,
+    header: &FixedArrayHeader,
+    dataset_dims: &[u64],
+    max_dims: Option<&[u64]>,
+    chunk_dimensions: &[u32],
+    element_size: u32,
+    offset_size: u8,
     _length_size: u8,
 ) -> Result<Vec<ChunkInfo>, FormatError> {
+    let file_len = len_usize(file);
     let db_offset = to_usize(header.data_block_address)?;
 
     // Parse data block header: FADB(4) + version(1) + client_id(1) + header_address(offset_size)
     let db_header_size = 4 + 1 + 1 + offset_size as usize;
-    ensure_len(file_data, db_offset, db_header_size)?;
-
-    let d = &file_data[db_offset..];
+    let d = read_exact_at(file, db_offset as u64, db_header_size)?;
     if &d[0..4] != b"FADB" {
         return Err(FormatError::ChunkedReadError(
             "invalid Fixed Array data block signature".into(),
@@ -179,7 +204,7 @@ pub fn read_fixed_array_chunks(
     // A chunk index cannot describe more elements than the file has bytes (each
     // element occupies at least `offset_size` bytes). Reject a corrupt count
     // before it can drive a huge loop or overflow an offset computation.
-    if num_elements > file_data.len() {
+    if num_elements > file_len {
         return Err(FormatError::ChunkedReadError(
             "Fixed Array element count exceeds file size".into(),
         ));
@@ -209,30 +234,34 @@ pub fn read_fixed_array_chunks(
         chunk_dimensions.iter().map(|&d| d as u64).product::<u64>() * element_size as u64;
 
     let mut chunks = Vec::new();
-    let push_element =
-        |i: usize, abs: usize, chunks: &mut Vec<ChunkInfo>| -> Result<(), FormatError> {
-            if let Some((address, chunk_size, filter_mask)) = parse_fa_element(
-                file_data,
-                abs,
-                header.client_id,
-                offset_size,
-                header.element_size,
-                chunk_byte_size,
-            )? {
-                // A slot beyond the current extent is ignored, as the
-                // library does.
-                let Some(offsets) = grid.offsets(i as u64) else {
-                    return Ok(());
-                };
-                chunks.push(ChunkInfo {
-                    chunk_size,
-                    filter_mask,
-                    offsets,
-                    address,
-                });
-            }
-            Ok(())
-        };
+    // `rel` is relative to the data block, whose bytes are in `w`.
+    let push_element = |w: &Window<'_>,
+                        i: usize,
+                        rel: usize,
+                        chunks: &mut Vec<ChunkInfo>|
+     -> Result<(), FormatError> {
+        if let Some((address, chunk_size, filter_mask)) = parse_fa_element(
+            w,
+            rel,
+            header.client_id,
+            offset_size,
+            header.element_size,
+            chunk_byte_size,
+        )? {
+            // A slot beyond the current extent is ignored, as the
+            // library does.
+            let Some(offsets) = grid.offsets(i as u64) else {
+                return Ok(());
+            };
+            chunks.push(ChunkInfo {
+                chunk_size,
+                filter_mask,
+                offsets,
+                address,
+            });
+        }
+        Ok(())
+    };
 
     // A data block is paged when it holds more elements than fit in one page.
     // `max_nelmts_bits` is an untrusted u8; a shift >= the pointer width would
@@ -247,10 +276,16 @@ pub fn read_fixed_array_chunks(
 
     if !is_paged {
         // Non-paged: prefix, then `num_elements` elements packed directly,
-        // then a checksum over both.
-        verify_checksum(file_data, db_offset, elem_at(elements_start, num_elements)?)?;
+        // then a checksum over both. One window holds all of it (or ends at
+        // the end of the file), so its bounds checks are the whole-file ones.
+        let end = elem_at(elements_start, num_elements)?;
+        // The checksum's bounds check comes first: make it before reading.
+        #[cfg(feature = "checksum")]
+        Window::check_extent(file, db_offset as u64, end - db_offset, 4)?;
+        let w = Window::read(file, db_offset as u64, end.saturating_add(4) - db_offset)?;
+        verify_checksum(&w, 0, end - db_offset)?;
         for i in 0..num_elements {
-            push_element(i, elem_at(elements_start, i)?, &mut chunks)?;
+            push_element(&w, i, elem_at(elements_start, i)? - db_offset, &mut chunks)?;
         }
         return Ok(chunks);
     }
@@ -273,22 +308,40 @@ pub fn read_fixed_array_chunks(
         .and_then(|x| x.checked_add(4))
         .ok_or_else(stride_overflow)?;
 
-    if bitmap_start + bitmap_size > file_data.len() {
+    if bitmap_start + bitmap_size > file_len {
         return Err(FormatError::UnexpectedEof {
             expected: bitmap_start + bitmap_size,
-            available: file_data.len(),
+            available: file_len,
         });
     }
+    // The whole data block in one window when it is small: every page slot
+    // is at most `page_stride` bytes, so every position checked below lies
+    // inside it (or past the end of the file). A larger block is read as its
+    // prefix and bitmap, then each page in use on its own.
+    let block_len = (pages_start - db_offset).saturating_add(npages.saturating_mul(page_stride));
+    let whole = if block_len <= PAGED_BLOCK_ONE_READ_MAX {
+        Some(Window::read(file, db_offset as u64, block_len)?)
+    } else {
+        None
+    };
+    let head_w;
+    let head = match &whole {
+        Some(w) => w,
+        None => {
+            head_w = Window::read(file, db_offset as u64, pages_start - db_offset)?;
+            &head_w
+        }
+    };
     // The prefix and page bitmap are covered by their own checksum, and each
     // initialised page by one of its own.
-    verify_checksum(file_data, db_offset, bitmap_start + bitmap_size)?;
+    verify_checksum(head, 0, bitmap_start + bitmap_size - db_offset)?;
 
     for p in 0..npages {
         let page_first = p * page_nelmts; // < num_elements, cannot overflow
         let page_count = core::cmp::min(page_nelmts, num_elements - page_first);
 
         // Check the page-init bit (MSB-first within each byte).
-        let bit_byte = file_data[bitmap_start + p / 8];
+        let bit_byte = head.bytes[bitmap_start + p / 8 - db_offset];
         let bit_mask = 1u8 << (7 - (p % 8));
         if bit_byte & bit_mask == 0 {
             continue; // entire page unallocated
@@ -298,21 +351,33 @@ pub fn read_fixed_array_chunks(
             .checked_mul(page_stride)
             .and_then(|o| pages_start.checked_add(o))
             .ok_or_else(stride_overflow)?;
-        verify_checksum(file_data, page_off, elem_at(page_off, page_count)?)?;
+        let page_end = elem_at(page_off, page_count)?;
+        // `w` holds the page from `base` on (positions below are relative
+        // to it).
+        let page_w;
+        let (w, base) = match &whole {
+            Some(w) => (w, db_offset),
+            None => {
+                page_w =
+                    Window::read(file, page_off as u64, page_end.saturating_add(4) - page_off)?;
+                (&page_w, page_off)
+            }
+        };
+        verify_checksum(w, page_off - base, page_end - base)?;
         for e in 0..page_count {
-            push_element(page_first + e, elem_at(page_off, e)?, &mut chunks)?;
+            push_element(w, page_first + e, elem_at(page_off, e)? - base, &mut chunks)?;
         }
     }
 
     Ok(chunks)
 }
 
-/// Parse a single Fixed Array element at absolute file offset `abs`.
+/// Parse a single Fixed Array element at offset `abs` of the window `w`.
 ///
 /// Returns `Some((address, chunk_size, filter_mask))` for an allocated chunk, or
 /// `None` if the element is undefined (an unallocated chunk, address all-`0xFF`).
 fn parse_fa_element(
-    file_data: &[u8],
+    w: &Window<'_>,
     abs: usize,
     client_id: u8,
     offset_size: u8,
@@ -322,12 +387,8 @@ fn parse_fa_element(
     let os = offset_size as usize;
     if client_id == 0 {
         // Non-filtered: element is just the chunk address.
-        if abs + os > file_data.len() {
-            return Err(FormatError::UnexpectedEof {
-                expected: abs + os,
-                available: file_data.len(),
-            });
-        }
+        w.ensure(abs, os)?;
+        let file_data: &[u8] = &w.bytes;
         if is_undefined(file_data, abs, offset_size) {
             return Ok(None);
         }
@@ -342,17 +403,14 @@ fn parse_fa_element(
             ));
         }
         let chunk_size_bytes = es - os - 4;
-        if abs + es > file_data.len() {
-            return Err(FormatError::UnexpectedEof {
-                expected: abs + es,
-                available: file_data.len(),
-            });
-        }
+        w.ensure(abs, es)?;
+        let file_data: &[u8] = &w.bytes;
         if is_undefined(file_data, abs, offset_size) {
             return Ok(None);
         }
         let address = read_offset(file_data, abs, offset_size)?;
-        let chunk_size = read_variable_length(&file_data[abs + os..], chunk_size_bytes)?;
+        let chunk_size =
+            read_variable_length(&file_data[abs + os..abs + es - 4], chunk_size_bytes)?;
         let fm_off = abs + os + chunk_size_bytes;
         let filter_mask = u32::from_le_bytes([
             file_data[fm_off],
@@ -814,4 +872,126 @@ mod tests {
             .collect();
         assert_eq!(got, expect);
     }
+
+    /// A fixed array (header at 0x100, data block at 0x200) of `n` chunks,
+    /// filtered or not, paged when `n` exceeds `1 << page_bits`; every
+    /// page initialised except page 1.
+    fn build_fixed_array(n: usize, filtered: bool, page_bits: u8) -> Vec<u8> {
+        let os = 8usize;
+        let es = if filtered { os + 4 + 4 } else { os };
+        let (fahd, db) = (0x100usize, 0x200usize);
+        let mut f = vec![0u8; 0x2000];
+        f[fahd..fahd + 4].copy_from_slice(b"FAHD");
+        f[fahd + 5] = u8::from(filtered);
+        f[fahd + 6] = es as u8;
+        f[fahd + 7] = page_bits;
+        f[fahd + 8..fahd + 16].copy_from_slice(&(n as u64).to_le_bytes());
+        f[fahd + 16..fahd + 24].copy_from_slice(&(db as u64).to_le_bytes());
+        stamp_checksum(&mut f, fahd, fahd + 24);
+        f[db..db + 4].copy_from_slice(b"FADB");
+        f[db + 5] = u8::from(filtered);
+        f[db + 6..db + 14].copy_from_slice(&(fahd as u64).to_le_bytes());
+        let elems = db + 6 + os;
+        let write = |f: &mut Vec<u8>, at: usize, i: usize| {
+            let addr = if i == 2 {
+                u64::MAX
+            } else {
+                0x1000 + i as u64 * 0x100
+            };
+            f[at..at + os].copy_from_slice(&addr.to_le_bytes());
+            if filtered {
+                f[at + os..at + os + 4].copy_from_slice(&(100 + i as u32).to_le_bytes());
+                f[at + os + 4..at + os + 8].copy_from_slice(&(i as u32 & 1).to_le_bytes());
+            }
+        };
+        let page = 1usize << page_bits;
+        if n <= page {
+            for i in 0..n {
+                write(&mut f, elems + i * es, i);
+            }
+            stamp_checksum(&mut f, db, elems + n * es);
+        } else {
+            let npages = n.div_ceil(page);
+            let bitmap = npages.div_ceil(8);
+            for p in 0..npages {
+                if p != 1 {
+                    f[elems + p / 8] |= 0x80 >> (p % 8);
+                }
+            }
+            stamp_checksum(&mut f, db, elems + bitmap);
+            let pages_start = elems + bitmap + 4;
+            for p in (0..npages).filter(|&p| p != 1) {
+                let at = pages_start + p * (page * es + 4);
+                let count = page.min(n - p * page);
+                for e in 0..count {
+                    write(&mut f, at + e * es, p * page + e);
+                }
+                stamp_checksum(&mut f, at, at + count * es);
+            }
+        }
+        f
+    }
+
+    /// Non-paged and paged, filtered and unfiltered arrays, cut at every
+    /// length through the data block and with a damaged byte, read
+    /// identically through a `read_at`-only storage.
+    #[test]
+    fn storage_reads_match_slice_reads() {
+        use crate::storage::CountingStorage;
+        for (n, filtered, bits) in [(3, false, 10), (3, true, 10), (11, false, 2), (11, true, 2)] {
+            let full = build_fixed_array(n, filtered, bits);
+            let es = if filtered { 16 } else { 8 };
+            let dims = [n as u64 * 20];
+            let h = FixedArrayHeader::parse(&full, 0x100, 8, 8).unwrap();
+            let chunks = read_fixed_array_chunks(&full, &h, &dims, None, &[20], 8, 8, 8).unwrap();
+            // Chunk 2 is unallocated, and so is page 1 of a paged array.
+            let expect = if n > 4 { n - 1 - 4 } else { n - 1 };
+            assert_eq!(chunks.len(), expect);
+            let mut files = Vec::new();
+            for cut in (0x100..0x200 + 40 + n * (es + 4) + 16).step_by(3) {
+                files.push(full[..cut].to_vec());
+            }
+            for at in [0x104, 0x210, 0x21a, 0x230] {
+                let mut damaged = full.clone();
+                damaged[at] ^= 1;
+                files.push(damaged);
+            }
+            files.push(full);
+            for f in files {
+                let storage = CountingStorage::new(f.clone());
+                let want = FixedArrayHeader::parse(&f, 0x100, 8, 8);
+                let got = FixedArrayHeader::parse_in(&storage, 0x100, 8, 8);
+                assert_eq!(format!("{got:?}"), format!("{want:?}"));
+                let Ok(h) = want else { continue };
+                let want = read_fixed_array_chunks(&f, &h, &dims, None, &[20], 8, 8, 8);
+                let got = read_fixed_array_chunks_in(&storage, &h, &dims, None, &[20], 8, 8, 8);
+                assert_eq!(format!("{got:?}"), format!("{want:?}"), "{} bytes", f.len());
+            }
+        }
+    }
+
+    /// A header whose element count stretches its data block (one checksum
+    /// over the whole block) far past the end of a 16 MiB file: the
+    /// checksum's bounds check fails before the block is read, with the
+    /// slice read's error.
+    #[cfg(feature = "checksum")]
+    #[test]
+    fn oversized_block_fails_before_reading() {
+        use crate::storage::CountingStorage;
+        let mut f = build_fixed_array(3, false, 10);
+        f.resize(16 << 20, 0);
+        let mut h = FixedArrayHeader::parse(&f, 0x100, 8, 8).unwrap();
+        h.max_nelmts_bits = 30;
+        h.num_elements = 4 << 20;
+        let dims = [h.num_elements * 20];
+        let want = read_fixed_array_chunks(&f, &h, &dims, None, &[20], 8, 8, 8);
+        assert!(
+            matches!(want, Err(FormatError::UnexpectedEof { .. })),
+            "{want:?}"
+        );
+        let storage = CountingStorage::new(f);
+        let got = read_fixed_array_chunks_in(&storage, &h, &dims, None, &[20], 8, 8, 8);
+        assert_eq!(format!("{got:?}"), format!("{want:?}"));
+        assert!(storage.bytes_read() < 64, "{} bytes", storage.bytes_read());
+    }
 }
diff --git a/crates/clawhdf5-format/src/fractal_heap.rs b/crates/clawhdf5-format/src/fractal_heap.rs
index d896156..7bdce6f 100644
--- a/crates/clawhdf5-format/src/fractal_heap.rs
+++ b/crates/clawhdf5-format/src/fractal_heap.rs
@@ -10,6 +10,7 @@ use crate::addr::to_usize;
 use crate::btree_v2::{BTreeV2Header, find_btree_v2_records};
 use crate::error::FormatError;
 use crate::filter_pipeline::FilterPipeline;
+use crate::storage::{Storage, Window, len_usize, read_exact_at, require_contiguous};
 
 /// Parsed fractal heap header (signature "FRHP").
 #[derive(Debug, Clone)]
@@ -139,6 +140,36 @@ impl FractalHeapHeader {
         offset_size: u8,
         length_size: u8,
     ) -> Result<FractalHeapHeader, FormatError> {
+        Self::parse_in(file_data, offset as u64, offset_size, length_size)
+    }
+
+    /// [`Self::parse`] over any [`Storage`]: one read of the header (two
+    /// when it holds an I/O filter pipeline).
+    pub fn parse_in<S: Storage + ?Sized>(
+        file: &S,
+        offset: u64,
+        offset_size: u8,
+        length_size: u8,
+    ) -> Result<FractalHeapHeader, FormatError> {
+        // Every field up to the checksum, without and with the filter
+        // information; the window holds all of it (or ends at the end of
+        // the file), so its bounds checks are the whole-file ones.
+        let (os, ls) = (usize::from(offset_size), usize::from(length_size));
+        let unfiltered_len = 26 + 12 * ls + 3 * os;
+        let mut w = Window::read(file, offset, unfiltered_len)?;
+        if w.bytes.len() == unfiltered_len {
+            let filter_len = usize::from(u16::from_le_bytes([w.bytes[7], w.bytes[8]]));
+            if filter_len > 0 {
+                w = Window::read(file, offset, unfiltered_len + ls + 4 + filter_len)?;
+            }
+        }
+        let ensure_len = |_: &[u8], pos: usize, needed: usize| w.ensure(pos, needed);
+        let read_offset = |_: &[u8], pos: usize, size: u8| {
+            w.ensure(pos, usize::from(size))?;
+            read_offset(&w.bytes, pos, size)
+        };
+        let file_data: &[u8] = &w.bytes;
+        let offset = 0usize;
         ensure_len(file_data, offset, 5)?;
         if &file_data[offset..offset + 4] != b"FRHP" {
             return Err(FormatError::InvalidFractalHeapSignature);
@@ -149,9 +180,6 @@ impl FractalHeapHeader {
             return Err(FormatError::InvalidFractalHeapVersion(version));
         }
 
-        let os = offset_size as usize;
-        let ls = length_size as usize;
-
         let mut pos = offset + 5;
         ensure_len(file_data, pos, 2)?;
         let heap_id_length = u16::from_le_bytes([file_data[pos], file_data[pos + 1]]);
@@ -354,6 +382,18 @@ impl FractalHeapHeader {
         file_data: &[u8],
         id_bytes: &[u8],
         offset_size: u8,
+    ) -> Result<Vec<u8>, FormatError> {
+        self.read_managed_object_in(file_data, id_bytes, offset_size)
+    }
+
+    /// [`Self::read_managed_object`] over any [`Storage`]. A huge object
+    /// found through the huge-object v2 B-tree still needs the whole file
+    /// in memory ([`FormatError::ContiguousStorageRequired`] otherwise).
+    pub fn read_managed_object_in<S: Storage + ?Sized>(
+        &self,
+        file_data: &S,
+        id_bytes: &[u8],
+        offset_size: u8,
     ) -> Result<Vec<u8>, FormatError> {
         crate::lookup_stats::heap_object_read();
         let Some(&first) = id_bytes.first() else {
@@ -385,7 +425,11 @@ impl FractalHeapHeader {
     }
 
     /// Read a huge object (heap ID type 1).
-    fn read_huge_object(&self, file_data: &[u8], id: &[u8]) -> Result<Vec<u8>, FormatError> {
+    fn read_huge_object<S: Storage + ?Sized>(
+        &self,
+        file: &S,
+        id: &[u8],
+    ) -> Result<Vec<u8>, FormatError> {
         let os = usize::from(self.offset_size);
         let ls = usize::from(self.length_size);
         // (address, stored length, filter mask, decoded length); the last two
@@ -416,18 +460,17 @@ impl FractalHeapHeader {
             let key_len = (usize::from(self.heap_id_length).saturating_sub(1)).min(8);
             ensure_len(id, 1, key_len)?;
             let key = le_uint(&id[1..1 + key_len]);
-            self.find_huge_record(file_data, key)?
+            self.find_huge_record(file, key)?
         };
 
         let start = usize::try_from(addr).map_err(|_| heap_error("huge object address"))?;
         let len = usize::try_from(stored_len).map_err(|_| heap_error("huge object length"))?;
-        ensure_len(file_data, start, len)?;
-        let stored = &file_data[start..start + len];
+        let stored = read_exact_at(file, start as u64, len)?;
         match &self.filter_pipeline {
-            None => Ok(stored.to_vec()),
+            None => Ok(stored.into_owned()),
             Some(pipeline) => {
                 let mem = usize::try_from(mem_len).map_err(|_| heap_error("huge object size"))?;
-                let out = crate::filters::decompress_chunk_masked(stored, pipeline, mem, 1, mask)?;
+                let out = crate::filters::decompress_chunk_masked(&stored, pipeline, mem, 1, mask)?;
                 if out.len() != mem {
                     return Err(heap_error("filtered huge object decoded to the wrong size"));
                 }
@@ -438,9 +481,9 @@ impl FractalHeapHeader {
 
     /// Look up huge object `key` in the huge-object v2 B-tree, returning
     /// (address, stored length, filter mask, decoded length).
-    fn find_huge_record(
+    fn find_huge_record<S: Storage + ?Sized>(
         &self,
-        file_data: &[u8],
+        file: &S,
         key: u64,
     ) -> Result<(u64, u64, u32, u64), FormatError> {
         if is_undefined(self.huge_btree_address, self.offset_size) {
@@ -448,6 +491,8 @@ impl FractalHeapHeader {
                 "huge object ID but the heap has no huge-object index",
             ));
         }
+        // The v2 B-tree is read from a slice until it is converted.
+        let file_data = require_contiguous(file, "a huge fractal-heap object's B-tree")?;
         let hdr = BTreeV2Header::parse(
             file_data,
             to_usize(self.huge_btree_address)?,
@@ -519,9 +564,9 @@ impl FractalHeapHeader {
     }
 
     /// Read a managed object (heap ID type 0).
-    fn read_heap_managed(
+    fn read_heap_managed<S: Storage + ?Sized>(
         &self,
-        file_data: &[u8],
+        file_data: &S,
         id_bytes: &[u8],
         offset_size: u8,
     ) -> Result<Vec<u8>, FormatError> {
@@ -569,9 +614,9 @@ impl FractalHeapHeader {
     /// header), so we just add it to the block address minus the block's heap
     /// offset. A filtered heap stores each direct block (header included)
     /// through its filter pipeline, so the block is decoded first.
-    fn read_from_direct_block(
+    fn read_from_direct_block<S: Storage + ?Sized>(
         &self,
-        file_data: &[u8],
+        file: &S,
         block: DirectBlock,
         target_offset: u64,
         length: usize,
@@ -587,9 +632,9 @@ impl FractalHeapHeader {
             let stored_len = usize::try_from(block.filtered_size)
                 .map_err(|_| heap_error("direct block size"))?;
             let size = usize::try_from(block.size).map_err(|_| heap_error("direct block size"))?;
-            ensure_len(file_data, block.addr, stored_len)?;
+            let stored = read_exact_at(file, block.addr as u64, stored_len)?;
             let decoded = crate::filters::decompress_chunk_masked(
-                &file_data[block.addr..block.addr + stored_len],
+                &stored,
                 pipeline,
                 size,
                 1,
@@ -603,17 +648,16 @@ impl FractalHeapHeader {
             .checked_add(local_offset)
             .ok_or(FormatError::UnexpectedEof {
                 expected: usize::MAX,
-                available: file_data.len(),
+                available: len_usize(file),
             })?;
-        ensure_len(file_data, pos, length)?;
-        Ok(file_data[pos..pos + length].to_vec())
+        Ok(read_exact_at(file, pos as u64, length)?.into_owned())
     }
 
     /// Read an object by traversing an indirect block to find the right direct block.
     #[allow(clippy::too_many_arguments)]
-    fn read_from_indirect_block(
+    fn read_from_indirect_block<S: Storage + ?Sized>(
         &self,
-        file_data: &[u8],
+        file: &S,
         iblock_addr: usize,
         nrows: u16,
         iblock_heap_offset: u64,
@@ -627,29 +671,169 @@ impl FractalHeapHeader {
                 "fractal heap: maximum recursion depth exceeded".into(),
             ));
         }
-        // Parse indirect block header
-        ensure_len(file_data, iblock_addr, 4)?;
-        if &file_data[iblock_addr..iblock_addr + 4] != b"FHIB" {
-            return Err(FormatError::InvalidFractalHeapSignature);
-        }
-
         let block_offset_bytes = (self.max_heap_size as usize).div_ceil(8);
         let iblock_header = 5 + offset_size as usize + block_offset_bytes;
-        let mut pos = iblock_addr + iblock_header;
-
-        let tw = self.table_width as u64;
         let nrows_usize = nrows as usize;
-        let mut current_heap_offset = iblock_heap_offset;
 
         // Rows below max_direct_rows hold direct blocks; rows at/above hold
         // child indirect blocks. (NOT the FRHP "starting rows" field.)
         let start_indirect = self.max_direct_rows();
         let max_direct_rows = nrows_usize.min(start_indirect);
 
+        // The block up to its last child entry. The walk below reads
+        // entries in order and stops at the one covering the target, which
+        // the geometry alone locates, so the first window ends there: a
+        // header claiming a huge table costs a read of the entries in front
+        // of the target, not of the rest of the file. Only when that entry
+        // is unallocated (or none covers the target) does the walk go on,
+        // over the whole block. Either window holds what it was asked for or
+        // ends at the end of the file, so its bounds checks are the
+        // whole-file ones.
+        let direct_entry = usize::from(offset_size)
+            + if self.filter_pipeline.is_some() {
+                usize::from(self.length_size) + 4
+            } else {
+                0
+            };
+        let direct_entries = max_direct_rows.saturating_mul(usize::from(self.table_width));
+        let entries_len = |n: usize| {
+            n.min(direct_entries)
+                .saturating_mul(direct_entry)
+                .saturating_add(
+                    n.saturating_sub(direct_entries)
+                        .saturating_mul(usize::from(offset_size)),
+                )
+        };
+        let all_entries = direct_entries.saturating_add(
+            nrows_usize
+                .saturating_sub(start_indirect)
+                .saturating_mul(usize::from(self.table_width)),
+        );
+        let block_len = iblock_header.saturating_add(entries_len(all_entries));
+        let target_entry = self.indirect_entry_for(nrows_usize, iblock_heap_offset, target_offset);
+        let first_len = target_entry.map_or(block_len, |i| {
+            iblock_header
+                .saturating_add(entries_len(i.saturating_add(1)))
+                .min(block_len)
+        });
+        let mut next = self.walk_indirect_block(
+            &Window::read(file, iblock_addr as u64, first_len)?,
+            nrows_usize,
+            iblock_heap_offset,
+            target_offset,
+            offset_size,
+            target_entry.map_or(usize::MAX, |i| i.saturating_add(1)),
+        )?;
+        if next.is_none() && first_len < block_len {
+            next = self.walk_indirect_block(
+                &Window::read(file, iblock_addr as u64, block_len)?,
+                nrows_usize,
+                iblock_heap_offset,
+                target_offset,
+                offset_size,
+                usize::MAX,
+            )?;
+        }
+        match next {
+            Some(IndirectChild::Direct(block)) => {
+                self.read_from_direct_block(file, block, target_offset, length)
+            }
+            Some(IndirectChild::Indirect {
+                addr,
+                nrows,
+                heap_offset,
+            }) => self.read_from_indirect_block(
+                file,
+                addr,
+                nrows,
+                heap_offset,
+                target_offset,
+                length,
+                offset_size,
+                depth_remaining - 1,
+            ),
+            None => Err(FormatError::UnexpectedEof {
+                expected: to_usize(target_offset)?.saturating_add(length),
+                available: len_usize(file),
+            }),
+        }
+    }
+
+    /// Which child entry of an indirect block (numbered in walk order:
+    /// direct rows, then indirect rows) covers `target_offset`, from the
+    /// doubling-table geometry alone — the entry
+    /// [`Self::walk_indirect_block`] stops at if it is allocated. `None`
+    /// when no entry does.
+    fn indirect_entry_for(&self, nrows: usize, heap_offset: u64, target: u64) -> Option<usize> {
+        // The walk adds block sizes with saturation; in u128 the same test
+        // is `cur <= target < cur + size` without it (a target of u64::MAX
+        // is never inside a saturated range).
+        if target == u64::MAX {
+            return None;
+        }
+        let (tw, target) = (u128::from(self.table_width), u128::from(target));
+        let mut cur = u128::from(heap_offset);
+        let mut before = 0usize;
+        for row in 0..nrows {
+            if target < cur {
+                return None;
+            }
+            // Direct and indirect rows alike span this row's block size per
+            // entry.
+            let size = u128::from(self.block_size_for_row(row));
+            let span = size * tw;
+            if size > 0 && target < cur + span {
+                let col = usize::try_from((target - cur) / size).ok()?;
+                return before.checked_add(col);
+            }
+            cur += span;
+            before = before.saturating_add(self.table_width as usize);
+        }
+        None
+    }
+
+    /// Walk an indirect block's child entries in order, in the window `w`
+    /// (the block from its signature on), and return the allocated child
+    /// covering `target_offset`, or `None` when no entry among the first
+    /// `limit` does.
+    fn walk_indirect_block(
+        &self,
+        w: &Window<'_>,
+        nrows: usize,
+        iblock_heap_offset: u64,
+        target_offset: u64,
+        offset_size: u8,
+        limit: usize,
+    ) -> Result<Option<IndirectChild>, FormatError> {
+        let ensure_len = |_: &[u8], pos: usize, needed: usize| w.ensure(pos, needed);
+        let read_offset = |_: &[u8], pos: usize, size: u8| {
+            w.ensure(pos, usize::from(size))?;
+            read_offset(&w.bytes, pos, size)
+        };
+        let file_data: &[u8] = &w.bytes;
+        let block_offset_bytes = (self.max_heap_size as usize).div_ceil(8);
+        let iblock_header = 5 + offset_size as usize + block_offset_bytes;
+        let tw = self.table_width as u64;
+        let mut current_heap_offset = iblock_heap_offset;
+        let start_indirect = self.max_direct_rows();
+        let max_direct_rows = nrows.min(start_indirect);
+        let mut walked = 0usize;
+
+        // Parse indirect block header
+        ensure_len(file_data, 0, 4)?;
+        if &file_data[..4] != b"FHIB" {
+            return Err(FormatError::InvalidFractalHeapSignature);
+        }
+        let mut pos = iblock_header;
+
         for row in 0..max_direct_rows {
             let block_size = self.block_size_for_row(row);
 
             for _col in 0..tw {
+                if walked == limit {
+                    return Ok(None);
+                }
+                walked += 1;
                 let child_addr = read_offset(file_data, pos, offset_size)?;
                 pos += offset_size as usize;
 
@@ -676,18 +860,13 @@ impl FractalHeapHeader {
                     && target_offset >= current_heap_offset
                     && target_offset < block_end
                 {
-                    return self.read_from_direct_block(
-                        file_data,
-                        DirectBlock {
-                            addr: to_usize(child_addr)?,
-                            size: block_size,
-                            heap_offset: current_heap_offset,
-                            filtered_size,
-                            filter_mask,
-                        },
-                        target_offset,
-                        length,
-                    );
+                    return Ok(Some(IndirectChild::Direct(DirectBlock {
+                        addr: to_usize(child_addr)?,
+                        size: block_size,
+                        heap_offset: current_heap_offset,
+                        filtered_size,
+                        filter_mask,
+                    })));
                 }
                 current_heap_offset = block_end;
             }
@@ -696,11 +875,15 @@ impl FractalHeapHeader {
         // Rows at and above `start_indirect` hold child indirect blocks. A
         // child in row r spans exactly that row's block size of heap space,
         // so it has as many rows as a table of that total size needs.
-        for row in start_indirect..nrows_usize {
+        for row in start_indirect..nrows {
             let child_space = self.block_size_for_row(row);
             let child_nrows = self.rows_for_size(child_space);
 
             for _col in 0..tw {
+                if walked == limit {
+                    return Ok(None);
+                }
+                walked += 1;
                 let child_addr = read_offset(file_data, pos, offset_size)?;
                 pos += offset_size as usize;
 
@@ -709,25 +892,16 @@ impl FractalHeapHeader {
                     && target_offset >= current_heap_offset
                     && target_offset < block_end
                 {
-                    return self.read_from_indirect_block(
-                        file_data,
-                        to_usize(child_addr)?,
-                        child_nrows,
-                        current_heap_offset,
-                        target_offset,
-                        length,
-                        offset_size,
-                        depth_remaining - 1,
-                    );
+                    return Ok(Some(IndirectChild::Indirect {
+                        addr: to_usize(child_addr)?,
+                        nrows: child_nrows,
+                        heap_offset: current_heap_offset,
+                    }));
                 }
                 current_heap_offset = block_end;
             }
         }
-
-        Err(FormatError::UnexpectedEof {
-            expected: to_usize(target_offset)?.saturating_add(length),
-            available: file_data.len(),
-        })
+        Ok(None)
     }
 
     /// Number of rows in the doubling table whose block size is at most the
@@ -771,6 +945,16 @@ impl FractalHeapHeader {
 
 /// A managed direct block's location, extent and (for a filtered heap) its
 /// stored size and filter mask.
+/// The child of an indirect block that covers a heap offset.
+enum IndirectChild {
+    Direct(DirectBlock),
+    Indirect {
+        addr: usize,
+        nrows: u16,
+        heap_offset: u64,
+    },
+}
+
 struct DirectBlock {
     addr: usize,
     size: u64,
@@ -1030,4 +1214,154 @@ mod tests {
         let id = [0x40u8, 0, 0, 0, 0, 0, 0];
         assert!(hdr.read_managed_object(&file_data, &id, 8).is_err());
     }
+
+    /// Headers, and managed (in a direct root and through an indirect
+    /// root), huge and tiny objects read identically through a
+    /// `read_at`-only storage, for every truncation of the file.
+    #[test]
+    fn storage_reads_match_slice_reads() {
+        use crate::storage::CountingStorage;
+        let (mut file, header_end) = build_simple_heap(8, 8);
+        // An indirect root block at 600: row 0 holds the direct block at
+        // 256, then three undefined blocks.
+        file[600..604].copy_from_slice(b"FHIB");
+        let mut at = 600 + 5 + 8 + 2;
+        for addr in [256u64, u64::MAX, u64::MAX, u64::MAX] {
+            file[at..at + 8].copy_from_slice(&addr.to_le_bytes());
+            at += 8;
+        }
+        file[900..905].copy_from_slice(b"huge!");
+        let managed_id = |offset: u64, len: u64| {
+            let payload = offset | (len << 16);
+            let mut id = vec![0u8];
+            id.extend_from_slice(&payload.to_le_bytes()[..6]);
+            id
+        };
+        let mut huge = vec![0x10u8];
+        huge.extend_from_slice(&900u64.to_le_bytes());
+        huge.extend_from_slice(&5u64.to_le_bytes());
+        let ids = [
+            managed_id(15, 13),
+            managed_id(15, 200),
+            managed_id(130, 4),
+            huge,
+            vec![0x22, b'a', b'b', b'c', 0, 0, 0],
+        ];
+        let mut cuts: Vec<usize> = (0..=header_end + 1).collect();
+        cuts.extend([256, 260, 271, 280, 600, 610, 620, 640, 900, 903, file.len()]);
+        for cut in cuts {
+            let f = &file[..cut];
+            let storage = CountingStorage::new(f.to_vec());
+            let want = FractalHeapHeader::parse(f, 0, 8, 8);
+            let got = FractalHeapHeader::parse_in(&storage, 0, 8, 8);
+            assert_eq!(format!("{got:?}"), format!("{want:?}"), "cut {cut}");
+            let Ok(direct) = want else { continue };
+            let mut indirect = direct.clone();
+            indirect.root_block_address = 600;
+            indirect.current_rows_in_root_indirect_block = 1;
+            let mut huge_ids = direct.clone();
+            huge_ids.heap_id_length = 17;
+            for hdr in [&direct, &indirect, &huge_ids] {
+                for id in &ids {
+                    assert_eq!(
+                        hdr.read_managed_object_in(&storage, id, 8),
+                        hdr.read_managed_object(f, id, 8),
+                        "cut {cut}"
+                    );
+                }
+            }
+        }
+    }
+
+    /// A header claiming a huge doubling table (width 0xFFFF, 0xFFFF rows in
+    /// the root indirect block) in a 16 MiB file: reading an object from the
+    /// table's first block reads the entries up to it, not the rest of the
+    /// file, and gives what the slice read gives. When the covering entry is
+    /// unallocated the walk goes on over the whole block, still identically.
+    #[test]
+    fn huge_table_claims_read_only_what_the_walk_needs() {
+        use crate::storage::CountingStorage;
+        let (mut file, _) = build_simple_heap(8, 8);
+        file.resize(16 << 20, 0);
+        file[600..604].copy_from_slice(b"FHIB");
+        let first_entry = 600 + 5 + 8 + 2;
+        file[first_entry..first_entry + 8].copy_from_slice(&256u64.to_le_bytes());
+        let mut hdr = FractalHeapHeader::parse(&file, 0, 8, 8).unwrap();
+        hdr.table_width = 0xFFFF;
+        hdr.root_block_address = 600;
+        hdr.current_rows_in_root_indirect_block = 0xFFFF;
+        let managed_id = |offset: u64, len: u64| {
+            let payload = offset | (len << 16);
+            let mut id = vec![0u8];
+            id.extend_from_slice(&payload.to_le_bytes()[..6]);
+            id
+        };
+        let storage = CountingStorage::new(file.clone());
+        let id = managed_id(15, 13);
+        let want = hdr.read_managed_object(&file, &id, 8);
+        assert!(want.is_ok(), "{want:?}");
+        storage.reset();
+        assert_eq!(hdr.read_managed_object_in(&storage, &id, 8), want);
+        assert!(
+            storage.bytes_read() < 1024,
+            "{} bytes in {} reads",
+            storage.bytes_read(),
+            storage.reads()
+        );
+        // The second entry (heap offsets 128..256) is unallocated (zero is
+        // not the undefined address, so make it all ones).
+        file[first_entry + 8..first_entry + 16].fill(0xFF);
+        let storage = CountingStorage::new(file.clone());
+        let id = managed_id(130, 4);
+        let want = hdr.read_managed_object(&file, &id, 8);
+        assert_eq!(hdr.read_managed_object_in(&storage, &id, 8), want);
+    }
+
+    /// A huge object found through the huge-object B-tree needs the whole
+    /// file in memory until the B-tree reader is converted: a clean error
+    /// on other storage.
+    #[test]
+    fn huge_object_btree_needs_contiguous_storage() {
+        use crate::storage::CountingStorage;
+        let (file, _) = build_simple_heap(8, 8);
+        let mut hdr = FractalHeapHeader::parse(&file, 0, 8, 8).unwrap();
+        hdr.huge_btree_address = 700;
+        let storage = CountingStorage::new(file);
+        assert_eq!(
+            hdr.read_managed_object_in(&storage, &[0x10, 1, 0, 0, 0, 0, 0], 8),
+            Err(FormatError::ContiguousStorageRequired(
+                "a huge fractal-heap object's B-tree"
+            ))
+        );
+    }
+
+    /// A header with an I/O filter pipeline (read in a second, longer
+    /// window) parses identically through a `read_at`-only storage, for
+    /// every truncation.
+    #[test]
+    fn filtered_header_parses_identically_through_storage() {
+        use crate::storage::CountingStorage;
+        let (simple, header_end) = build_simple_heap(8, 8);
+        let pipeline = [2u8, 1, 1, 0, 0, 0, 1, 0, 6, 0, 0, 0]; // deflate, level 6
+        let mut header = simple[..header_end - 4].to_vec();
+        header[7..9].copy_from_slice(&(pipeline.len() as u16).to_le_bytes());
+        header.extend_from_slice(&100u64.to_le_bytes()); // root block's stored size
+        header.extend_from_slice(&0u32.to_le_bytes()); // its filter mask
+        header.extend_from_slice(&pipeline);
+        let sum = crate::checksum::jenkins_lookup3(&header);
+        header.extend_from_slice(&sum.to_le_bytes());
+        let mut file = header.clone();
+        file.resize(256, 0);
+        let hdr = FractalHeapHeader::parse(&file, 0, 8, 8).unwrap();
+        assert!(hdr.filter_pipeline.is_some());
+        for cut in 0..=file.len() {
+            let f = &file[..cut];
+            let storage = CountingStorage::new(f.to_vec());
+            assert_eq!(
+                format!("{:?}", FractalHeapHeader::parse_in(&storage, 0, 8, 8)),
+                format!("{:?}", FractalHeapHeader::parse(f, 0, 8, 8)),
+                "cut {cut}"
+            );
+        }
+    }
 }
diff --git a/crates/clawhdf5-format/src/global_heap.rs b/crates/clawhdf5-format/src/global_heap.rs
index 474c7fd..9861e8f 100644
--- a/crates/clawhdf5-format/src/global_heap.rs
+++ b/crates/clawhdf5-format/src/global_heap.rs
@@ -1,9 +1,12 @@
 //! HDF5 Global Heap collection parsing.
 
 #[cfg(not(feature = "std"))]
-use alloc::{format, string::String, vec::Vec};
+use alloc::{borrow::Cow, format, string::String, vec::Vec};
+#[cfg(feature = "std")]
+use std::borrow::Cow;
 
 use crate::error::FormatError;
+use crate::storage::{Storage, len_usize, read_exact_at};
 
 /// Magic signature for global heap collections.
 const GCOL_SIGNATURE: [u8; 4] = *b"GCOL";
@@ -28,19 +31,20 @@ pub struct GlobalHeapObject {
     pub data: Vec<u8>,
 }
 
-fn ensure_len(data: &[u8], offset: usize, needed: usize) -> Result<(), FormatError> {
+/// Checks that `[offset, offset + needed)` ends by `data_len`.
+fn ensure_len(data_len: usize, offset: usize, needed: usize) -> Result<(), FormatError> {
     match offset.checked_add(needed) {
-        Some(end) if end <= data.len() => Ok(()),
+        Some(end) if end <= data_len => Ok(()),
         _ => Err(FormatError::UnexpectedEof {
             expected: offset.saturating_add(needed),
-            available: data.len(),
+            available: data_len,
         }),
     }
 }
 
 fn read_length(data: &[u8], offset: usize, length_size: u8) -> Result<u64, FormatError> {
     let s = length_size as usize;
-    ensure_len(data, offset, s)?;
+    ensure_len(data.len(), offset, s)?;
     let slice = &data[offset..offset + s];
     Ok(match length_size {
         2 => u16::from_le_bytes([slice[0], slice[1]]) as u64,
@@ -95,7 +99,17 @@ impl GlobalHeapCollection {
         offset: usize,
         length_size: u8,
     ) -> Result<GlobalHeapCollection, FormatError> {
-        let index = Self::parse_index(file_data, offset, length_size)?;
+        Self::parse_in(file_data, offset as u64, length_size)
+    }
+
+    /// [`Self::parse`] over any [`Storage`]: one read of the header, one of
+    /// the collection.
+    pub fn parse_in<S: Storage + ?Sized>(
+        file: &S,
+        offset: u64,
+        length_size: u8,
+    ) -> Result<GlobalHeapCollection, FormatError> {
+        let (bytes, base, index) = Self::read_collection(file, offset, length_size)?;
         Ok(GlobalHeapCollection {
             collection_size: index.collection_size,
             objects: index
@@ -104,7 +118,7 @@ impl GlobalHeapCollection {
                 .map(|o| GlobalHeapObject {
                     index: o.index,
                     reference_count: o.reference_count,
-                    data: file_data[o.offset..o.offset + o.size].to_vec(),
+                    data: bytes[o.offset - base..o.offset - base + o.size].to_vec(),
                 })
                 .collect(),
         })
@@ -122,43 +136,72 @@ impl GlobalHeapCollection {
         offset: usize,
         length_size: u8,
     ) -> Result<GlobalHeapIndex, FormatError> {
+        Self::parse_index_in(file_data, offset as u64, length_size)
+    }
+
+    /// [`Self::parse_index`] over any [`Storage`]: one read of the header,
+    /// one of the collection. The object offsets are file offsets.
+    pub fn parse_index_in<S: Storage + ?Sized>(
+        file: &S,
+        offset: u64,
+        length_size: u8,
+    ) -> Result<GlobalHeapIndex, FormatError> {
+        Ok(Self::read_collection(file, offset, length_size)?.2)
+    }
+
+    /// Read the collection at `offset` and index its objects: the
+    /// collection's bytes, its offset as a `usize`, and the index (with
+    /// file offsets).
+    fn read_collection<S: Storage + ?Sized>(
+        file: &S,
+        offset: u64,
+        length_size: u8,
+    ) -> Result<(Cow<'_, [u8]>, usize, GlobalHeapIndex), FormatError> {
+        let file_len = len_usize(file);
         // signature(4) + version(1) + reserved(3) + collection_size(length_size),
         // padded to a multiple of 8 as libhdf5 lays it out (`H5HG_SIZEOF_HDR`).
         // With 8-byte lengths the padding is 0; with 4-byte lengths it is 4,
         // and reading without it put every object 4 bytes early.
         let header_size = pad8(8 + length_size as usize);
-        ensure_len(file_data, offset, header_size)?;
+        let header = read_exact_at(file, offset, header_size)?;
+        let offset = usize::try_from(offset).map_err(|_| FormatError::UnexpectedEof {
+            expected: usize::MAX,
+            available: file_len,
+        })?;
 
-        if file_data[offset..offset + 4] != GCOL_SIGNATURE {
+        if header[..4] != GCOL_SIGNATURE {
             return Err(FormatError::InvalidGlobalHeapSignature);
         }
 
-        let version = file_data[offset + 4];
+        let version = header[4];
         if version != 1 {
             return Err(FormatError::InvalidGlobalHeapVersion(version));
         }
 
-        let collection_size = read_length(file_data, offset + 8, length_size)?;
+        let collection_size = read_length(&header, 8, length_size)?;
         let collection_end = usize::try_from(collection_size)
             .ok()
             .and_then(|size| offset.checked_add(size))
             .ok_or(FormatError::UnexpectedEof {
                 expected: usize::MAX,
-                available: file_data.len(),
+                available: file_len,
             })?;
-        if collection_end > file_data.len() {
+        if collection_end > file_len {
             return Err(FormatError::UnexpectedEof {
                 expected: collection_end,
-                available: file_data.len(),
+                available: file_len,
             });
         }
+        let collection = read_exact_at(file, offset as u64, collection_end - offset)?;
+        // Positions below are file offsets; `file_data(p)` is the byte at `p`.
+        let file_data = |p: usize| collection[p - offset];
 
         let mut pos = offset + header_size;
         let mut objects = Vec::new();
 
         // Parse objects until we hit index 0 (free space) or run out of space
         while pos + 2 <= collection_end {
-            let object_index = u16::from_le_bytes([file_data[pos], file_data[pos + 1]]);
+            let object_index = u16::from_le_bytes([file_data(pos), file_data(pos + 1)]);
 
             if object_index == 0 {
                 // Free space marker — done
@@ -168,11 +211,12 @@ impl GlobalHeapCollection {
             // object_index(2) + reference_count(2) + reserved(4) +
             // object_size(length_size), padded to 8 (`H5HG_SIZEOF_OBJHDR`).
             let obj_header_size = pad8(8 + length_size as usize);
-            ensure_len(&file_data[..collection_end], pos, obj_header_size)?;
+            ensure_len(collection_end, pos, obj_header_size)?;
 
-            let reference_count = u16::from_le_bytes([file_data[pos + 2], file_data[pos + 3]]);
-            let object_size = usize::try_from(read_length(file_data, pos + 8, length_size)?)
-                .map_err(|_| FormatError::Overflow("global heap object size".into()))?;
+            let reference_count = u16::from_le_bytes([file_data(pos + 2), file_data(pos + 3)]);
+            let object_size =
+                usize::try_from(read_length(&collection[pos - offset..], 8, length_size)?)
+                    .map_err(|_| FormatError::Overflow("global heap object size".into()))?;
 
             pos += obj_header_size;
             if pos
@@ -197,10 +241,11 @@ impl GlobalHeapCollection {
             pos = pos.saturating_add(pad8(object_size));
         }
 
-        Ok(GlobalHeapIndex {
+        let index = GlobalHeapIndex {
             collection_size,
             objects,
-        })
+        };
+        Ok((collection, offset, index))
     }
 
     /// Get an object by its index.
@@ -226,7 +271,7 @@ mod tests {
         let mut obj_size_total = 0usize;
         for (_, _, data) in objects {
             let obj_header = pad8(8 + ls);
-            obj_size_total += obj_header + pad8(data.len());
+            obj_size_total += obj_header + pad8(<[u8]>::len(data));
         }
         // Free space marker (2 bytes for index 0)
         obj_size_total += 2;
@@ -251,15 +296,17 @@ mod tests {
             buf.extend_from_slice(&ref_count.to_le_bytes());
             buf.extend_from_slice(&[0u8; 4]); // reserved
             match length_size {
-                4 => buf.extend_from_slice(&(data.len() as u32).to_le_bytes()),
-                8 => buf.extend_from_slice(&(data.len() as u64).to_le_bytes()),
+                // `<[u8]>::len`: with `Storage` in scope `data.len()` on a
+                // `&&[u8]` resolves to `Storage::len` (a `u64`).
+                4 => buf.extend_from_slice(&(<[u8]>::len(data) as u32).to_le_bytes()),
+                8 => buf.extend_from_slice(&(<[u8]>::len(data) as u64).to_le_bytes()),
                 _ => panic!("unsupported"),
             }
             buf.resize(buf.len() + (pad8(8 + ls) - (8 + ls)), 0);
             buf.extend_from_slice(data);
             // Pad to 8 bytes
-            let padded = pad8(data.len());
-            buf.resize(buf.len() + (padded - data.len()), 0);
+            let padded = pad8(<[u8]>::len(data));
+            buf.resize(buf.len() + (padded - <[u8]>::len(data)), 0);
         }
 
         // Free space marker
@@ -327,4 +374,44 @@ mod tests {
         assert_eq!(coll.objects.len(), 1);
         assert_eq!(coll.objects[0].data, b"test");
     }
+
+    /// Collections, and every truncation of them, index and parse
+    /// identically through a `read_at`-only storage: two reads each.
+    #[test]
+    fn storage_parse_matches_slice_parse() {
+        use crate::storage::CountingStorage;
+        let objs: &[(u16, u16, &[u8])] = &[(1, 1, b"hello"), (2, 3, b"a longer object")];
+        for ls in [4u8, 8] {
+            let coll = build_collection(objs, ls);
+            let mut corrupt = coll.clone();
+            corrupt[8] = 200; // collection size past the end of the file
+            let mut overrun = coll.clone();
+            let size_at = pad8(8 + ls as usize) + 8;
+            overrun[size_at] = 250; // first object runs past the collection
+            for full in [coll, corrupt, overrun] {
+                for at in [0usize, 5] {
+                    for cut in 0..=full.len() {
+                        let mut f = vec![0u8; at];
+                        f.extend_from_slice(&full[..cut]);
+                        let storage = CountingStorage::new(f.clone());
+                        let want = GlobalHeapCollection::parse(&f, at, ls);
+                        let got = GlobalHeapCollection::parse_in(&storage, at as u64, ls);
+                        assert_eq!(format!("{got:?}"), format!("{want:?}"));
+                        let want = GlobalHeapCollection::parse_index(&f, at, ls);
+                        let got = GlobalHeapCollection::parse_index_in(&storage, at as u64, ls);
+                        assert_eq!(format!("{got:?}"), format!("{want:?}"));
+                    }
+                }
+            }
+        }
+        let storage = CountingStorage::new(build_collection(objs, 8));
+        assert_eq!(
+            GlobalHeapCollection::parse_in(&storage, 0, 8)
+                .unwrap()
+                .objects
+                .len(),
+            2
+        );
+        assert_eq!(storage.reads(), 2);
+    }
 }
diff --git a/crates/clawhdf5-format/src/lib.rs b/crates/clawhdf5-format/src/lib.rs
index 345c923..9d42a68 100644
--- a/crates/clawhdf5-format/src/lib.rs
+++ b/crates/clawhdf5-format/src/lib.rs
@@ -122,6 +122,7 @@ pub mod property_list;
 pub mod selection;
 pub mod shared_message;
 pub mod signature;
+pub mod storage;
 pub mod superblock;
 pub mod superblock_ext;
 pub mod symbol_table;
diff --git a/crates/clawhdf5-format/src/local_heap.rs b/crates/clawhdf5-format/src/local_heap.rs
index b2ef923..01a1594 100644
--- a/crates/clawhdf5-format/src/local_heap.rs
+++ b/crates/clawhdf5-format/src/local_heap.rs
@@ -5,6 +5,7 @@ use alloc::string::String;
 
 use crate::addr::to_usize;
 use crate::error::FormatError;
+use crate::storage::{Storage, len_usize, read_exact_at};
 
 /// Parsed HDF5 Local Heap header.
 #[derive(Debug, Clone)]
@@ -17,21 +18,6 @@ pub struct LocalHeap {
     pub data_segment_address: u64,
 }
 
-/// Checks that `[offset, offset + needed)` fits within `data`, guarding the
-/// addition against `usize` overflow from a crafted near-`usize::MAX` offset.
-fn ensure_len(data: &[u8], offset: usize, needed: usize) -> Result<(), FormatError> {
-    if offset
-        .checked_add(needed)
-        .is_none_or(|end| end > data.len())
-    {
-        return Err(FormatError::UnexpectedEof {
-            expected: offset.saturating_add(needed),
-            available: data.len(),
-        });
-    }
-    Ok(())
-}
-
 fn read_offset(data: &[u8], pos: usize, size: u8) -> Result<u64, FormatError> {
     let s = size as usize;
     if pos.checked_add(s).is_none_or(|end| end > data.len()) {
@@ -51,6 +37,10 @@ fn read_offset(data: &[u8], pos: usize, size: u8) -> Result<u64, FormatError> {
     })
 }
 
+/// First read of a name on a backend without the file in memory: most link
+/// names are shorter than this.
+const NAME_READ_START: usize = 64;
+
 impl LocalHeap {
     /// Parse a local heap header at the given offset in the file data.
     pub fn parse(
@@ -58,12 +48,24 @@ impl LocalHeap {
         offset: usize,
         offset_size: u8,
         length_size: u8,
+    ) -> Result<LocalHeap, FormatError> {
+        Self::parse_in(file_data, offset as u64, offset_size, length_size)
+    }
+
+    /// [`Self::parse`] over any [`Storage`]: one read of the header.
+    pub fn parse_in<S: Storage + ?Sized>(
+        file: &S,
+        offset: u64,
+        offset_size: u8,
+        length_size: u8,
     ) -> Result<LocalHeap, FormatError> {
         // signature(4) + version(1) + reserved(3) = 8, then length_size*2 + offset_size
         let ls = length_size as usize;
         let os = offset_size as usize;
         let total = 8 + ls * 2 + os;
-        ensure_len(file_data, offset, total)?;
+        let header = read_exact_at(file, offset, total)?;
+        let file_data: &[u8] = &header;
+        let offset = 0usize;
 
         if &file_data[offset..offset + 4] != b"HEAP" {
             return Err(FormatError::InvalidLocalHeapSignature);
@@ -100,6 +102,16 @@ impl LocalHeap {
     /// The end of the list is `H5HL_FREE_NULL` (1); an all-ones value (the
     /// undefined address) is accepted as "no free list" too.
     pub fn validate_free_list(&self, file_data: &[u8], length_size: u8) -> Result<(), FormatError> {
+        self.validate_free_list_in(file_data, length_size)
+    }
+
+    /// [`Self::validate_free_list`] over any [`Storage`]: two small reads
+    /// per free block.
+    pub fn validate_free_list_in<S: Storage + ?Sized>(
+        &self,
+        file: &S,
+        length_size: u8,
+    ) -> Result<(), FormatError> {
         const FREE_NULL: u64 = 1;
         let ls = length_size as usize;
         let undefined = if ls >= 8 {
@@ -124,11 +136,12 @@ impl LocalHeap {
                 .and_then(|a| usize::try_from(a).ok())
                 .ok_or(FormatError::InvalidLocalHeapFreeList)?;
             let block_offset = next;
-            next = read_offset(file_data, at, length_size)?;
+            next = read_offset(&read_exact_at(file, at as u64, ls)?, 0, length_size)?;
             if next == 0 {
                 return Err(FormatError::InvalidLocalHeapFreeList);
             }
-            let block_size = read_offset(file_data, at + ls, length_size)?;
+            let block_size =
+                read_offset(&read_exact_at(file, (at + ls) as u64, ls)?, 0, length_size)?;
             if block_offset
                 .checked_add(block_size)
                 .is_none_or(|end| end > size)
@@ -141,6 +154,18 @@ impl LocalHeap {
 
     /// Read a null-terminated string from the heap's data segment at the given byte offset.
     pub fn read_string(&self, file_data: &[u8], string_offset: u64) -> Result<String, FormatError> {
+        self.read_string_in(file_data, string_offset)
+    }
+
+    /// [`Self::read_string`] over any [`Storage`]: one read of up to 64
+    /// bytes for a short name, more (each four times the last) up to the end
+    /// of the data segment for a longer one.
+    pub fn read_string_in<S: Storage + ?Sized>(
+        &self,
+        file: &S,
+        string_offset: u64,
+    ) -> Result<String, FormatError> {
+        let file_len = len_usize(file);
         let seg_addr = to_usize(self.data_segment_address)?;
         let str_start =
             seg_addr
@@ -154,30 +179,40 @@ impl LocalHeap {
                 "local heap seg_addr + data_segment_size overflow".into(),
             ))?;
 
-        if str_start >= file_data.len() || str_start >= seg_end {
+        if str_start >= file_len || str_start >= seg_end {
             return Err(FormatError::UnexpectedEof {
                 expected: str_start + 1,
-                available: file_data.len(),
+                available: file_len,
             });
         }
 
-        // Find null terminator
-        let search_end = seg_end.min(file_data.len());
-        let mut end = str_start;
-        while end < search_end && file_data[end] != 0 {
-            end += 1;
+        // Find the null terminator, which lies before the end of the data
+        // segment (or of the file). In memory that is one borrowed slice;
+        // otherwise the bytes are read in growing pieces, so a name costs a
+        // read of about its own length, not of the rest of the segment
+        // (whose size is an untrusted header field).
+        let search_end = seg_end.min(file_len);
+        let total = search_end - str_start;
+        let mut want = if file.as_contiguous().is_some() {
+            total
+        } else {
+            total.min(NAME_READ_START)
+        };
+        loop {
+            let rest = read_exact_at(file, str_start as u64, want)?;
+            if let Some(len) = rest.iter().position(|&b| b == 0) {
+                let s = core::str::from_utf8(&rest[..len])
+                    .map_err(|_| FormatError::InvalidLocalHeapSignature)?;
+                return Ok(String::from(s));
+            }
+            if want == total {
+                return Err(FormatError::UnexpectedEof {
+                    expected: search_end + 1,
+                    available: search_end,
+                });
+            }
+            want = want.saturating_mul(4).min(total);
         }
-
-        if end >= search_end {
-            return Err(FormatError::UnexpectedEof {
-                expected: end + 1,
-                available: search_end,
-            });
-        }
-
-        let s = core::str::from_utf8(&file_data[str_start..end])
-            .map_err(|_| FormatError::InvalidLocalHeapSignature)?;
-        Ok(String::from(s))
     }
 }
 
@@ -346,4 +381,77 @@ mod tests {
         let err = LocalHeap::parse(&file, 0, 8, 8).unwrap_err();
         assert_eq!(err, FormatError::InvalidLocalHeapVersion(1));
     }
+
+    /// Header, free list and strings read identically through a
+    /// `read_at`-only storage, for every truncation of the file.
+    #[test]
+    fn storage_reads_match_slice_reads() {
+        use crate::storage::CountingStorage;
+        let plain = build_heap_file(0, 64, &["", "alpha", "beta"], 8, 8);
+        // A free block of 16 bytes at segment offset 12, ending the list.
+        let mut free = build_heap_file(0, 64, &["", "alpha", "beta", &"x".repeat(20)], 8, 8);
+        free[16..24].copy_from_slice(&12u64.to_le_bytes());
+        free[64 + 12..64 + 20].copy_from_slice(&1u64.to_le_bytes());
+        free[64 + 20..64 + 28].copy_from_slice(&16u64.to_le_bytes());
+        let mut bad_free = free.clone();
+        bad_free[64 + 20..64 + 28].copy_from_slice(&99u64.to_le_bytes());
+        for full in [plain, free, bad_free] {
+            for cut in 0..=full.len() {
+                let f = &full[..cut];
+                let storage = CountingStorage::new(f.to_vec());
+                let want = LocalHeap::parse(f, 0, 8, 8);
+                let got = LocalHeap::parse_in(&storage, 0, 8, 8);
+                assert_eq!(format!("{got:?}"), format!("{want:?}"));
+                let Ok(heap) = want else { continue };
+                assert_eq!(
+                    heap.validate_free_list_in(&storage, 8),
+                    heap.validate_free_list(f, 8)
+                );
+                for off in [0u64, 1, 2, 6, 7, 11, 100] {
+                    assert_eq!(heap.read_string_in(&storage, off), heap.read_string(f, off));
+                }
+            }
+        }
+    }
+
+    /// Names of every length around the first read's size, and one with no
+    /// terminator, read identically through a `read_at`-only storage; a
+    /// short name in a heap whose header claims a huge data segment costs
+    /// one small read, not a read of the rest of the file.
+    #[test]
+    fn long_names_and_hostile_segment_sizes() {
+        use crate::storage::CountingStorage;
+        let names: Vec<String> = [0usize, 1, 63, 64, 65, 255, 256, 257, 1000, 5000]
+            .iter()
+            .map(|&n| "n".repeat(n))
+            .collect();
+        let refs: Vec<&str> = names.iter().map(String::as_str).collect();
+        let mut file = build_heap_file(0, 64, &refs, 8, 8);
+        let heap = LocalHeap::parse(&file, 0, 8, 8).unwrap();
+        let storage = CountingStorage::new(file.clone());
+        let mut off = 0u64;
+        for name in &names {
+            let got = heap.read_string_in(&storage, off);
+            assert_eq!(got, heap.read_string(&file, off));
+            assert_eq!(got.unwrap(), *name);
+            off += name.len() as u64 + 1;
+        }
+        // The last name loses its terminator: both report the same error.
+        let seg_end = 64 + heap.data_segment_size as usize;
+        file[seg_end - 1] = b'n';
+        let storage = CountingStorage::new(file.clone());
+        let last = off - names[names.len() - 1].len() as u64 - 1;
+        let want = heap.read_string(&file, last);
+        assert!(want.is_err());
+        assert_eq!(heap.read_string_in(&storage, last), want);
+
+        // A 64 MiB file whose heap claims a data segment reaching its end.
+        let mut big = build_heap_file(0, 64, &["short", "names"], 8, 8);
+        big.resize(64 << 20, 0);
+        big[8..16].copy_from_slice(&((64u64 << 20) - 64).to_le_bytes());
+        let heap = LocalHeap::parse(&big, 0, 8, 8).unwrap();
+        let storage = CountingStorage::new(big.clone());
+        assert_eq!(heap.read_string_in(&storage, 6).unwrap(), "names");
+        assert_eq!((storage.reads(), storage.bytes_read()), (1, 64));
+    }
 }
diff --git a/crates/clawhdf5-format/src/object_header.rs b/crates/clawhdf5-format/src/object_header.rs
index 3d7b7f7..0d031ed 100644
--- a/crates/clawhdf5-format/src/object_header.rs
+++ b/crates/clawhdf5-format/src/object_header.rs
@@ -8,6 +8,7 @@ use byteorder::{ByteOrder, LittleEndian};
 use crate::addr::to_usize;
 use crate::error::FormatError;
 use crate::message_type::MessageType;
+use crate::storage::{Storage, Window, len_usize, read_exact_at};
 
 /// OHDR signature for v2 object headers.
 const OHDR_SIGNATURE: [u8; 4] = *b"OHDR";
@@ -119,32 +120,52 @@ impl ObjectHeader {
         offset_size: u8,
         length_size: u8,
     ) -> Result<ObjectHeader, FormatError> {
-        ensure_len(data, offset, 4)?;
-        if data[offset..offset + 4] == OHDR_SIGNATURE {
-            Self::parse_v2(data, offset, offset_size, length_size)
+        Self::parse_in(data, offset as u64, offset_size, length_size)
+    }
+
+    /// [`Self::parse`] over any [`Storage`].
+    ///
+    /// Reads the prefix (at most [`V2_PREFIX_MAX`] bytes, signature
+    /// included), then each chunk as one bounded read, continuation chunks
+    /// included.
+    pub fn parse_in<S: Storage + ?Sized>(
+        file: &S,
+        offset: u64,
+        offset_size: u8,
+        length_size: u8,
+    ) -> Result<ObjectHeader, FormatError> {
+        // The longest prefix of either version, in one read. It holds the
+        // whole prefix or ends at the end of the file, so its bounds checks
+        // are the whole-file ones.
+        let prefix = Window::read(file, offset, V2_PREFIX_MAX)?;
+        prefix.ensure(0, 4)?;
+        if prefix.bytes[..4] == OHDR_SIGNATURE {
+            Self::parse_v2(file, offset, &prefix, offset_size, length_size)
         } else {
-            Self::parse_v1(data, offset, offset_size, length_size)
+            Self::parse_v1(file, offset, &prefix, offset_size, length_size)
         }
     }
 
-    fn parse_v1(
-        data: &[u8],
-        offset: usize,
+    fn parse_v1<S: Storage + ?Sized>(
+        file: &S,
+        offset: u64,
+        prefix: &Window<'_>,
         offset_size: u8,
         length_size: u8,
     ) -> Result<ObjectHeader, FormatError> {
         // version(1) + reserved(1) + num_messages(2) + ref_count(4) + header_size(4) = 12
         // then pad to 8-byte alignment from start of header
-        ensure_len(data, offset, 12)?;
+        prefix.ensure(0, 12)?;
+        let prefix = &prefix.bytes[..12];
 
-        let version = data[offset];
+        let version = prefix[0];
         if version != 1 {
             return Err(FormatError::InvalidObjectHeaderVersion(version));
         }
 
-        let num_messages = LittleEndian::read_u16(&data[offset + 2..offset + 4]) as usize;
-        let reference_count = LittleEndian::read_u32(&data[offset + 4..offset + 8]);
-        let header_data_size = LittleEndian::read_u32(&data[offset + 8..offset + 12]) as usize;
+        let num_messages = LittleEndian::read_u16(&prefix[2..4]) as usize;
+        let reference_count = LittleEndian::read_u32(&prefix[4..8]);
+        let header_data_size = LittleEndian::read_u32(&prefix[8..12]) as usize;
 
         // libhdf5 (H5O__prefix_deserialize): a header with messages needs room
         // for at least one message header, and one without has an empty chunk.
@@ -162,14 +183,15 @@ impl ObjectHeader {
             .checked_add(12 + padding)
             .ok_or(FormatError::UnexpectedEof {
                 expected: usize::MAX,
-                available: data.len(),
+                available: len_usize(file),
             })?;
 
-        ensure_len(data, msg_start, header_data_size)?;
-
-        let mut messages = Vec::new();
+        // parse_v1_chunk reads the chunk, with the bounds check that was here.
+        // The prefix's count (NIL messages included, capped: it is untrusted)
+        // sizes the list once instead of growing it message by message.
+        let mut messages = Vec::with_capacity(num_messages.min(64));
         let chunk0_count = Self::parse_v1_chunk(
-            data,
+            file,
             msg_start,
             header_data_size,
             offset_size,
@@ -209,9 +231,9 @@ impl ObjectHeader {
     /// end of the chunk, or leftover bytes too few for a message header (a
     /// "gap", which only version 2 allows).
     #[allow(clippy::too_many_arguments)]
-    fn parse_v1_chunk(
-        data: &[u8],
-        offset: usize,
+    fn parse_v1_chunk<S: Storage + ?Sized>(
+        file: &S,
+        offset: u64,
         length: usize,
         offset_size: u8,
         length_size: u8,
@@ -221,9 +243,10 @@ impl ObjectHeader {
         if depth_remaining == 0 {
             return Err(FormatError::NestingDepthExceeded);
         }
-        ensure_len(data, offset, length)?;
-        let end = offset + length;
-        let mut pos = offset;
+        let chunk = read_exact_at(file, offset, length)?;
+        let data: &[u8] = &chunk;
+        let end = length;
+        let mut pos = 0usize;
         let mut count = 0usize;
 
         while pos < end {
@@ -268,8 +291,8 @@ impl ObjectHeader {
                 let cont_offset = to_usize(read_offset(body, 0, offset_size)?)?;
                 let cont_length = to_usize(read_offset(body, offset_size as usize, length_size)?)?;
                 Self::parse_v1_chunk(
-                    data,
-                    cont_offset,
+                    file,
+                    cont_offset as u64,
                     cont_length,
                     offset_size,
                     length_size,
@@ -282,12 +305,22 @@ impl ObjectHeader {
         Ok(count)
     }
 
-    fn parse_v2(
-        data: &[u8],
-        offset: usize,
+    fn parse_v2<S: Storage + ?Sized>(
+        file: &S,
+        offset: u64,
+        prefix: &Window<'_>,
         offset_size: u8,
         length_size: u8,
     ) -> Result<ObjectHeader, FormatError> {
+        // `ensure_len` checks positions relative to the header against the
+        // prefix window and reports them as the whole-file check did, with
+        // absolute positions and the file's length.
+        let data: &[u8] = &prefix.bytes;
+        let file_len = len_usize(file);
+        let base = usize::try_from(offset).unwrap_or(usize::MAX);
+        let abs = |rel: usize| base.saturating_add(rel);
+        let ensure_len = |_: &[u8], rel: usize, needed: usize| prefix.ensure(rel, needed);
+        let offset = 0usize;
         // signature(4) + version(1) + flags(1) = 6
         ensure_len(data, offset, 6)?;
 
@@ -352,15 +385,20 @@ impl ObjectHeader {
         }
 
         let chunk0_msg_start = pos;
-        let chunk0_msg_end = pos
-            .checked_add(chunk0_size)
-            .ok_or(FormatError::UnexpectedEof {
+        let Some(chunk0_abs_end) = abs(pos).checked_add(chunk0_size) else {
+            return Err(FormatError::UnexpectedEof {
                 expected: usize::MAX,
-                available: data.len(),
-            })?;
+                available: file_len,
+            });
+        };
+        let chunk0_msg_end = chunk0_abs_end - base;
+
+        // The whole first chunk, prefix to checksum, in one read (its
+        // bounds check is the one on the checksum's 4 bytes).
+        let chunk0 = read_exact_at(file, base as u64, chunk0_msg_end.saturating_add(4))?;
+        let data: &[u8] = &chunk0;
 
         // Validate checksum: from OHDR signature through all messages (before checksum)
-        ensure_len(data, chunk0_msg_end, 4)?;
         #[cfg(feature = "checksum")]
         {
             let stored = LittleEndian::read_u32(&data[chunk0_msg_end..chunk0_msg_end + 4]);
@@ -395,8 +433,8 @@ impl ObjectHeader {
             }
             cont_remaining -= 1;
             Self::parse_v2_continuation(
-                data,
-                cont_offset,
+                file,
+                cont_offset as u64,
                 cont_length,
                 has_creation_order,
                 offset_size,
@@ -495,9 +533,9 @@ impl ObjectHeader {
     }
 
     #[allow(clippy::too_many_arguments)]
-    fn parse_v2_continuation(
-        data: &[u8],
-        offset: usize,
+    fn parse_v2_continuation<S: Storage + ?Sized>(
+        file: &S,
+        offset: u64,
         length: usize,
         has_creation_order: bool,
         offset_size: u8,
@@ -506,7 +544,9 @@ impl ObjectHeader {
         continuations: &mut Vec<(usize, usize)>,
     ) -> Result<(), FormatError> {
         // OCHK signature(4) + messages + checksum(4)
-        ensure_len(data, offset, length)?;
+        let chunk = read_exact_at(file, offset, length)?;
+        let data: &[u8] = &chunk;
+        let offset = 0usize;
         if length < 8 {
             return Err(FormatError::UnexpectedEof {
                 expected: 8,
@@ -547,6 +587,10 @@ impl ObjectHeader {
     }
 }
 
+/// Longest version-2 object header prefix: signature(4) + version(1) +
+/// flags(1) + times(16) + attribute phase change(4) + chunk-0 size(8).
+const V2_PREFIX_MAX: usize = 34;
+
 /// Size of a version-1 message header: type(2) + size(2) + flags(1) + reserved(3).
 const V1_MSG_HEADER_SIZE: usize = 8;
 
@@ -755,13 +799,13 @@ mod tests {
         let mut msg_bytes = Vec::new();
         for (mtype, mdata, mflags) in messages {
             // v1 message sizes are multiples of 8 (the data is zero-padded).
-            let padded = mdata.len().div_ceil(8) * 8;
+            let padded = <[u8]>::len(mdata).div_ceil(8) * 8;
             msg_bytes.extend_from_slice(&mtype.to_le_bytes()); // type(2)
             msg_bytes.extend_from_slice(&(padded as u16).to_le_bytes()); // size(2)
             msg_bytes.push(*mflags); // flags(1)
             msg_bytes.extend_from_slice(&[0u8; 3]); // reserved(3)
             msg_bytes.extend_from_slice(mdata); // data
-            msg_bytes.resize(msg_bytes.len() + padded - mdata.len(), 0);
+            msg_bytes.resize(msg_bytes.len() + padded - <[u8]>::len(mdata), 0);
         }
 
         let mut buf = Vec::new();
@@ -1281,4 +1325,56 @@ mod tests {
         let err = ObjectHeader::parse(&data, 0, 8, 8).unwrap_err();
         assert!(matches!(err, FormatError::UnexpectedEof { .. }));
     }
+
+    /// Every header, and every truncation of it, parses to the same result
+    /// (or the same error) through a `read_at`-only storage as from a slice;
+    /// a header in one chunk takes two reads (prefix, chunk).
+    #[test]
+    fn parse_in_matches_slice_parse() {
+        use crate::storage::CountingStorage;
+        let mut headers = vec![
+            build_v1_header(&[], 8, 8),
+            build_v1_header(&[(0x0001, &[1, 2, 3], 0), (0x0003, &[9; 8], 0)], 8, 8),
+            build_v2_header(0x00, &[(0x01, &[42], 0)], None),
+            build_v2_header(0x03, &[(0x01, &[1, 2], 0), (0x03, &[3], 0)], None),
+            build_v2_header(0x24, &[(0x01, &[1], 0)], Some((1, 2, 3, 4))),
+            build_v2_header(0x35, &[(0x01, &[1], 0)], Some((5, 6, 7, 8))),
+        ];
+        // A v2 header with a continuation chunk at 256.
+        let mut ochk = OCHK_SIGNATURE.to_vec();
+        ochk.extend_from_slice(&[0x03, 2, 0, 0, 0xDE, 0xAD]);
+        let sum = crate::checksum::jenkins_lookup3(&ochk);
+        ochk.extend_from_slice(&sum.to_le_bytes());
+        let mut cont = 256u64.to_le_bytes().to_vec();
+        cont.extend_from_slice(&(ochk.len() as u64).to_le_bytes());
+        let main = build_v2_header(0x00, &[(0x01, &[42], 0), (0x10, &cont, 0)], None);
+        let mut with_cont = vec![0u8; 256 + ochk.len()];
+        with_cont[..main.len()].copy_from_slice(&main);
+        with_cont[256..].copy_from_slice(&ochk);
+        headers.push(with_cont);
+
+        for h in headers {
+            for at in [0usize, 3] {
+                for cut in 0..=h.len() {
+                    let mut f = vec![0u8; at];
+                    f.extend_from_slice(&h[..cut]);
+                    if at == 0 && cut == h.len() {
+                        f.resize(f.len() + 64, 0);
+                    }
+                    let want = ObjectHeader::parse(&f, at, 8, 8);
+                    let storage = CountingStorage::new(f.clone());
+                    let got = ObjectHeader::parse_in(&storage, at as u64, 8, 8);
+                    assert_eq!(
+                        format!("{got:?}"),
+                        format!("{want:?}"),
+                        "at {at}, cut {cut}"
+                    );
+                }
+            }
+        }
+        let one_chunk = build_v2_header(0x00, &[(0x01, &[42], 0)], None);
+        let storage = CountingStorage::new(one_chunk);
+        ObjectHeader::parse_in(&storage, 0, 8, 8).unwrap();
+        assert_eq!(storage.reads(), 2);
+    }
 }
diff --git a/crates/clawhdf5-format/src/shared_message.rs b/crates/clawhdf5-format/src/shared_message.rs
index 5c3227f..05ee649 100644
--- a/crates/clawhdf5-format/src/shared_message.rs
+++ b/crates/clawhdf5-format/src/shared_message.rs
@@ -23,12 +23,12 @@ use alloc::vec::Vec;
 #[cfg(feature = "std")]
 use std::borrow::Cow;
 
-use crate::addr::to_usize;
 use crate::btree_v2::{BTreeV2Header, collect_btree_v2_records};
 use crate::error::FormatError;
 use crate::fractal_heap::FractalHeapHeader;
 use crate::message_type::MessageType;
 use crate::object_header::ObjectHeader;
+use crate::storage::{Storage, Window, read_exact_at, require_contiguous};
 
 /// Fractal heap ID length for SOHM entries (fixed at 8 bytes).
 const FHEAP_ID_LEN: usize = 8;
@@ -254,17 +254,31 @@ pub fn parse_sohm_table(
     nindexes: u8,
     offset_size: u8,
 ) -> Result<SohmTable, FormatError> {
-    ensure_len(file_data, table_addr, 4)?;
-    if &file_data[table_addr..table_addr + 4] != b"SMTB" {
+    parse_sohm_table_in(file_data, table_addr as u64, nindexes, offset_size)
+}
+
+/// [`parse_sohm_table`] over any [`Storage`]: one read of the signature,
+/// one of every index entry.
+pub fn parse_sohm_table_in<S: Storage + ?Sized>(
+    file: &S,
+    table_addr: u64,
+    nindexes: u8,
+    offset_size: u8,
+) -> Result<SohmTable, FormatError> {
+    let sig = read_exact_at(file, table_addr, 4)?;
+    if *sig != *b"SMTB" {
         return Err(FormatError::InvalidSohmTableSignature);
     }
-    let mut pos = table_addr + 4;
     let os = offset_size as usize;
     let entry_size = 1 + 1 + 2 + 4 + 2 + 2 + 2 + os + os; // 14 + 2*offset_size
+    // Positions below are relative to the table.
+    let w = Window::read(file, table_addr, 4 + nindexes as usize * entry_size)?;
+    let file_data: &[u8] = &w.bytes;
+    let mut pos = 4;
 
     let mut indexes = Vec::with_capacity(nindexes as usize);
     for _ in 0..nindexes {
-        ensure_len(file_data, pos, entry_size)?;
+        w.ensure(pos, entry_size)?;
         let version = file_data[pos];
         if version != 0 {
             return Err(FormatError::InvalidSohmTableVersion(version));
@@ -370,16 +384,29 @@ pub fn parse_sohm_list(
     num_messages: u16,
     offset_size: u8,
 ) -> Result<Vec<SohmEntry>, FormatError> {
-    ensure_len(file_data, list_addr, 4)?;
-    if &file_data[list_addr..list_addr + 4] != b"SMLI" {
+    parse_sohm_list_in(file_data, list_addr as u64, num_messages, offset_size)
+}
+
+/// [`parse_sohm_list`] over any [`Storage`]: one read of the signature, one
+/// of every entry.
+pub fn parse_sohm_list_in<S: Storage + ?Sized>(
+    file: &S,
+    list_addr: u64,
+    num_messages: u16,
+    offset_size: u8,
+) -> Result<Vec<SohmEntry>, FormatError> {
+    let sig = read_exact_at(file, list_addr, 4)?;
+    if *sig != *b"SMLI" {
         return Err(FormatError::InvalidSohmListSignature);
     }
     let entry_sz = sohm_entry_size(offset_size);
-    let mut pos = list_addr + 4;
+    // Positions below are relative to the list.
+    let w = Window::read(file, list_addr, 4 + num_messages as usize * entry_sz)?;
+    let mut pos = 4;
     let mut entries = Vec::with_capacity(num_messages as usize);
     for _ in 0..num_messages {
-        ensure_len(file_data, pos, entry_sz)?;
-        let entry = parse_sohm_entry(&file_data[pos..], offset_size)?;
+        w.ensure(pos, entry_sz)?;
+        let entry = parse_sohm_entry(&w.bytes[pos..], offset_size)?;
         entries.push(entry);
         pos += entry_sz;
     }
@@ -393,6 +420,20 @@ pub fn parse_sohm_btree_entries(
     offset_size: u8,
     length_size: u8,
 ) -> Result<Vec<SohmEntry>, FormatError> {
+    parse_sohm_btree_entries_in(file_data, btree_addr as u64, offset_size, length_size)
+}
+
+/// [`parse_sohm_btree_entries`] over any [`Storage`]. The v2 B-tree is not
+/// read over [`Storage`] yet, so this needs the whole file in memory
+/// ([`FormatError::ContiguousStorageRequired`] otherwise).
+pub fn parse_sohm_btree_entries_in<S: Storage + ?Sized>(
+    file: &S,
+    btree_addr: u64,
+    offset_size: u8,
+    length_size: u8,
+) -> Result<Vec<SohmEntry>, FormatError> {
+    let file_data = require_contiguous(file, "a shared-message B-tree index")?;
+    let btree_addr = usize::try_from(btree_addr).unwrap_or(usize::MAX);
     let header = BTreeV2Header::parse(file_data, btree_addr, offset_size, length_size)?;
     let records = collect_btree_v2_records(file_data, &header, offset_size, length_size)?;
     let mut entries = Vec::with_capacity(records.len());
@@ -414,15 +455,24 @@ pub fn load_sohm_table(
     offset_size: u8,
     length_size: u8,
 ) -> Result<Option<SohmTable>, FormatError> {
-    let sig = crate::signature::find_signature(file_data)?;
-    let sb = crate::superblock::Superblock::parse(file_data, sig)?;
+    load_sohm_table_in(file_data, offset_size, length_size)
+}
+
+/// [`load_sohm_table`] over any [`Storage`].
+pub fn load_sohm_table_in<S: Storage + ?Sized>(
+    file_data: &S,
+    offset_size: u8,
+    length_size: u8,
+) -> Result<Option<SohmTable>, FormatError> {
+    let sig = crate::signature::find_signature_in(file_data)?;
+    let sb = crate::superblock::Superblock::parse_in(file_data, sig)?;
     let Some(ext_addr) = sb
         .superblock_extension_address
         .filter(|&a| !is_undefined(a, offset_size))
     else {
         return Ok(None);
     };
-    let ext = ObjectHeader::parse(file_data, to_usize(ext_addr)?, offset_size, length_size)?;
+    let ext = ObjectHeader::parse_in(file_data, ext_addr, offset_size, length_size)?;
     let Some(msg) = ext
         .messages
         .iter()
@@ -431,9 +481,9 @@ pub fn load_sohm_table(
         return Ok(None);
     };
     let table_msg = parse_sohm_table_message(&msg.data, offset_size)?;
-    parse_sohm_table(
+    parse_sohm_table_in(
         file_data,
-        to_usize(table_msg.table_address)?,
+        table_msg.table_address,
         table_msg.nindexes,
         offset_size,
     )
@@ -447,17 +497,27 @@ pub fn message_data_with_sohm<'a>(
     msg: &'a crate::object_header::HeaderMessage,
     offset_size: u8,
     length_size: u8,
+) -> Result<Cow<'a, [u8]>, FormatError> {
+    message_data_with_sohm_in(file_data, msg, offset_size, length_size)
+}
+
+/// [`message_data_with_sohm`] over any [`Storage`].
+pub fn message_data_with_sohm_in<'a, S: Storage + ?Sized>(
+    file_data: &S,
+    msg: &'a crate::object_header::HeaderMessage,
+    offset_size: u8,
+    length_size: u8,
 ) -> Result<Cow<'a, [u8]>, FormatError> {
     if !is_shared(msg.flags) {
         return Ok(Cow::Borrowed(&msg.data));
     }
     let shared_ref = parse_shared_ref_sized(&msg.data, offset_size, length_size)?;
     let table = if shared_ref.heap_id.is_some() {
-        load_sohm_table(file_data, offset_size, length_size)?
+        load_sohm_table_in(file_data, offset_size, length_size)?
     } else {
         None
     };
-    resolve_shared_message_with_sohm(
+    resolve_shared_message_with_sohm_in(
         file_data,
         &shared_ref,
         msg.msg_type,
@@ -496,6 +556,25 @@ pub fn resolve_sohm_message(
     target_msg_type: MessageType,
     offset_size: u8,
     length_size: u8,
+) -> Result<Vec<u8>, FormatError> {
+    resolve_sohm_message_in(
+        &file_data,
+        heap_id,
+        sohm_table,
+        target_msg_type,
+        offset_size,
+        length_size,
+    )
+}
+
+/// [`resolve_sohm_message`] over any [`Storage`].
+pub fn resolve_sohm_message_in<S: Storage + ?Sized>(
+    file_data: &S,
+    heap_id: &[u8; FHEAP_ID_LEN],
+    sohm_table: &SohmTable,
+    target_msg_type: MessageType,
+    offset_size: u8,
+    length_size: u8,
 ) -> Result<Vec<u8>, FormatError> {
     let index = find_index_for_msg_type(sohm_table, target_msg_type)
         .ok_or(FormatError::InvalidSharedMessageVersion(2))?;
@@ -504,13 +583,9 @@ pub fn resolve_sohm_message(
         return Err(FormatError::InvalidSharedMessageVersion(2));
     }
 
-    let fh_header = FractalHeapHeader::parse(
-        file_data,
-        to_usize(index.heap_addr)?,
-        offset_size,
-        length_size,
-    )?;
-    fh_header.read_managed_object(file_data, heap_id, offset_size)
+    let fh_header =
+        FractalHeapHeader::parse_in(file_data, index.heap_addr, offset_size, length_size)?;
+    fh_header.read_managed_object_in(file_data, heap_id, offset_size)
 }
 
 /// The payload of an object-header message, following the indirection if the
@@ -527,12 +602,22 @@ pub fn message_data<'a>(
     msg: &'a crate::object_header::HeaderMessage,
     offset_size: u8,
     length_size: u8,
+) -> Result<Cow<'a, [u8]>, FormatError> {
+    message_data_in(file_data, msg, offset_size, length_size)
+}
+
+/// [`message_data`] over any [`Storage`].
+pub fn message_data_in<'a, S: Storage + ?Sized>(
+    file_data: &S,
+    msg: &'a crate::object_header::HeaderMessage,
+    offset_size: u8,
+    length_size: u8,
 ) -> Result<Cow<'a, [u8]>, FormatError> {
     if !is_shared(msg.flags) {
         return Ok(Cow::Borrowed(&msg.data));
     }
     let shared_ref = parse_shared_ref_sized(&msg.data, offset_size, length_size)?;
-    resolve_shared_message(
+    resolve_shared_message_in(
         file_data,
         &shared_ref,
         msg.msg_type,
@@ -554,13 +639,30 @@ pub fn resolve_shared_message(
     target_msg_type: MessageType,
     offset_size: u8,
     length_size: u8,
+) -> Result<Vec<u8>, FormatError> {
+    resolve_shared_message_in(
+        &file_data,
+        shared_ref,
+        target_msg_type,
+        offset_size,
+        length_size,
+    )
+}
+
+/// [`resolve_shared_message`] over any [`Storage`].
+pub fn resolve_shared_message_in<S: Storage + ?Sized>(
+    file_data: &S,
+    shared_ref: &SharedMessageRef,
+    target_msg_type: MessageType,
+    offset_size: u8,
+    length_size: u8,
 ) -> Result<Vec<u8>, FormatError> {
     let table = if shared_ref.heap_id.is_some() {
-        load_sohm_table(file_data, offset_size, length_size)?
+        load_sohm_table_in(file_data, offset_size, length_size)?
     } else {
         None
     };
-    resolve_shared_message_with_sohm(
+    resolve_shared_message_with_sohm_in(
         file_data,
         shared_ref,
         target_msg_type,
@@ -578,6 +680,25 @@ pub fn resolve_shared_message_with_sohm(
     offset_size: u8,
     length_size: u8,
     sohm_table: Option<&SohmTable>,
+) -> Result<Vec<u8>, FormatError> {
+    resolve_shared_message_with_sohm_in(
+        &file_data,
+        shared_ref,
+        target_msg_type,
+        offset_size,
+        length_size,
+        sohm_table,
+    )
+}
+
+/// [`resolve_shared_message_with_sohm`] over any [`Storage`].
+pub fn resolve_shared_message_with_sohm_in<S: Storage + ?Sized>(
+    file_data: &S,
+    shared_ref: &SharedMessageRef,
+    target_msg_type: MessageType,
+    offset_size: u8,
+    length_size: u8,
+    sohm_table: Option<&SohmTable>,
 ) -> Result<Vec<u8>, FormatError> {
     // Dispatch on what the reference carries rather than on `ref_type`: v1/v2
     // references are always an object-header address whatever their type
@@ -587,8 +708,7 @@ pub fn resolve_shared_message_with_sohm(
         shared_ref.heap_id.as_ref(),
     ) {
         (Some(addr), _) => {
-            let target_header =
-                ObjectHeader::parse(file_data, to_usize(addr)?, offset_size, length_size)?;
+            let target_header = ObjectHeader::parse_in(file_data, addr, offset_size, length_size)?;
             for msg in &target_header.messages {
                 if msg.msg_type == target_msg_type && !is_shared(msg.flags) {
                     return Ok(msg.data.clone());
@@ -615,7 +735,7 @@ pub fn resolve_shared_message_with_sohm(
         }
         (None, Some(heap_id)) => {
             let table = sohm_table.ok_or(FormatError::InvalidSharedMessageVersion(2))?;
-            resolve_sohm_message(
+            resolve_sohm_message_in(
                 file_data,
                 heap_id,
                 table,
@@ -1053,4 +1173,81 @@ mod tests {
         // With 2-byte offsets: OH=2+2=4, heap=12, entry=1+4+12=17
         assert_eq!(sohm_entry_size(2), 17);
     }
+
+    /// SOHM tables and lists parse identically through a read_at-only
+    /// CountingStorage: at two offsets, with 4- and 8-byte offsets, cut at
+    /// every length and with a bad signature.
+    #[test]
+    fn storage_reads_match_slice_reads() {
+        use crate::storage::CountingStorage;
+        let idx = |t: u8, n: u16| SohmIndex {
+            index_type: t,
+            mesg_types: 0x0008,
+            min_mesg_size: 50,
+            list_max: 50,
+            btree_min: 40,
+            num_messages: n,
+            index_addr: 0x3000,
+            heap_addr: 0x4000,
+        };
+        let heap_entry = |h: u32| SohmEntry {
+            location: 0,
+            hash: h,
+            heap_id: Some([1, 2, 3, 4, 5, 6, 7, h as u8]),
+            ref_count: Some(h),
+            mesg_index: None,
+            oh_addr: None,
+        };
+        let oh_entry = SohmEntry {
+            location: 1,
+            hash: 9,
+            heap_id: None,
+            ref_count: None,
+            mesg_index: Some(3),
+            oh_addr: Some(0x7000),
+        };
+        let mut compared = 0;
+        for os in [4u8, 8] {
+            let smtb = build_smtb(&[idx(0, 2), idx(1, 7)], os);
+            let smli = build_smli(&[heap_entry(1), oh_entry.clone(), heap_entry(2)], os);
+            for (body, n) in [(smtb, 2u16), (smli, 3)] {
+                let is_table = &body[..4] == b"SMTB";
+                for at in [0usize, 0x40] {
+                    let mut full = vec![0u8; at];
+                    full.extend_from_slice(&body);
+                    let mut files = Vec::new();
+                    for cut in at..=full.len() {
+                        files.push(full[..cut].to_vec());
+                    }
+                    let mut bad = full.clone();
+                    bad[at] = b'X';
+                    files.push(bad);
+                    for f in files {
+                        let st = CountingStorage::new(f.clone());
+                        let (want, got) = if is_table {
+                            (
+                                format!("{:?}", parse_sohm_table(&f, at, n as u8, os)),
+                                format!("{:?}", parse_sohm_table_in(&st, at as u64, n as u8, os)),
+                            )
+                        } else {
+                            (
+                                format!("{:?}", parse_sohm_list(&f, at, n, os)),
+                                format!("{:?}", parse_sohm_list_in(&st, at as u64, n, os)),
+                            )
+                        };
+                        assert_eq!(got, want, "{} bytes", f.len());
+                        assert!(st.reads() <= 2);
+                        compared += 1;
+                    }
+                }
+            }
+        }
+        assert!(compared > 200);
+        // The B-tree index is not read over Storage yet: a clean error.
+        let st = CountingStorage::new(vec![0u8; 64]);
+        assert_eq!(
+            parse_sohm_btree_entries_in(&st, 0, 8, 8).unwrap_err(),
+            FormatError::ContiguousStorageRequired("a shared-message B-tree index")
+        );
+    }
 }
diff --git a/crates/clawhdf5-format/src/signature.rs b/crates/clawhdf5-format/src/signature.rs
index 600b650..3a2e5a1 100644
--- a/crates/clawhdf5-format/src/signature.rs
+++ b/crates/clawhdf5-format/src/signature.rs
@@ -1,6 +1,7 @@
 //! HDF5 file signature (magic bytes) detection.
 
 use crate::error::FormatError;
+use crate::storage::{Storage, read_exact_at};
 
 /// The 8-byte HDF5 magic signature.
 pub const HDF5_SIGNATURE: [u8; 8] = [0x89, b'H', b'D', b'F', b'\r', b'\n', 0x1A, b'\n'];
@@ -39,6 +40,20 @@ pub fn find_signature(data: &[u8]) -> Result<usize, FormatError> {
     Err(FormatError::SignatureNotFound)
 }
 
+/// [`find_signature`] over any [`Storage`]: one 8-byte read per candidate
+/// offset.
+pub fn find_signature_in<S: Storage + ?Sized>(file: &S) -> Result<u64, FormatError> {
+    let len = file.len();
+    let mut offset = 0u64;
+    while offset.checked_add(8).is_some_and(|end| end <= len) {
+        if *read_exact_at(file, offset, 8)? == HDF5_SIGNATURE {
+            return Ok(offset);
+        }
+        offset = if offset == 0 { 512 } else { offset * 2 };
+    }
+    Err(FormatError::SignatureNotFound)
+}
+
 /// Split a file into its user block and its HDF5 bytes.
 ///
 /// Returns `(user_block, hdf5)`: `user_block` is everything before the
@@ -132,4 +147,26 @@ mod tests {
         data[512..520].copy_from_slice(&HDF5_SIGNATURE);
         assert_eq!(find_signature(&data), Ok(0));
     }
+
+    #[test]
+    fn find_signature_in_matches_slice_search() {
+        use crate::storage::CountingStorage;
+        for (len, at) in [
+            (0, None),
+            (7, None),
+            (8, Some(0)),
+            (600, Some(512)),
+            (5000, Some(4096)),
+            (3000, Some(2048)),
+            (3000, None),
+        ] {
+            let mut data = vec![0u8; len];
+            if let Some(at) = at {
+                data[at..at + 8].copy_from_slice(&HDF5_SIGNATURE);
+            }
+            let want = find_signature(&data).map(|o| o as u64);
+            let got = find_signature_in(&CountingStorage::new(data));
+            assert_eq!(got, want, "{len} {at:?}");
+        }
+    }
 }
diff --git a/crates/clawhdf5-format/src/storage.rs b/crates/clawhdf5-format/src/storage.rs
new file mode 100644
index 0000000..555ac78
--- /dev/null
+++ b/crates/clawhdf5-format/src/storage.rs
@@ -0,0 +1,466 @@
+//! Where the parsers read the file from: the [`Storage`] trait.
+//!
+//! Every parser used to take the whole file as one `&[u8]`. [`Storage`] is
+//! the abstraction that replaces it (see `docs/design/range-reads.md`,
+//! option (a)): a parser asks for the bytes it needs, `[offset, offset +
+//! len)`, with 64-bit offsets, and gets them back as a [`Cow`] — borrowed
+//! when the backend holds the file in memory (a `Vec`, an mmap), owned when
+//! it had to fetch them (a range request, a block cache).
+//!
+//! `impl Storage for [u8]` serves the in-memory case with no copy, and
+//! [`Storage::as_contiguous`] lets a hot loop borrow the whole file at once
+//! when the backend has it. Modules are converted one at a time: a converted
+//! parser has an `*_in<S: Storage + ?Sized>(file: &S, ..)` core and keeps
+//! its old `&[u8]` signature as a thin wrapper, so callers do not change.
+//!
+//! The cores are generic rather than taking `&dyn Storage` so that the
+//! wrappers monomorphise for `[u8]`: the bounds check of each structure read
+//! inlines to what the slice code did, with no indirect call and no copy,
+//! which keeps local files as fast as before the migration. A `&dyn Storage`
+//! still works (`S = dyn Storage`), and a remote backend pays one indirect
+//! call per structure read.
+//!
+//! The trait is synchronous and `no_std`: parsing is CPU work, and a remote
+//! backend bridges to its own I/O.
+
+#[cfg(not(feature = "std"))]
+use alloc::{borrow::Cow, boxed::Box, vec::Vec};
+#[cfg(feature = "std")]
+use std::{borrow::Cow, boxed::Box, vec::Vec};
+
+use core::ops::Range;
+
+use crate::error::FormatError;
+
+/// A random-access source of file bytes.
+///
+/// Offsets are relative to the start of the HDF5 data (the superblock), like
+/// every address in the file.
+pub trait Storage {
+    /// Bytes `[offset, offset + len)`.
+    ///
+    /// The result is shorter than `len` only when the range runs past the
+    /// end of the storage (and empty when `offset` is at or past the end);
+    /// a backend that cannot serve a range returns an error instead of a
+    /// short read.
+    fn read_at(&self, offset: u64, len: usize) -> Result<Cow<'_, [u8]>, FormatError>;
+
+    /// Current length of the storage in bytes.
+    fn len(&self) -> u64;
+
+    /// Whether the storage holds no bytes.
+    fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    /// Several reads at once, in the order given. Backends that talk to a
+    /// remote store coalesce and parallelise these; the default reads them
+    /// one by one with [`Storage::read_at`].
+    fn read_ranges(&self, ranges: &[Range<u64>]) -> Result<Vec<Cow<'_, [u8]>>, FormatError> {
+        ranges
+            .iter()
+            .map(|r| {
+                let len = usize::try_from(r.end.saturating_sub(r.start)).map_err(|_| {
+                    FormatError::Overflow("read range longer than the address space".into())
+                })?;
+                self.read_at(r.start, len)
+            })
+            .collect()
+    }
+
+    /// The whole storage as one slice, when the backend has it in memory
+    /// (a `Vec`, an mmap). Hot loops use this to keep their zero-copy path;
+    /// `None` means every byte has to go through [`Storage::read_at`].
+    fn as_contiguous(&self) -> Option<&[u8]> {
+        None
+    }
+}
+
+impl Storage for [u8] {
+    #[inline]
+    fn read_at(&self, offset: u64, len: usize) -> Result<Cow<'_, [u8]>, FormatError> {
+        let n = self.len();
+        let start = usize::try_from(offset).map_or(n, |o| o.min(n));
+        let end = start.saturating_add(len).min(n);
+        Ok(Cow::Borrowed(&self[start..end]))
+    }
+
+    #[inline]
+    fn len(&self) -> u64 {
+        <[u8]>::len(self) as u64
+    }
+
+    #[inline]
+    fn as_contiguous(&self) -> Option<&[u8]> {
+        Some(self)
+    }
+}
+
+impl Storage for Vec<u8> {
+    #[inline]
+    fn read_at(&self, offset: u64, len: usize) -> Result<Cow<'_, [u8]>, FormatError> {
+        self.as_slice().read_at(offset, len)
+    }
+
+    #[inline]
+    fn len(&self) -> u64 {
+        Vec::len(self) as u64
+    }
+
+    #[inline]
+    fn as_contiguous(&self) -> Option<&[u8]> {
+        Some(self.as_slice())
+    }
+}
+
+impl<T: Storage + ?Sized> Storage for &T {
+    #[inline]
+    fn read_at(&self, offset: u64, len: usize) -> Result<Cow<'_, [u8]>, FormatError> {
+        (**self).read_at(offset, len)
+    }
+
+    #[inline]
+    fn len(&self) -> u64 {
+        (**self).len()
+    }
+
+    #[inline]
+    fn read_ranges(&self, ranges: &[Range<u64>]) -> Result<Vec<Cow<'_, [u8]>>, FormatError> {
+        (**self).read_ranges(ranges)
+    }
+
+    #[inline]
+    fn as_contiguous(&self) -> Option<&[u8]> {
+        (**self).as_contiguous()
+    }
+}
+
+impl<T: Storage + ?Sized> Storage for Box<T> {
+    #[inline]
+    fn read_at(&self, offset: u64, len: usize) -> Result<Cow<'_, [u8]>, FormatError> {
+        (**self).read_at(offset, len)
+    }
+
+    #[inline]
+    fn len(&self) -> u64 {
+        (**self).len()
+    }
+
+    #[inline]
+    fn read_ranges(&self, ranges: &[Range<u64>]) -> Result<Vec<Cow<'_, [u8]>>, FormatError> {
+        (**self).read_ranges(ranges)
+    }
+
+    #[inline]
+    fn as_contiguous(&self) -> Option<&[u8]> {
+        (**self).as_contiguous()
+    }
+}
+
+#[cfg(feature = "std")]
+impl<T: Storage + ?Sized> Storage for std::sync::Arc<T> {
+    #[inline]
+    fn read_at(&self, offset: u64, len: usize) -> Result<Cow<'_, [u8]>, FormatError> {
+        (**self).read_at(offset, len)
+    }
+
+    #[inline]
+    fn len(&self) -> u64 {
+        (**self).len()
+    }
+
+    #[inline]
+    fn read_ranges(&self, ranges: &[Range<u64>]) -> Result<Vec<Cow<'_, [u8]>>, FormatError> {
+        (**self).read_ranges(ranges)
+    }
+
+    #[inline]
+    fn as_contiguous(&self) -> Option<&[u8]> {
+        (**self).as_contiguous()
+    }
+}
+
+/// `storage.len()` as the `usize` the parsers' end-of-file errors report
+/// (saturating on targets where the file is larger than the address space).
+#[inline]
+pub(crate) fn len_usize<S: Storage + ?Sized>(file: &S) -> usize {
+    usize::try_from(file.len()).unwrap_or(usize::MAX)
+}
+
+/// Bytes `[offset, offset + len)`, all of them.
+///
+/// A range that runs past the end of the storage is
+/// [`FormatError::UnexpectedEof`] with `expected = offset + len` and
+/// `available = storage length` — the error the `&[u8]` parsers give for
+/// the same bounds check (`offset + len > file_data.len()`).
+#[inline]
+pub fn read_exact_at<S: Storage + ?Sized>(
+    file: &S,
+    offset: u64,
+    len: usize,
+) -> Result<Cow<'_, [u8]>, FormatError> {
+    let eof = || FormatError::UnexpectedEof {
+        expected: usize::try_from(offset)
+            .unwrap_or(usize::MAX)
+            .saturating_add(len),
+        available: len_usize(file),
+    };
+    // In-memory fast path: plain slicing (for `S = [u8]` this inlines to
+    // the slice code's bounds check).
+    if let Some(all) = file.as_contiguous() {
+        return usize::try_from(offset)
+            .ok()
+            .and_then(|start| all.get(start..start.checked_add(len)?))
+            .map(Cow::Borrowed)
+            .ok_or_else(eof);
+    }
+    match offset.checked_add(len as u64) {
+        Some(end) if end <= file.len() => {}
+        _ => return Err(eof()),
+    }
+    let bytes = file.read_at(offset, len)?;
+    if bytes.len() < len {
+        // The storage shrank or the backend served a short read inside the
+        // file: never parse a partial structure.
+        return Err(short_read());
+    }
+    Ok(bytes)
+}
+
+#[cold]
+#[inline(never)]
+fn short_read() -> FormatError {
+    FormatError::Storage(
+        "short read inside the file (the storage shrank or the backend failed)".into(),
+    )
+}
+
+/// Largest paged data block (fixed or extensible array) read in one piece.
+/// A bigger one is read as its prefix and then page by page, only the pages
+/// in use, so a block whose size fields claim more than the file holds
+/// costs no more than the pages it really has.
+pub(crate) const PAGED_BLOCK_ONE_READ_MAX: usize = 1 << 20;
+
+/// A window of the file: up to `max` bytes read at `base`, fewer only at
+/// the end of the file. Its [`Window::ensure`] reports a bounds failure
+/// exactly as the whole-file check `ensure_len(file_data, base + rel, n)`
+/// did — with the absolute position and the file's length — as long as
+/// every position checked lies within the `max` bytes the window was asked
+/// for: then a position past the window is past the end of the file.
+pub(crate) struct Window<'a> {
+    /// The bytes, from `base` on.
+    pub bytes: Cow<'a, [u8]>,
+    base: usize,
+    file_len: usize,
+}
+
+impl<'a> Window<'a> {
+    /// Read up to `max` bytes at `base`.
+    pub fn read<S: Storage + ?Sized>(
+        file: &'a S,
+        base: u64,
+        max: usize,
+    ) -> Result<Self, FormatError> {
+        Ok(Window {
+            bytes: read_upto(file, base, max)?,
+            base: usize::try_from(base).unwrap_or(usize::MAX),
+            file_len: len_usize(file),
+        })
+    }
+
+    /// A whole in-memory file as one window (base 0).
+    #[cfg(test)]
+    pub fn whole(bytes: &'a [u8]) -> Self {
+        Window {
+            bytes: Cow::Borrowed(bytes),
+            base: 0,
+            file_len: bytes.len(),
+        }
+    }
+
+    /// [`Window::ensure`] for a window at `base` that has not been read:
+    /// whether `[rel, rel + needed)` lies in the file, with the same error.
+    /// Lets a parser whose first step is to check a structure's whole extent
+    /// (a checksum at its end) fail before reading a structure that a
+    /// hostile size field has stretched past the end of the file.
+    pub fn check_extent<S: Storage + ?Sized>(
+        file: &S,
+        base: u64,
+        rel: usize,
+        needed: usize,
+    ) -> Result<(), FormatError> {
+        let base = usize::try_from(base).unwrap_or(usize::MAX);
+        let file_len = len_usize(file);
+        match base.checked_add(rel).and_then(|p| p.checked_add(needed)) {
+            Some(end) if end <= file_len => Ok(()),
+            _ => Err(FormatError::UnexpectedEof {
+                expected: base.saturating_add(rel).saturating_add(needed),
+                available: file_len,
+            }),
+        }
+    }
+
+    /// Check that `[rel, rel + needed)` (relative to `base`) is in the file.
+    #[inline]
+    pub fn ensure(&self, rel: usize, needed: usize) -> Result<(), FormatError> {
+        match rel.checked_add(needed) {
+            Some(end) if end <= self.bytes.len() => Ok(()),
+            _ => Err(FormatError::UnexpectedEof {
+                expected: self.base.saturating_add(rel).saturating_add(needed),
+                available: self.file_len,
+            }),
+        }
+    }
+}
+
+/// Up to `max` bytes from `offset` on: fewer only at the end of the
+/// storage. For structures whose size is only known once their prefix has
+/// been parsed and whose parsers bound-check what they are given.
+#[inline]
+pub fn read_upto<S: Storage + ?Sized>(
+    file: &S,
+    offset: u64,
+    max: usize,
+) -> Result<Cow<'_, [u8]>, FormatError> {
+    if let Some(all) = file.as_contiguous() {
+        let start = usize::try_from(offset).map_or(all.len(), |o| o.min(all.len()));
+        let end = start.saturating_add(max).min(all.len());
+        return Ok(Cow::Borrowed(&all[start..end]));
+    }
+    let avail = file.len().saturating_sub(offset);
+    let len = usize::try_from(avail).map_or(max, |a| a.min(max));
+    let bytes = file.read_at(offset, len)?;
+    if bytes.len() < len {
+        return Err(short_read());
+    }
+    Ok(bytes)
+}
+
+/// Borrow the whole file for a code path that has not been converted to
+/// [`Storage`] yet. On a backend without a contiguous view this is the
+/// clean [`FormatError::ContiguousStorageRequired`] error, never a guess.
+#[inline]
+pub fn require_contiguous<'a, S: Storage + ?Sized>(
+    file: &'a S,
+    what: &'static str,
+) -> Result<&'a [u8], FormatError> {
+    file.as_contiguous()
+        .ok_or(FormatError::ContiguousStorageRequired(what))
+}
+
+/// A [`Storage`] over an in-memory buffer that serves every byte through
+/// [`Storage::read_at`] (its [`Storage::as_contiguous`] is `None`, so no
+/// parser can take the whole-slice shortcut), copies what it serves (as a
+/// remote backend would), and counts the reads and bytes.
+///
+/// It is the equivalence harness of the range-read migration: parsing a
+/// file through it must give exactly what parsing the `&[u8]` gives, and
+/// the counters are the request counts a cacheless range reader would make.
+#[derive(Debug)]
+pub struct CountingStorage {
+    data: Vec<u8>,
+    reads: portable_atomic::AtomicU64,
+    bytes: portable_atomic::AtomicU64,
+}
+
+impl CountingStorage {
+    /// Serve `data` (the file from the superblock on).
+    pub fn new(data: Vec<u8>) -> Self {
+        CountingStorage {
+            data,
+            reads: portable_atomic::AtomicU64::new(0),
+            bytes: portable_atomic::AtomicU64::new(0),
+        }
+    }
+
+    /// Number of `read_at` calls served so far.
+    pub fn reads(&self) -> u64 {
+        self.reads.load(portable_atomic::Ordering::Relaxed)
+    }
+
+    /// Number of bytes served so far.
+    pub fn bytes_read(&self) -> u64 {
+        self.bytes.load(portable_atomic::Ordering::Relaxed)
+    }
+
+    /// Reset both counters.
+    pub fn reset(&self) {
+        self.reads.store(0, portable_atomic::Ordering::Relaxed);
+        self.bytes.store(0, portable_atomic::Ordering::Relaxed);
+    }
+}
+
+impl Storage for CountingStorage {
+    fn read_at(&self, offset: u64, len: usize) -> Result<Cow<'_, [u8]>, FormatError> {
+        let got = self.data.as_slice().read_at(offset, len)?;
+        self.reads.fetch_add(1, portable_atomic::Ordering::Relaxed);
+        self.bytes
+            .fetch_add(got.len() as u64, portable_atomic::Ordering::Relaxed);
+        Ok(Cow::Owned(got.into_owned()))
+    }
+
+    fn len(&self) -> u64 {
+        self.data.len() as u64
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn slice_reads_are_borrowed_and_clamped() {
+        let data: Vec<u8> = (0u8..10).collect();
+        let s: &[u8] = &data;
+        let dynamic: &dyn Storage = &s;
+        assert_eq!(dynamic.len(), 10);
+        let r = dynamic.read_at(2, 3).unwrap();
+        assert!(matches!(r, Cow::Borrowed(_)));
+        assert_eq!(&*r, &[2, 3, 4]);
+        assert_eq!(&*dynamic.read_at(8, 5).unwrap(), &[8, 9]);
+        assert!(dynamic.read_at(10, 5).unwrap().is_empty());
+        assert!(dynamic.read_at(u64::MAX, 5).unwrap().is_empty());
+        assert_eq!(dynamic.as_contiguous(), Some(&data[..]));
+        let v: &dyn Storage = &data;
+        assert_eq!(v.as_contiguous(), Some(&data[..]));
+    }
+
+    #[test]
+    fn read_exact_matches_slice_bounds_errors() {
+        let data = [0u8; 10];
+        let s: &[u8] = &data;
+        assert_eq!(&*read_exact_at(&s, 4, 6).unwrap(), &[0; 6]);
+        assert_eq!(
+            read_exact_at(&s, 4, 7).unwrap_err(),
+            FormatError::UnexpectedEof {
+                expected: 11,
+                available: 10
+            }
+        );
+        assert!(read_exact_at(&s, u64::MAX, 1).is_err());
+        assert_eq!(read_upto(&s, 7, 100).unwrap().len(), 3);
+        assert_eq!(read_upto(&s, 70, 100).unwrap().len(), 0);
+    }
+
+    #[test]
+    fn counting_storage_counts_and_hides_the_slice() {
+        let c = CountingStorage::new((0u8..10).collect());
+        assert!(c.as_contiguous().is_none());
+        let r = c.read_at(3, 4).unwrap();
+        assert!(matches!(r, Cow::Owned(_)));
+        assert_eq!(&*r, &[3, 4, 5, 6]);
+        c.read_at(8, 4).unwrap();
+        assert_eq!((c.reads(), c.bytes_read()), (2, 6));
+        c.reset();
+        assert_eq!((c.reads(), c.bytes_read()), (0, 0));
+    }
+
+    #[test]
+    fn read_ranges_default_loops() {
+        let data: Vec<u8> = (0u8..10).collect();
+        let s: &[u8] = &data;
+        let got = s.read_ranges(&[1..3, 5..9]).unwrap();
+        assert_eq!(&*got[0], &[1, 2]);
+        assert_eq!(&*got[1], &[5, 6, 7, 8]);
+    }
+}
diff --git a/crates/clawhdf5-format/src/superblock.rs b/crates/clawhdf5-format/src/superblock.rs
index a565321..dc21f7c 100644
--- a/crates/clawhdf5-format/src/superblock.rs
+++ b/crates/clawhdf5-format/src/superblock.rs
@@ -7,6 +7,11 @@ use byteorder::{ByteOrder, LittleEndian};
 
 use crate::error::FormatError;
 use crate::signature::HDF5_SIGNATURE;
+use crate::storage::{Storage, read_upto};
+
+/// Bytes read to parse a superblock: more than the largest one (version 1
+/// with 8-byte offsets and lengths, 100 bytes).
+const SUPERBLOCK_READ_LEN: usize = 128;
 
 /// Parsed HDF5 superblock (all versions).
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -161,7 +166,16 @@ impl Superblock {
         file_data: &[u8],
         signature_offset: usize,
     ) -> Result<u64, FormatError> {
-        let refreshed = Superblock::parse(file_data, signature_offset)?;
+        self.refresh_eof_in(file_data, signature_offset as u64)
+    }
+
+    /// [`Self::refresh_eof`] over any [`Storage`].
+    pub fn refresh_eof_in<S: Storage + ?Sized>(
+        &mut self,
+        file: &S,
+        signature_offset: u64,
+    ) -> Result<u64, FormatError> {
+        let refreshed = Superblock::parse_in(file, signature_offset)?;
         self.eof_address = refreshed.eof_address;
         self.consistency_flags = refreshed.consistency_flags;
         Ok(self.eof_address)
@@ -219,15 +233,23 @@ impl Superblock {
     /// [`FormatError::UserBlockNotStripped`] because the addresses in the
     /// returned superblock would otherwise be applied to the wrong bytes.
     pub fn parse(data: &[u8], signature_offset: usize) -> Result<Superblock, FormatError> {
+        Self::parse_in(data, signature_offset as u64)
+    }
+
+    /// [`Self::parse`] over any [`Storage`]: one read of the first
+    /// [`SUPERBLOCK_READ_LEN`] bytes (fewer when the file is shorter, which
+    /// is then refused with the same end-of-file errors as a short slice).
+    pub fn parse_in<S: Storage + ?Sized>(
+        file: &S,
+        signature_offset: u64,
+    ) -> Result<Superblock, FormatError> {
         if signature_offset != 0 {
-            return Err(FormatError::UserBlockNotStripped(signature_offset as u64));
+            return Err(FormatError::UserBlockNotStripped(signature_offset));
         }
-        let d = data
-            .get(signature_offset..)
-            .ok_or(FormatError::UnexpectedEof {
-                expected: signature_offset + 1,
-                available: data.len(),
-            })?;
+        // Every bounds check below needs at most 100 bytes, so on a longer
+        // file none of them can fail and the window's length does not show.
+        let window = read_upto(file, 0, SUPERBLOCK_READ_LEN)?;
+        let d: &[u8] = &window;
         ensure_len(d, 9)?; // signature(8) + version(1)
 
         // Verify signature
@@ -894,4 +916,34 @@ mod tests {
         assert_eq!(parsed.version, 3);
         assert_eq!(parsed.page_size, None);
     }
+
+    /// Through a storage that serves only `read_at`, every version parses
+    /// to the same superblock, and every truncation to the same error, as
+    /// from a slice — in one read.
+    #[test]
+    fn parse_in_matches_slice_parse() {
+        use crate::storage::CountingStorage;
+        let mut files = vec![
+            build_v0_bytes(8),
+            build_v0_bytes(4),
+            build_v1_bytes(8),
+            build_v1_bytes(4),
+            build_v2_bytes(8, 2),
+            build_v2_bytes(4, 3),
+        ];
+        for f in files.clone() {
+            let mut long = f.clone();
+            long.resize(4096, 0xAB);
+            files.push(long);
+            for cut in [0, 5, 9, 13, 20, 30, f.len() - 1] {
+                files.push(f[..cut.min(f.len())].to_vec());
+            }
+        }
+        for f in files {
+            let want = Superblock::parse(&f, 0);
+            let storage = CountingStorage::new(f.clone());
+            assert_eq!(Superblock::parse_in(&storage, 0), want, "{} bytes", f.len());
+            assert_eq!(storage.reads(), 1);
+        }
+    }
 }
diff --git a/crates/clawhdf5-format/src/superblock_ext.rs b/crates/clawhdf5-format/src/superblock_ext.rs
index e1d1cf3..4a66473 100644
--- a/crates/clawhdf5-format/src/superblock_ext.rs
+++ b/crates/clawhdf5-format/src/superblock_ext.rs
@@ -21,13 +21,14 @@
 //! file is never copied whole.
 
 #[cfg(not(feature = "std"))]
-use alloc::{collections::BTreeSet, vec::Vec};
+use alloc::{borrow::Cow, collections::BTreeSet, vec::Vec};
 #[cfg(feature = "std")]
-use std::collections::BTreeSet;
+use std::{borrow::Cow, collections::BTreeSet};
 
 use crate::error::FormatError;
 use crate::message_type::MessageType;
 use crate::object_header::ObjectHeader;
+use crate::storage::{Storage, read_exact_at};
 use crate::superblock::Superblock;
 
 /// Message type of the File Space Info message.
@@ -169,6 +170,15 @@ impl<'a> Cursor<'a> {
 pub fn read_superblock_extension(
     data: &[u8],
     sb: &Superblock,
+) -> Result<Option<SuperblockExtension>, FormatError> {
+    read_superblock_extension_in(data, sb)
+}
+
+/// [`read_superblock_extension`] over any [`Storage`]; its length is the
+/// end of file.
+pub fn read_superblock_extension_in<S: Storage + ?Sized>(
+    file: &S,
+    sb: &Superblock,
 ) -> Result<Option<SuperblockExtension>, FormatError> {
     let os = sb.offset_size;
     let ls = sb.length_size;
@@ -181,8 +191,8 @@ pub fn read_superblock_extension(
         return Ok(None);
     };
     let addr = usize::try_from(addr).map_err(|_| ext_err("address out of range"))?;
-    let header = ObjectHeader::parse(data, addr, os, ls)?;
-    let eoa = data.len() as u64;
+    let header = ObjectHeader::parse_in(file, addr as u64, os, ls)?;
+    let eoa = file.len();
 
     let mut ext = SuperblockExtension::default();
     for msg in &header.messages {
@@ -340,12 +350,21 @@ impl CacheImage {
         data: &[u8],
         location: CacheImageLocation,
         sb: &Superblock,
+    ) -> Result<Self, FormatError> {
+        Self::decode_in(data, location, sb)
+    }
+
+    /// [`Self::decode`] over any [`Storage`]: one read of the image block.
+    pub fn decode_in<S: Storage + ?Sized>(
+        file: &S,
+        location: CacheImageLocation,
+        sb: &Superblock,
     ) -> Result<Self, FormatError> {
         let (offset_size, length_size) = (sb.offset_size, sb.length_size);
         let bad = FormatError::InvalidCacheImage;
-        let block = image_block(data, location)?;
-        let eoa = data.len() as u64;
-        let mut c = Cursor::new(block, bad(RAN_OFF));
+        let block = image_block_in(file, location)?;
+        let eoa = file.len();
+        let mut c = Cursor::new(&block, bad(RAN_OFF));
 
         // Header: signature, version, flags, image data length, entry count.
         if c.take(4)? != MDCI_SIGNATURE {
@@ -463,6 +482,14 @@ impl CacheImage {
         image_block(data, self.location)
     }
 
+    /// [`Self::block`] over any [`Storage`].
+    pub fn block_in<'a, S: Storage + ?Sized>(
+        &self,
+        file: &'a S,
+    ) -> Result<Cow<'a, [u8]>, FormatError> {
+        image_block_in(file, self.location)
+    }
+
     /// Write every entry over `dst`, the file's bytes from the superblock
     /// on (as long as the `data` the image was decoded from), taking the
     /// entries from `block` (the image block, see [`Self::block`]). `block`
@@ -483,13 +510,32 @@ impl CacheImage {
 }
 
 fn image_block(data: &[u8], location: CacheImageLocation) -> Result<&[u8], FormatError> {
+    let (start, len) = image_block_range(data.len() as u64, location)?;
+    let start = crate::addr::to_usize(start)?;
+    Ok(&data[start..start + len])
+}
+
+fn image_block_in<S: Storage + ?Sized>(
+    file: &S,
+    location: CacheImageLocation,
+) -> Result<Cow<'_, [u8]>, FormatError> {
+    let (start, len) = image_block_range(file.len(), location)?;
+    read_exact_at(file, start, len)
+}
+
+/// Where the image block is, checked against a file of `file_len` bytes.
+fn image_block_range(
+    file_len: u64,
+    location: CacheImageLocation,
+) -> Result<(u64, usize), FormatError> {
     let bad = FormatError::InvalidCacheImage;
     let start = usize::try_from(location.address).map_err(|_| bad("address out of range"))?;
     let len = usize::try_from(location.length).map_err(|_| bad("length out of range"))?;
     start
         .checked_add(len)
-        .and_then(|end| data.get(start..end))
-        .ok_or(bad("image block extends past the end of the file"))
+        .filter(|&end| end as u64 <= file_len)
+        .ok_or(bad("image block extends past the end of the file"))?;
+    Ok((start as u64, len))
 }
 
 /// What an opener must do before reading a file's metadata: check the
@@ -498,11 +544,19 @@ fn image_block(data: &[u8], location: CacheImageLocation) -> Result<&[u8], Forma
 /// ([`CacheImage::decode`]). `data` is the file from the superblock on, up
 /// to its recorded end of file.
 pub fn cache_image_state(data: &[u8], sb: &Superblock) -> Result<CacheImageState, FormatError> {
-    match read_superblock_extension(data, sb)? {
+    cache_image_state_in(data, sb)
+}
+
+/// [`cache_image_state`] over any [`Storage`].
+pub fn cache_image_state_in<S: Storage + ?Sized>(
+    file: &S,
+    sb: &Superblock,
+) -> Result<CacheImageState, FormatError> {
+    match read_superblock_extension_in(file, sb)? {
         Some(SuperblockExtension {
             cache_image: Some(location),
             ..
-        }) => Ok(match CacheImage::decode(data, location, sb) {
+        }) => Ok(match CacheImage::decode_in(file, location, sb) {
             Ok(image) => CacheImageState::Loaded(image),
             Err(e) => CacheImageState::Unloadable(e),
         }),
@@ -566,7 +620,7 @@ mod tests {
     /// holds the given messages, padded to `len` bytes.
     fn file_with_ext(messages: &[(u16, &[u8])], len: usize) -> Vec<u8> {
         let mut body = Vec::new();
-        for (t, d) in messages {
+        for &(t, d) in messages {
             let padded = d.len().div_ceil(8) * 8;
             body.extend_from_slice(&t.to_le_bytes());
             body.extend_from_slice(&(padded as u16).to_le_bytes());
@@ -809,4 +863,43 @@ mod tests {
         // An entry cannot be its own parent.
         assert!(load(image_with_deps(&[(40, b"C", 1, Some(40))])).is_err());
     }
+
+    /// The extension and cache image decode identically through a
+    /// `read_at`-only storage, errors included.
+    #[test]
+    fn storage_parse_matches_slice_parse() {
+        use crate::storage::CountingStorage;
+        let img = image(&[(16, b"HEADER"), (40, b"NODE")]);
+        let mut with_image = file_with_ext(&[(MSG_MDCI, &mdci(256, img.len() as u64))], 256);
+        with_image.extend_from_slice(&img);
+        let mut bad_image = with_image.clone();
+        bad_image[256] = b'X';
+        let files = [
+            file_with_ext(&[(MSG_FSINFO, &fsinfo_v1(4096, false, 0))], 256),
+            file_with_ext(&[(MSG_FSINFO, &fsinfo_v1(256, false, 0))], 256),
+            file_with_ext(&[(MSG_MDCI, &mdci(128, 64))], 192),
+            file_with_ext(&[(MSG_MDCI, &mdci(0x10100, 0x1000_0000))], 2565),
+            file_with_ext(&[(MSG_FSINFO, &fsinfo_v1(4096, true, 12))], 60),
+            with_image,
+            bad_image,
+        ];
+        for f in files {
+            let storage = CountingStorage::new(f.clone());
+            let sb = sb_v2(48);
+            assert_eq!(
+                read_superblock_extension_in(&storage, &sb),
+                read_superblock_extension(&f, &sb)
+            );
+            assert_eq!(
+                cache_image_state_in(&storage, &sb),
+                cache_image_state(&f, &sb)
+            );
+            if let Ok(CacheImageState::Loaded(image)) = cache_image_state(&f, &sb) {
+                assert_eq!(
+                    &*image.block_in(&storage).unwrap(),
+                    image.block(&f).unwrap()
+                );
+            }
+        }
+    }
 }
diff --git a/crates/clawhdf5-format/src/symbol_table.rs b/crates/clawhdf5-format/src/symbol_table.rs
index 9809c75..467f43b 100644
--- a/crates/clawhdf5-format/src/symbol_table.rs
+++ b/crates/clawhdf5-format/src/symbol_table.rs
@@ -4,6 +4,7 @@
 use alloc::vec::Vec;
 
 use crate::error::FormatError;
+use crate::storage::{Storage, read_exact_at};
 
 /// Symbol Table message (type 0x0011) found in v1 group object headers.
 #[derive(Debug, Clone, PartialEq)]
@@ -79,65 +80,49 @@ impl SymbolTableNode {
         offset: usize,
         offset_size: u8,
     ) -> Result<SymbolTableNode, FormatError> {
-        // signature(4) + version(1) + reserved(1) + number_of_symbols(2) = 8
-        if offset
-            .checked_add(8)
-            .is_none_or(|end| end > file_data.len())
-        {
-            return Err(FormatError::UnexpectedEof {
-                expected: offset.saturating_add(8),
-                available: file_data.len(),
-            });
-        }
+        Self::parse_in(file_data, offset as u64, offset_size)
+    }
 
-        if &file_data[offset..offset + 4] != b"SNOD" {
+    /// [`Self::parse`] over any [`Storage`]: one read of the node's header,
+    /// one of its entries.
+    pub fn parse_in<S: Storage + ?Sized>(
+        file: &S,
+        offset: u64,
+        offset_size: u8,
+    ) -> Result<SymbolTableNode, FormatError> {
+        // signature(4) + version(1) + reserved(1) + number_of_symbols(2) = 8
+        let header = read_exact_at(file, offset, 8)?;
+
+        if &header[..4] != b"SNOD" {
             return Err(FormatError::InvalidSymbolTableNodeSignature);
         }
 
-        let version = file_data[offset + 4];
+        let version = header[4];
         if version != 1 {
             return Err(FormatError::InvalidSymbolTableNodeVersion(version));
         }
 
-        let num_symbols =
-            u16::from_le_bytes([file_data[offset + 6], file_data[offset + 7]]) as usize;
+        let num_symbols = u16::from_le_bytes([header[6], header[7]]) as usize;
 
         let os = offset_size as usize;
         // Each entry: link_name_offset(os) + obj_hdr_addr(os) + cache_type(4) + reserved(4) + scratch(16)
         let entry_size = os + os + 4 + 4 + 16;
-        let entries_start = offset + 8;
-        let needed = entries_start.checked_add(num_symbols * entry_size).ok_or(
-            FormatError::UnexpectedEof {
-                expected: usize::MAX,
-                available: file_data.len(),
-            },
-        )?;
-        if needed > file_data.len() {
-            return Err(FormatError::UnexpectedEof {
-                expected: needed,
-                available: file_data.len(),
-            });
-        }
+        // `offset + 8` fits: the header's read checked it. The entries'
+        // read is the bounds check (`offset + 8 + entries > file length`,
+        // which cannot overflow: at most 65535 entries of 40 bytes).
+        let body = read_exact_at(file, offset + 8, num_symbols * entry_size)?;
+        let file_data: &[u8] = &body;
 
         let mut entries = Vec::with_capacity(num_symbols);
-        let mut pos = entries_start;
-        for _ in 0..num_symbols {
-            let link_name_offset = read_offset(file_data, pos, offset_size)?;
-            pos += os;
-            let object_header_address = read_offset(file_data, pos, offset_size)?;
-            pos += os;
-            let cache_type = u32::from_le_bytes([
-                file_data[pos],
-                file_data[pos + 1],
-                file_data[pos + 2],
-                file_data[pos + 3],
-            ]);
-            pos += 4;
+        for entry in file_data.chunks_exact(entry_size) {
+            let link_name_offset = read_offset(entry, 0, offset_size)?;
+            let object_header_address = read_offset(entry, os, offset_size)?;
+            let pos = 2 * os;
+            let cache_type =
+                u32::from_le_bytes([entry[pos], entry[pos + 1], entry[pos + 2], entry[pos + 3]]);
             // reserved 4 bytes
-            pos += 4;
             let mut scratch_pad = [0u8; 16];
-            scratch_pad.copy_from_slice(&file_data[pos..pos + 16]);
-            pos += 16;
+            scratch_pad.copy_from_slice(&entry[pos + 8..pos + 24]);
 
             entries.push(SymbolTableEntry {
                 link_name_offset,
@@ -256,4 +241,28 @@ mod tests {
         let result = SymbolTableNode::parse(&data, usize::MAX / 2, 8);
         assert!(result.is_err());
     }
+
+    /// Nodes, cut at every length and at an offset, parse identically
+    /// through a `read_at`-only storage.
+    #[test]
+    fn storage_parse_matches_slice_parse() {
+        use crate::storage::CountingStorage;
+        for os in [4u8, 8] {
+            let node = build_snod(&[(0, 0x100, 0), (8, 0x200, 1), (16, 0x300, 2)], os);
+            let mut bad = node.clone();
+            bad[4] = 2;
+            for full in [node, bad] {
+                for at in [0usize, 7] {
+                    for cut in 0..=full.len() {
+                        let mut f = vec![0u8; at];
+                        f.extend_from_slice(&full[..cut]);
+                        let storage = CountingStorage::new(f.clone());
+                        let want = SymbolTableNode::parse(&f, at, os);
+                        let got = SymbolTableNode::parse_in(&storage, at as u64, os);
+                        assert_eq!(format!("{got:?}"), format!("{want:?}"));
+                    }
+                }
+            }
+        }
+    }
 }
diff --git a/crates/clawhdf5-format/tests/fixtures/v1_groups_400.h5 b/crates/clawhdf5-format/tests/fixtures/v1_groups_400.h5
new file mode 100644
index 0000000..5237559
Binary files /dev/null and b/crates/clawhdf5-format/tests/fixtures/v1_groups_400.h5 differ
diff --git a/crates/clawhdf5-format/tests/storage_equivalence.rs b/crates/clawhdf5-format/tests/storage_equivalence.rs
new file mode 100644
index 0000000..5cc71b9
--- /dev/null
+++ b/crates/clawhdf5-format/tests/storage_equivalence.rs
@@ -0,0 +1,701 @@
+//! Equivalence harness for the range-read migration
+//! (`docs/design/range-reads.md`, milestone M1).
+//!
+//! Every metadata parser converted to [`Storage`] must give exactly what its
+//! `&[u8]` form gives. This walks real files — the fixtures, files h5py
+//! writes to exercise the less common structures, and optionally the
+//! conformance corpus — and, for every object, runs each converted parser
+//! twice: over the file as a slice, and over a [`CountingStorage`] that
+//! serves the same bytes through `read_at` only (`as_contiguous()` is
+//! `None`, so no parser can fall back to the whole slice). The results must
+//! be identical, value for value and error for error.
+//!
+//! The one allowed difference is [`FormatError::ContiguousStorageRequired`]
+//! from the storage path, and only from the structures still indexed by a v2
+//! B-tree (dense attributes, a SOHM B-tree index, huge fractal-heap objects;
+//! see `CONTIGUOUS_REQUIRED`), which fail cleanly instead of reading the
+//! whole file. Those are counted; the error from any other site or check
+//! fails the harness.
+//!
+//! Milestones M2/M3 extend `check_object` with the raw-data and group
+//! parsers as they are converted.
+//!
+//! - `CLAWHDF5_STORAGE_CORPUS=dir[:dir...]` adds every `.h5`/`.hdf5`/`.he5`/
+//!   `.nc`/`.h5ad` file under those directories (the conformance corpus is
+//!   `conformance/.cache/corpus`); `CLAWHDF5_STORAGE_REPORT=1` prints the
+//!   per-file read counts.
+//! - The h5py-written files honour `CLAWHDF5_PYTHON` and
+//!   `CLAWHDF5_REQUIRE_INTEROP` like the facade's interop tests.
+
+use std::collections::{HashSet, VecDeque};
+use std::fmt::Debug;
+use std::path::{Path, PathBuf};
+use std::process::Command;
+
+use clawhdf5_format::attribute::{
+    extract_attributes_full, extract_attributes_full_in, extract_attributes_tolerant,
+    extract_attributes_tolerant_in,
+};
+use clawhdf5_format::attribute_info::AttributeInfoMessage;
+use clawhdf5_format::btree_v1::{collect_symbol_table_nodes, collect_symbol_table_nodes_in};
+use clawhdf5_format::btree_v2::{BTreeV2Header, collect_btree_v2_records};
+use clawhdf5_format::data_layout::DataLayout;
+use clawhdf5_format::dataspace::Dataspace;
+use clawhdf5_format::datatype::Datatype;
+use clawhdf5_format::error::FormatError;
+use clawhdf5_format::extensible_array::{
+    ExtensibleArrayHeader, read_extensible_array_chunks, read_extensible_array_chunks_in,
+};
+use clawhdf5_format::fill_value::{dataset_fill_value_from_storage, dataset_fill_value_in};
+use clawhdf5_format::fixed_array::{
+    FixedArrayHeader, read_fixed_array_chunks, read_fixed_array_chunks_in,
+};
+use clawhdf5_format::fractal_heap::FractalHeapHeader;
+use clawhdf5_format::link_info::LinkInfoMessage;
+use clawhdf5_format::local_heap::LocalHeap;
+use clawhdf5_format::message_type::MessageType;
+use clawhdf5_format::object_header::ObjectHeader;
+use clawhdf5_format::shared_message::{
+    self, load_sohm_table, load_sohm_table_in, message_data_with_sohm, message_data_with_sohm_in,
+    parse_sohm_btree_entries, parse_sohm_btree_entries_in, parse_sohm_list, parse_sohm_list_in,
+};
+use clawhdf5_format::signature::split_user_block;
+use clawhdf5_format::storage::{CountingStorage, Storage};
+use clawhdf5_format::superblock::Superblock;
+use clawhdf5_format::superblock_ext::{
+    cache_image_state, cache_image_state_in, read_superblock_extension,
+    read_superblock_extension_in,
+};
+use clawhdf5_format::symbol_table::{SymbolTableMessage, SymbolTableNode};
+
+/// Objects visited per file, heap objects read per heap: enough to cover
+/// every structure kind while keeping a 35 000-group file fast.
+const MAX_OBJECTS: usize = 1500;
+const MAX_HEAP_IDS: usize = 200;
+
+/// The structures that still need the whole file in memory, because they
+/// are found through a version-2 B-tree (not converted yet), and the checks
+/// that can reach each of them. Anything else answering
+/// [`FormatError::ContiguousStorageRequired`] is a converted parser falling
+/// back to the whole file, and fails the harness.
+const CONTIGUOUS_REQUIRED: &[(&str, &[&str])] = &[
+    (
+        "dense attribute storage (a v2 B-tree)",
+        &["attributes", "attributes (tolerant)"],
+    ),
+    (
+        "a shared-message B-tree index",
+        &[
+            "SOHM B-tree",
+            "shared message",
+            "fill value",
+            "attributes",
+            "attributes (tolerant)",
+        ],
+    ),
+    (
+        "a huge fractal-heap object's B-tree",
+        &["heap object", "attributes", "attributes (tolerant)"],
+    ),
+];
+
+fn may_require_contiguous(check: &str, site: &str) -> bool {
+    CONTIGUOUS_REQUIRED
+        .iter()
+        .any(|(s, checks)| *s == site && checks.contains(&check))
+}
+
+#[derive(Default, Debug)]
+struct Tally {
+    files: usize,
+    objects: usize,
+    checks: usize,
+    contiguous_required: usize,
+    reads: u64,
+    bytes: u64,
+    /// Chunk indexes (fixed and extensible arrays) read, and the most bytes
+    /// one of them took through the storage.
+    chunk_indexes: usize,
+    max_chunk_index_bytes: u64,
+}
+
+struct Walk<'a> {
+    slice: &'a [u8],
+    storage: &'a CountingStorage,
+    name: String,
+    tally: &'a mut Tally,
+}
+
+impl Walk<'_> {
+    /// The storage result must equal the slice result, or be the clean
+    /// "needs the whole file" error.
+    fn same<T: Debug>(
+        &mut self,
+        what: &str,
+        want: &Result<T, FormatError>,
+        got: &Result<T, FormatError>,
+    ) {
+        self.tally.checks += 1;
+        if let Err(FormatError::ContiguousStorageRequired(site)) = got {
+            assert!(
+                may_require_contiguous(what, site),
+                "{}: {what} fell back to the whole file ({site}), which only the \
+                 v2-B-tree-indexed structures may do",
+                self.name
+            );
+            self.tally.contiguous_required += 1;
+            return;
+        }
+        let (w, g) = (format!("{want:?}"), format!("{got:?}"));
+        assert!(
+            w == g,
+            "{}: {what} differs\n slice:   {}\n storage: {}",
+            self.name,
+            &w[..w.len().min(600)],
+            &g[..g.len().min(600)]
+        );
+    }
+
+    fn index_read(&mut self, bytes_before: u64) {
+        self.tally.chunk_indexes += 1;
+        let bytes = self.storage.bytes_read() - bytes_before;
+        self.tally.max_chunk_index_bytes = self.tally.max_chunk_index_bytes.max(bytes);
+    }
+
+    fn st(&self) -> &dyn Storage {
+        self.storage
+    }
+
+    fn run(&mut self) {
+        let slice = self.slice;
+        let sb = Superblock::parse(slice, 0);
+        self.same("superblock", &sb, &Superblock::parse_in(self.st(), 0));
+        let Ok(sb) = sb else { return };
+        let (os, ls) = (sb.offset_size, sb.length_size);
+
+        let want = read_superblock_extension(slice, &sb);
+        self.same(
+            "superblock extension",
+            &want,
+            &read_superblock_extension_in(self.st(), &sb),
+        );
+        let want = cache_image_state(slice, &sb);
+        self.same("cache image", &want, &cache_image_state_in(self.st(), &sb));
+        let table = load_sohm_table(slice, os, ls);
+        self.same("SOHM table", &table, &load_sohm_table_in(self.st(), os, ls));
+        if let Ok(Some(table)) = &table {
+            for idx in &table.indexes {
+                if idx.index_type == 0 {
+                    let want =
+                        parse_sohm_list(slice, idx.index_addr as usize, idx.num_messages, os);
+                    let got = parse_sohm_list_in(self.st(), idx.index_addr, idx.num_messages, os);
+                    self.same("SOHM list", &want, &got);
+                } else {
+                    let want = parse_sohm_btree_entries(slice, idx.index_addr as usize, os, ls);
+                    let got = parse_sohm_btree_entries_in(self.st(), idx.index_addr, os, ls);
+                    self.same("SOHM B-tree", &want, &got);
+                }
+            }
+        }
+
+        let mut seen = HashSet::new();
+        let mut queue = VecDeque::from([sb.root_group_address]);
+        while let Some(addr) = queue.pop_front() {
+            if seen.len() >= MAX_OBJECTS || !seen.insert(addr) {
+                continue;
+            }
+            self.tally.objects += 1;
+            self.check_object(&sb, addr);
+            // Traversal only (group lookups are milestone M0/M3 work).
+            if let Ok(children) =
+                clawhdf5_format::group_v2::resolve_group_children(slice, &sb, addr)
+            {
+                queue.extend(children.iter().map(|c| c.object_header_address));
+            }
+        }
+    }
+
+    fn check_object(&mut self, sb: &Superblock, addr: u64) {
+        let slice = self.slice;
+        let (os, ls) = (sb.offset_size, sb.length_size);
+        let header = ObjectHeader::parse(slice, addr as usize, os, ls);
+        self.same(
+            "object header",
+            &header,
+            &ObjectHeader::parse_in(self.st(), addr, os, ls),
+        );
+        let Ok(header) = header else { return };
+
+        let want = extract_attributes_full(slice, &header, os, ls);
+        self.same(
+            "attributes",
+            &want,
+            &extract_attributes_full_in(self.st(), &header, os, ls),
+        );
+        let want = extract_attributes_tolerant(slice, &header, os, ls);
+        let got = extract_attributes_tolerant_in(self.st(), &header, os, ls);
+        self.same("attributes (tolerant)", &want, &got);
+        let want = dataset_fill_value_in(slice, &header.messages, os, ls);
+        self.same(
+            "fill value",
+            &want,
+            &dataset_fill_value_from_storage(self.st(), &header.messages, os, ls),
+        );
+
+        for msg in &header.messages {
+            if shared_message::is_shared(msg.flags) {
+                let want = message_data_with_sohm(slice, msg, os, ls);
+                let got = message_data_with_sohm_in(self.st(), msg, os, ls);
+                self.same("shared message", &want, &got);
+            }
+            match msg.msg_type {
+                MessageType::SymbolTable => {
+                    if let Ok(stm) = SymbolTableMessage::parse(&msg.data, os) {
+                        self.check_v1_group(&stm, os, ls);
+                    }
+                }
+                MessageType::LinkInfo => {
+                    if let Ok(li) = LinkInfoMessage::parse(&msg.data, os) {
+                        self.check_heap(
+                            li.fractal_heap_address,
+                            li.btree_name_index_address,
+                            4,
+                            os,
+                            ls,
+                        );
+                    }
+                }
+                MessageType::AttributeInfo => {
+                    if let Ok(ai) = AttributeInfoMessage::parse(&msg.data, os) {
+                        self.check_heap(
+                            ai.fractal_heap_address,
+                            ai.btree_name_index_address,
+                            0,
+                            os,
+                            ls,
+                        );
+                    }
+                }
+                _ => {}
+            }
+        }
+        self.check_layout(&header, os, ls);
+    }
+
+    /// A symbol-table group: its local heap, B-tree, nodes and names.
+    fn check_v1_group(&mut self, stm: &SymbolTableMessage, os: u8, ls: u8) {
+        let slice = self.slice;
+        let heap = LocalHeap::parse(slice, stm.local_heap_address as usize, os, ls);
+        self.same(
+            "local heap",
+            &heap,
+            &LocalHeap::parse_in(self.st(), stm.local_heap_address, os, ls),
+        );
+        let nodes = collect_symbol_table_nodes(slice, stm.btree_address, os, ls);
+        let got = collect_symbol_table_nodes_in(self.st(), stm.btree_address, os, ls);
+        self.same("group B-tree", &nodes, &got);
+        let Ok(heap) = heap else { return };
+        let want = heap.validate_free_list(slice, ls);
+        self.same(
+            "local heap free list",
+            &want,
+            &heap.validate_free_list_in(self.st(), ls),
+        );
+        let Ok(nodes) = nodes else { return };
+        for &node in nodes.iter().take(MAX_HEAP_IDS) {
+            let snod = SymbolTableNode::parse(slice, node as usize, os);
+            self.same(
+                "symbol table node",
+                &snod,
+                &SymbolTableNode::parse_in(self.st(), node, os),
+            );
+            let Ok(snod) = snod else { continue };
+            for e in &snod.entries {
+                let want = heap.read_string(slice, e.link_name_offset);
+                self.same(
+                    "link name",
+                    &want,
+                    &heap.read_string_in(self.st(), e.link_name_offset),
+                );
+            }
+        }
+    }
+
+    /// A dense group's or dense attributes' fractal heap: the header, and
+    /// the objects its name index points at. `id_at` is where the heap ID
+    /// starts in a name-index record (after the hash for links).
+    fn check_heap(&mut self, heap: Option<u64>, index: Option<u64>, id_at: usize, os: u8, ls: u8) {
+        let slice = self.slice;
+        let Some(heap_addr) = heap else { return };
+        let fh = FractalHeapHeader::parse(slice, heap_addr as usize, os, ls);
+        self.same(
+            "fractal heap",
+            &fh,
+            &FractalHeapHeader::parse_in(self.st(), heap_addr, os, ls),
+        );
+        let (Ok(fh), Some(index)) = (fh, index) else {
+            return;
+        };
+        let Ok(bt) = BTreeV2Header::parse(slice, index as usize, os, ls) else {
+            return;
+        };
+        let Ok(records) = collect_btree_v2_records(slice, &bt, os, ls) else {
+            return;
+        };
+        let id_len = fh.heap_id_length as usize;
+        for rec in records.iter().take(MAX_HEAP_IDS) {
+            let Some(id) = rec.data.get(id_at..id_at + id_len) else {
+                continue;
+            };
+            let want = fh.read_managed_object(slice, id, os);
+            self.same(
+                "heap object",
+                &want,
+                &fh.read_managed_object_in(self.st(), id, os),
+            );
+        }
+    }
+
+    /// A dataset's layout: VDS mappings, and fixed/extensible array chunk
+    /// indexes.
+    fn check_layout(&mut self, header: &ObjectHeader, os: u8, ls: u8) {
+        let slice = self.slice;
+        let find = |t: MessageType| {
+            header
+                .messages
+                .iter()
+                .find(|m| m.msg_type == t)
+                .and_then(|m| shared_message::message_data_with_sohm(slice, m, os, ls).ok())
+        };
+        let Some(layout) = find(MessageType::DataLayout) else {
+            return;
+        };
+        let Ok(layout) = DataLayout::parse(&layout, os, ls) else {
+            return;
+        };
+        match &layout {
+            DataLayout::Virtual { .. } => {
+                let (mut want, mut got) = (layout.clone(), layout.clone());
+                let w = want.resolve_vds_mappings(slice, ls).map(|()| want);
+                let g = got.resolve_vds_mappings_in(self.st(), ls).map(|()| got);
+                self.same("VDS mappings", &w, &g);
+            }
+            DataLayout::Chunked {
+                chunk_dimensions,
+                btree_address: Some(addr),
+                version: 4,
+                chunk_index_type: Some(kind @ (3 | 4)),
+                ..
+            } => {
+                let (Some(ds), Some(dt)) =
+                    (find(MessageType::Dataspace), find(MessageType::Datatype))
+                else {
+                    return;
+                };
+                let (Ok(ds), Ok((dt, _))) = (Dataspace::parse(&ds, ls), Datatype::parse(&dt))
+                else {
+                    return;
+                };
+                let rank = ds.dimensions.len();
+                if chunk_dimensions.len() < rank {
+                    return;
+                }
+                let dims = &chunk_dimensions[..rank];
+                let max = ds.max_dimensions.as_deref();
+                let es = dt.type_size();
+                if *kind == 3 {
+                    let h = FixedArrayHeader::parse(slice, *addr as usize, os, ls);
+                    self.same(
+                        "fixed array header",
+                        &h,
+                        &FixedArrayHeader::parse_in(self.st(), *addr, os, ls),
+                    );
+                    let Ok(h) = h else { return };
+                    let want =
+                        read_fixed_array_chunks(slice, &h, &ds.dimensions, max, dims, es, os, ls);
+                    let before = self.storage.bytes_read();
+                    let got = read_fixed_array_chunks_in(
+                        self.st(),
+                        &h,
+                        &ds.dimensions,
+                        max,
+                        dims,
+                        es,
+                        os,
+                        ls,
+                    );
+                    self.index_read(before);
+                    self.same("fixed array chunks", &want, &got);
+                } else {
+                    let h = ExtensibleArrayHeader::parse(slice, *addr as usize, os, ls);
+                    let got = ExtensibleArrayHeader::parse_in(self.st(), *addr, os, ls);
+                    self.same("extensible array header", &h, &got);
+                    let Ok(h) = h else { return };
+                    let want = read_extensible_array_chunks(
+                        slice,
+                        &h,
+                        &ds.dimensions,
+                        max,
+                        dims,
+                        es,
+                        os,
+                        ls,
+                    );
+                    let before = self.storage.bytes_read();
+                    let got = read_extensible_array_chunks_in(
+                        self.st(),
+                        &h,
+                        &ds.dimensions,
+                        max,
+                        dims,
+                        es,
+                        os,
+                        ls,
+                    );
+                    self.index_read(before);
+                    self.same("extensible array chunks", &want, &got);
+                }
+            }
+            _ => {}
+        }
+    }
+}
+
+fn check_file(path: &Path, tally: &mut Tally) {
+    let Ok(bytes) = std::fs::read(path) else {
+        return;
+    };
+    check_bytes(&path.display().to_string(), &bytes, tally);
+}
+
+fn check_bytes(name: &str, bytes: &[u8], tally: &mut Tally) {
+    let Ok((_, hdf5)) = split_user_block(bytes) else {
+        return;
+    };
+    let storage = CountingStorage::new(hdf5.to_vec());
+    let before = (tally.checks, tally.objects);
+    let mut walk = Walk {
+        slice: hdf5,
+        storage: &storage,
+        name: name.to_string(),
+        tally,
+    };
+    walk.run();
+    tally.files += 1;
+    tally.reads += storage.reads();
+    tally.bytes += storage.bytes_read();
+    if std::env::var("CLAWHDF5_STORAGE_REPORT").is_ok_and(|v| v == "1") {
+        eprintln!(
+            "{:>6} objects {:>7} checks {:>8} reads {:>12} bytes  {}",
+            tally.objects - before.1,
+            tally.checks - before.0,
+            storage.reads(),
+            storage.bytes_read(),
+            name
+        );
+    }
+}
+
+fn hdf5_files(dir: &Path, out: &mut Vec<PathBuf>) {
+    let Ok(entries) = std::fs::read_dir(dir) else {
+        return;
+    };
+    for e in entries.flatten() {
+        let p = e.path();
+        if p.is_dir() {
+            hdf5_files(&p, out);
+        } else if p
+            .extension()
+            .and_then(|x| x.to_str())
+            .is_some_and(|x| matches!(x, "h5" | "hdf5" | "he5" | "nc" | "h5ad" | "hdf"))
+        {
+            out.push(p);
+        }
+    }
+}
+
+#[test]
+fn fixtures_parse_identically_through_storage() {
+    let mut files = Vec::new();
+    hdf5_files(
+        &Path::new(env!("CARGO_MANIFEST_DIR")).join("tests/fixtures"),
+        &mut files,
+    );
+    files.sort();
+    assert!(files.len() >= 40, "{} fixtures", files.len());
+    let mut tally = Tally::default();
+    for f in &files {
+        check_file(f, &mut tally);
+    }
+    eprintln!("fixtures: {tally:?}");
+    assert!(tally.objects >= 150 && tally.checks >= 1000, "{tally:?}");
+    // Reads really went through read_at.
+    assert!(tally.reads > tally.objects as u64);
+}
+
+#[test]
+fn corpus_parses_identically_through_storage() {
+    let Ok(dirs) = std::env::var("CLAWHDF5_STORAGE_CORPUS") else {
+        eprintln!("CLAWHDF5_STORAGE_CORPUS not set; skipping the corpus");
+        return;
+    };
+    let mut files = Vec::new();
+    for d in std::env::split_paths(&dirs) {
+        hdf5_files(&d, &mut files);
+    }
+    files.sort();
+    let mut tally = Tally::default();
+    for f in &files {
+        check_file(f, &mut tally);
+    }
+    eprintln!("corpus: {tally:?}");
+    assert!(tally.files > 0);
+}
+
+fn python() -> String {
+    std::env::var("CLAWHDF5_PYTHON").unwrap_or_else(|_| "python3".to_string())
+}
+
+fn interop_required() -> bool {
+    std::env::var("CLAWHDF5_REQUIRE_INTEROP").is_ok_and(|v| v == "1")
+}
+
+/// Files h5py writes to cover what the fixtures do not: extensible arrays
+/// deep enough for super blocks and paged data blocks, paged fixed arrays,
+/// big symbol-table and dense groups, dense and shared attributes, a SOHM
+/// list and a SOHM B-tree, committed datatypes, and a user block.
+const GENERATE: &str = r#"
+import ctypes, glob, os, sys, h5py, numpy as np
+out = sys.argv[1]
+def p(n): return os.path.join(out, n)
+
+with h5py.File(p('ea.h5'), 'w', libver='latest') as f:
+    # One unlimited dimension: extensible array. 3000 chunks reach super
+    # blocks; 2-element chunks of int8 keep data small.
+    d = f.create_dataset('ea', shape=(6000,), maxshape=(None,), chunks=(2,), dtype='i1')
+    d[:] = np.arange(6000) % 100
+    d2 = f.create_dataset('ea_deflate', shape=(4000, 3), maxshape=(None, 3), chunks=(2, 3),
+                          dtype='f4', compression='gzip')
+    d2[:] = np.random.default_rng(1).random((4000, 3))
+    d3 = f.create_dataset('ea_sparse', shape=(100000,), maxshape=(None,), chunks=(4,), dtype='i2')
+    d3[0:8] = 1; d3[50000:50004] = 2; d3[99996:] = 3
+    fa = f.create_dataset('fa_paged', shape=(5000,), chunks=(1,), dtype='u1')
+    fa[::3] = 7
+    fa2 = f.create_dataset('fa', shape=(40, 40), chunks=(8, 8), dtype='f8', compression='gzip')
+    fa2[:] = 1.5
+    for i in range(30):
+        f.attrs[f'a{i}'] = np.arange(i + 1)
+    g = f.create_group('dense')
+    for i in range(300):
+        g.create_group(f'child{i:04d}').attrs['i'] = i
+    f['committed'] = np.dtype([('x', 'i4'), ('y', 'f8')])
+    f.create_dataset('uses_committed', shape=(3,), dtype=f['committed'])
+    f['uses_committed'].attrs.create('ta', data=np.zeros(2, dtype=f['committed'].dtype), dtype=f['committed'])
+    f.attrs['vl'] = ['alpha', 'beta', 'gamma']
+
+with h5py.File(p('v1_groups.h5'), 'w', libver='earliest', userblock_size=512) as f:
+    for i in range(400):
+        g = f.create_group(f'g{i:04d}')
+        g.attrs['n'] = i
+    f.create_dataset('x', data=np.arange(10))
+
+# Paged chunk indexes whose data blocks are bigger than the storage reads
+# in one piece (1 MiB), with two chunks written: a fixed array of 300 000
+# chunks (a 2.4 MB data block) and an extensible array grown to 1.2e9 (its
+# last data block holds 131 072 chunks: over 1 MiB).
+with h5py.File(p('big_paged.h5'), 'w', libver='latest') as f:
+    d = f.create_dataset('fa', shape=(300000,), chunks=(1,), dtype='u1')
+    d[5] = 1; d[250000] = 2
+    e = f.create_dataset('ea', shape=(1,), maxshape=(None,), chunks=(1,), dtype='u1')
+    e.resize((1200000000,)); e[10] = 1; e[1100000000] = 3
+
+libs = glob.glob(os.path.join(os.path.dirname(h5py.__file__), '..', 'h5py.libs', 'libhdf5-*.so*'))
+if libs:
+    lib = ctypes.CDLL(libs[0])
+    lib.H5Pset_shared_mesg_nindexes.argtypes = [ctypes.c_int64, ctypes.c_uint]
+    lib.H5Pset_shared_mesg_index.argtypes = [ctypes.c_int64, ctypes.c_uint, ctypes.c_uint, ctypes.c_uint]
+    lib.H5Pset_shared_mesg_phase_change.argtypes = [ctypes.c_int64, ctypes.c_uint, ctypes.c_uint]
+    for name, list_max in [('sohm_list.h5', 50), ('sohm_btree.h5', 0)]:
+        fcpl = h5py.h5p.create(h5py.h5p.FILE_CREATE)
+        assert lib.H5Pset_shared_mesg_nindexes(fcpl.id, 1) >= 0
+        # datatype, dataspace, fill value, filter pipeline, attribute
+        assert lib.H5Pset_shared_mesg_index(fcpl.id, 0, 0x02 | 0x04 | 0x08 | 0x10 | 0x20, 1) >= 0
+        assert lib.H5Pset_shared_mesg_phase_change(fcpl.id, list_max, 0) >= 0
+        fapl = h5py.h5p.create(h5py.h5p.FILE_ACCESS)
+        fapl.set_libver_bounds(h5py.h5f.LIBVER_LATEST, h5py.h5f.LIBVER_LATEST)
+        fid = h5py.h5f.create(p(name).encode(), h5py.h5f.ACC_TRUNC, fcpl=fcpl, fapl=fapl)
+        with h5py.File(fid) as f:
+            for i in range(20):
+                d = f.create_dataset(f'd{i}', shape=(10, i + 1), dtype='f4', fillvalue=-1.0,
+                                     chunks=(5, 1), compression='gzip')
+                d.attrs['units'] = 'metres per second, a long enough string to share'
+                d.attrs['scale'] = np.arange(20, dtype='f8')
+print('ok')
+"#;
+
+#[test]
+fn h5py_files_parse_identically_through_storage() {
+    let dir = Path::new(env!("CARGO_TARGET_TMPDIR")).join("storage_equivalence");
+    let _ = std::fs::remove_dir_all(&dir);
+    std::fs::create_dir_all(&dir).unwrap();
+    let out = Command::new(python())
+        .args(["-c", GENERATE, dir.to_str().unwrap()])
+        .output();
+    match out {
+        Ok(o) if o.status.success() => {}
+        Ok(o) if interop_required() => panic!(
+            "h5py generation failed:\n{}\n{}",
+            String::from_utf8_lossy(&o.stdout),
+            String::from_utf8_lossy(&o.stderr)
+        ),
+        Err(e) if interop_required() => panic!("python not available: {e}"),
+        _ => {
+            eprintln!("python3 with h5py unavailable; skipping");
+            return;
+        }
+    }
+    let mut files = Vec::new();
+    hdf5_files(&dir, &mut files);
+    files.sort();
+    let names: Vec<_> = files
+        .iter()
+        .map(|f| f.file_name().unwrap().to_string_lossy().into_owned())
+        .collect();
+    for want in ["ea.h5", "v1_groups.h5", "big_paged.h5"] {
+        assert!(names.iter().any(|n| n == want), "{names:?}");
+    }
+    if interop_required() {
+        assert!(names.iter().any(|n| n == "sohm_btree.h5"), "{names:?}");
+    }
+    let mut tally = Tally::default();
+    for f in &files {
+        if f.ends_with("big_paged.h5") {
+            // Only the pages in use are read, not the whole data blocks:
+            // read in one piece, the fixed array took 2.4 MB and the
+            // extensible array 1.2 MB (its super block's page bitmap and
+            // block addresses are most of what remains).
+            let mut big = Tally::default();
+            check_file(f, &mut big);
+            eprintln!("big paged blocks: {big:?}");
+            assert_eq!(big.chunk_indexes, 2, "{big:?}");
+            assert!(big.max_chunk_index_bytes < 256 << 10, "{big:?}");
+            // Truncated anywhere, the page-by-page reads still agree with
+            // the slice reads (errors included).
+            let bytes = std::fs::read(f).unwrap();
+            for cut in (0..bytes.len()).step_by(bytes.len() / 97) {
+                check_bytes(
+                    &format!("big_paged.h5 cut at {cut}"),
+                    &bytes[..cut],
+                    &mut big,
+                );
+            }
+            eprintln!("big paged blocks, truncated: {big:?}");
+            assert!(big.chunk_indexes > 100, "{big:?}");
+        }
+        check_file(f, &mut tally);
+    }
+    eprintln!("h5py files: {tally:?}");
+    assert!(tally.objects >= 700, "{tally:?}");
+    // Dense attributes and the SOHM B-tree are the known clean errors.
+    assert!(tally.contiguous_required > 0, "{tally:?}");
+}
diff --git a/crates/clawhdf5-py/src/lib.rs b/crates/clawhdf5-py/src/lib.rs
index e3b5619..0d3873c 100644
--- a/crates/clawhdf5-py/src/lib.rs
+++ b/crates/clawhdf5-py/src/lib.rs
@@ -82,6 +82,7 @@ pub(crate) fn to_py_err(e: clawhdf5_rs::Error) -> PyErr {
         | Error::ZeroCopyUnaligned { .. } => {
             PyErr::new::<pyo3::exceptions::PyValueError, _>(e.to_string())
         }
+        _ => PyErr::new::<pyo3::exceptions::PyOSError, _>(e.to_string()),
     }
 }
 
diff --git a/crates/clawhdf5/Cargo.toml b/crates/clawhdf5/Cargo.toml
index 4f68e58..6a95ced 100644
--- a/crates/clawhdf5/Cargo.toml
+++ b/crates/clawhdf5/Cargo.toml
@@ -31,6 +31,10 @@ harness = false
 name = "parallel_bench"
 harness = false
 
+[[bench]]
+name = "local_metadata_bench"
+harness = false
+
 [features]
 default = ["mmap", "provenance", "lzf"]
 mmap = ["clawhdf5-io/mmap"]
diff --git a/crates/clawhdf5/benches/local_metadata_bench.rs b/crates/clawhdf5/benches/local_metadata_bench.rs
new file mode 100644
index 0000000..0b2af1f
--- /dev/null
+++ b/crates/clawhdf5/benches/local_metadata_bench.rs
@@ -0,0 +1,91 @@
+//! Metadata parsing over an in-memory file: the local fast path that the
+//! range-read `Storage` migration must not slow down
+//! (`docs/design/range-reads.md`, "Keeping the local fast path").
+//!
+//! Only the `&[u8]` APIs are used, so the same file builds against older
+//! revisions for an A/B comparison. The input is a version-1 (symbol table)
+//! file with 400 groups, written by h5py with `libver='earliest'` and a
+//! 512-byte user block (`clawhdf5-format/tests/fixtures/v1_groups_400.h5`).
+
+use clawhdf5::{File, Group};
+use clawhdf5_format::btree_v1::collect_symbol_table_nodes;
+use clawhdf5_format::message_type::MessageType;
+use clawhdf5_format::object_header::ObjectHeader;
+use clawhdf5_format::superblock::Superblock;
+use clawhdf5_format::symbol_table::{SymbolTableMessage, SymbolTableNode};
+use criterion::{Criterion, criterion_group, criterion_main};
+use std::hint::black_box;
+
+const FIXTURE: &str = concat!(
+    env!("CARGO_MANIFEST_DIR"),
+    "/../clawhdf5-format/tests/fixtures/v1_groups_400.h5"
+);
+
+fn walk(g: &Group<'_>, objs: &mut usize) {
+    for name in g.datasets().unwrap_or_default() {
+        *objs += 1;
+        if let Ok(ds) = g.dataset(&name) {
+            let _ = black_box(ds.shape());
+            let _ = black_box(ds.dtype());
+            let _ = black_box(ds.attrs());
+        }
+    }
+    for name in g.groups().unwrap_or_default() {
+        *objs += 1;
+        if let Ok(sub) = g.group(&name) {
+            walk(&sub, objs);
+        }
+    }
+}
+
+fn bench_local_metadata(c: &mut Criterion) {
+    let bytes = std::fs::read(FIXTURE).unwrap();
+    let (_, f) = clawhdf5_format::signature::split_user_block(&bytes).unwrap();
+    let sb = Superblock::parse(f, 0).unwrap();
+    let (os, ls) = (sb.offset_size, sb.length_size);
+    let root = ObjectHeader::parse(f, sb.root_group_address as usize, os, ls).unwrap();
+    let stm = root
+        .messages
+        .iter()
+        .find(|m| m.msg_type == MessageType::SymbolTable)
+        .map(|m| SymbolTableMessage::parse(&m.data, os).unwrap())
+        .unwrap();
+    let nodes = collect_symbol_table_nodes(f, stm.btree_address, os, ls).unwrap();
+    let headers: Vec<u64> = nodes
+        .iter()
+        .flat_map(|&a| SymbolTableNode::parse(f, a as usize, os).unwrap().entries)
+        .map(|e| e.object_header_address)
+        .collect();
+    assert_eq!(headers.len(), 401);
+
+    let mut g = c.benchmark_group("local_metadata");
+    g.bench_function("object_header_parse_x401", |b| {
+        b.iter(|| {
+            for &a in &headers {
+                black_box(ObjectHeader::parse(f, a as usize, os, ls).unwrap());
+            }
+        })
+    });
+    g.bench_function("snod_parse_all", |b| {
+        b.iter(|| {
+            for &a in &nodes {
+                black_box(SymbolTableNode::parse(f, a as usize, os).unwrap());
+            }
+        })
+    });
+    g.bench_function("btree_v1_walk", |b| {
+        b.iter(|| black_box(collect_symbol_table_nodes(f, stm.btree_address, os, ls).unwrap()))
+    });
+    let file = File::open(FIXTURE).unwrap();
+    g.bench_function("facade_list_400_groups", |b| {
+        b.iter(|| {
+            let mut n = 0;
+            walk(&file.root(), &mut n);
+            assert_eq!(n, 401);
+        })
+    });
+    g.finish();
+}
+
+criterion_group!(benches, bench_local_metadata);
+criterion_main!(benches);
diff --git a/crates/clawhdf5/src/error.rs b/crates/clawhdf5/src/error.rs
index de34355..08aabbd 100644
--- a/crates/clawhdf5/src/error.rs
+++ b/crates/clawhdf5/src/error.rs
@@ -6,7 +6,12 @@ use clawhdf5_format::error::FormatError;
 use clawhdf5_format::message_type::MessageType;
 
 /// Errors that can occur when using the high-level API.
+///
+/// Non-exhaustive: new failure modes add variants, so a `match` needs a
+/// wildcard arm. Failures of a [`Storage`](clawhdf5_format::storage::Storage)
+/// backend arrive as `Error::Format(FormatError::Storage(..))`.
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum Error {
     /// I/O error from the filesystem.
     Io(std::io::Error),
diff --git a/docs/design/range-reads.md b/docs/design/range-reads.md
index 9629a21..1c7fa55 100644
--- a/docs/design/range-reads.md
+++ b/docs/design/range-reads.md
@@ -1,7 +1,9 @@
 # Design: range reads (reading HDF5 without holding the whole file)
 
-Status: proposal, 2026-09-26. No library code has changed; this document is
-the plan for Phase 3's largest architectural change. Every count below was
+Status: proposal, 2026-09-26; the plan for Phase 3's largest architectural
+change. Progress: M1, first part (the `Storage` trait and the metadata
+parsers listed in `CHANGELOG.md` under "Range reads, milestone M1") is done;
+group B-tree v2 lookups, dense groups and the facade are not converted yet. Every count below was
 taken on `tank` on 2026-09-26 at commit `de2a53f`, with the commands given
 next to it. No timing numbers appear here on purpose: the machine was shared
 with other build jobs when this was written.
@@ -259,6 +261,11 @@ every `file_data[a..b]` becomes `file.read_at(a, b - a)?`.
   (binary size matters for wasm); `&dyn Storage` costs one indirect call per
   structure read, negligible next to parsing. Hot raw-data loops keep their
   speed through `as_contiguous()`.
+  *M1 outcome:* `&dyn` was not negligible for small structures — with it,
+  `ObjectHeader::parse` was ~25% and a 400-group facade listing ~14% slower
+  than the slice code (provisional, shared machine). The cores are now
+  generic (`S: Storage + ?Sized`), so the `&[u8]` wrappers get a `[u8]`
+  instance and `dyn Storage` is one more instance, not one per backend.
 
 ### (b) A page-cache "virtual slice"
 
@@ -400,7 +407,7 @@ fast path within benchmark noise.
   B-tree v1/v2, fractal heap, fixed/extensible array, symbol table, group
   v1/v2, shared messages, attributes, fill value, data layout — one commit
   each. The old `&[u8]` signature stays as a thin wrapper over the new one
-  (`fn parse(data: &[u8], ..) { parse_in(data as &dyn Storage, ..) }`), so
+  (`fn parse(data: &[u8], ..) { parse_in(data, ..) }`, generic core), so
   callers and the other crates don't move yet.
 - Replace the 5 open-ended slices and 38 `len()` checks with bounded reads.
 
@@ -441,8 +448,13 @@ Total: roughly 6–10 engineer-weeks for M0–M4 (estimate, not measured).
 - `impl Storage for [u8]` returns `Cow::Borrowed` — no copy, no allocation.
 - Hot loops (raw-data copies, contiguous typed reads, `read_selection_native`)
   branch once on `as_contiguous()` and then run today's code.
-- `&dyn` dispatch is per structure, not per byte; parse code keeps working on
+- The parser cores are generic over `S: Storage + ?Sized`, so the in-memory
+  instance has no dispatch at all; a remote backend behind `&dyn Storage`
+  pays one indirect call per structure read. Parse code keeps working on
   the returned slice.
+- Reads sized by untrusted fields cover what the parser uses (see
+  `CHANGELOG.md`, M1), so a hostile size costs no read of the rest of the
+  file.
 - Gate: `crates/clawhdf5/benches/mmap_bench.rs`, the concurrent-read benches in
   `clawhdf5-bench`, and the conformance run time, before and after each M1/M2
   commit, on an otherwise idle machine. Anything outside noise blocks the