Chunk dimensions of 2^32 or more; unfiltered 4 GiB chunks written without copies

- DataLayout::Chunked::chunk_dimensions is Vec<u64> (was Vec<u32>), and the chunk index readers, writers and serializers take &[u64]: layout messages of version 4/5 store each dimension in up to 8 bytes, and libhdf5 2.x writes dimensions of 2^32 or more (layout version 5). Such dimensions were refused on read (InvalidChunkDimensions) and write. A version-3 layout (4-byte dimensions) is never written for them; a chunk whose size overflows 64 bits is refused when opened. - The file writer lays chunked datasets out as pieces referring to the chunks instead of copying them into one buffer per pass, and an unfiltered chunk that is a contiguous run of the dataset's data (a dataset stored as one chunk of its shape, row blocks) borrows it; a filtered one is compressed straight from it. Contiguous datasets are not copied either. FileWriter::finish_with streams the file to a callback; FileBuilder::write uses it, so the file is never assembled in memory. DatasetBuilder::with_u8_data_owned takes the data without a copy. Peak RSS writing one unfiltered 1 GiB chunk (with_u8_data_owned + write): 5.0 GiB before, 1.0 GiB after; with_u8_data: 6.0 -> 2.0 GiB. - extract_chunk no longer panics on data shorter than the shape. - Tests: huge_chunk_dims.h5 fixture (libhdf5 2.0.0 via h5py 3.16, u8 chunks of 2^32 + 7), and opt-in end-to-end tests of chunk dims >= 2^32 (filtered and unfiltered, read and written, h5py and h5dump 2.2.0), of an unfiltered 4 GiB+ chunk written by clawhdf5, and of LZ4/Zstd chunks of that size; example write_one_chunk for memory measurements. Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
2026-09-29 20:47:33 -05:00
parent e4ba09946f
commit e4b3a76dc7
17 changed files with 1156 additions and 256 deletions
@@ -555,7 +555,7 @@ pub(crate) fn checked_byte_len(elements: u64, elem_size: usize) -> Result<usize,
 /// HDF5 2.0 writes them with layout version 5. A zero chunk dimension used
 /// to read as all fill values, and a huge one to hang the reader.
 pub(crate) fn chunk_geometry(
-    chunk_dimensions: &[u32],
+    chunk_dimensions: &[u64],
    layout_version: u8,
    dataspace: &Dataspace,
    elem_size: usize,
@@ -576,15 +576,26 @@ pub(crate) fn chunk_geometry(
            "chunk size must be > 0, dim = {d}"
        )));
    }
-    let bytes = spatial
+    // Saturating: 33 dimensions of up to 64 bits each can overflow u128.
-        .iter()
+    let bytes = spatial.iter().fold(elem_size as u128, |acc, &c| {
-        .fold(elem_size as u128, |acc, &c| acc * u128::from(c));
+        acc.saturating_mul(u128::from(c))
    });
    // So the indexes can compute a chunk's size in 64 bits.
    if bytes > u128::from(u64::MAX) {
        return Err(FormatError::InvalidChunkDimensions(format!(
            "chunk size overflows 64 bits (chunk {spatial:?} of {elem_size}-byte elements)"
        )));
    }
    if layout_version < 4 && bytes > u128::from(u32::MAX) {
        return Err(FormatError::InvalidChunkDimensions(format!(
            "chunk size must be < 4GB with v1 b-tree index (chunk {spatial:?} of {elem_size}-byte elements)"
        )));
    }
-    Ok((rank, spatial.iter().map(|&c| c as usize).collect()))
+    let spatial = spatial
        .iter()
        .map(|&c| to_usize(c))
        .collect::<Result<Vec<usize>, _>>()?;
    Ok((rank, spatial))
 }
 /// The size of one element of `dt` as stored in the file: a
@@ -625,7 +636,7 @@ pub fn check_chunk_element_size(
        return Ok(());
    };
    let expected = stored_element_size(datatype, offset_size);
-    if u64::from(stored) != expected {
+    if stored != expected {
        return Err(FormatError::InvalidChunkDimensions(format!(
            "stored datatype size in chunk layout does not match datatype description \
             (layout {stored} bytes, datatype {expected})"
@@ -759,7 +770,7 @@ pub fn collect_chunk_info_in<S: Storage + ?Sized>(
 pub fn collect_chunk_info_checked(
    file_data: &[u8],
    btree_address: u64,
-    chunk_dimensions: &[u32],
+    chunk_dimensions: &[u64],
    offset_size: u8,
    length_size: u8,
 ) -> Result<Vec<ChunkInfo>, FormatError> {
@@ -776,7 +787,7 @@ pub fn collect_chunk_info_checked(
 pub fn collect_chunk_info_checked_in<S: Storage + ?Sized>(
    file_data: &S,
    btree_address: u64,
-    chunk_dimensions: &[u32],
+    chunk_dimensions: &[u64],
    offset_size: u8,
    length_size: u8,
 ) -> Result<Vec<ChunkInfo>, FormatError> {
@@ -808,7 +819,7 @@ pub fn collect_chunk_info_checked_in<S: Storage + ?Sized>(
            .iter_mut()
            .zip(chunk.offsets.iter().zip(chunk_dimensions))
        {
-            *w = o / u64::from(d);
+            *w = o / d;
        }
        wanted[ndims - 1] = 0;
        if let Some(i) = root.find(&wanted) {
@@ -849,9 +860,9 @@ impl ChunkNode {
        }
    }
-    fn scale_keys(&mut self, dims: &[u32]) {
+    fn scale_keys(&mut self, dims: &[u64]) {
        for (k, &d) in self.keys.iter_mut().zip(dims.iter().cycle()) {
-            *k /= u64::from(d);
+            *k /= d;
        }
        if let ChunkChildren::Nodes(nodes) = &mut self.children {
            for n in nodes {
@@ -919,9 +930,9 @@ fn btree_cmp3(lt: &[u64], scaled: &[u64], rt: &[u64]) -> core::cmp::Ordering {
 /// Check one v1 B-tree chunk key's offsets (see
 /// [`collect_chunk_info_checked`]).
-fn check_key_offsets(offsets: &[u64], chunk_dimensions: &[u32]) -> Result<(), FormatError> {
+fn check_key_offsets(offsets: &[u64], chunk_dimensions: &[u64]) -> Result<(), FormatError> {
    for (&offset, &dim) in offsets.iter().zip(chunk_dimensions) {
-        if dim == 0 || offset % u64::from(dim) != 0 {
+        if dim == 0 || offset % dim != 0 {
            return Err(FormatError::ChunkedReadError(format!(
                "bad coordinate offset {offsets:?} for chunk dimensions {chunk_dimensions:?}"
            )));
@@ -937,7 +948,7 @@ fn read_key_offsets(
    file_data: &[u8],
    pos: usize,
    ndims: usize,
-    chunk_dimensions: Option<&[u32]>,
+    chunk_dimensions: Option<&[u64]>,
    out: &mut Vec<u64>,
 ) -> Result<(), FormatError> {
    let start = out.len();
@@ -966,7 +977,7 @@ fn parse_chunk_node<S: Storage + ?Sized>(
    file_data: &S,
    btree_address: u64,
    ndims: usize,
-    chunk_dimensions: Option<&[u32]>,
+    chunk_dimensions: Option<&[u64]>,
    offset_size: u8,
    depth: usize,
    stored: &mut Vec<ChunkInfo>,
@@ -1075,7 +1086,7 @@ fn parse_chunk_node<S: Storage + ?Sized>(
 pub fn generate_implicit_chunks(
    base_address: u64,
    dataset_dims: &[u64],
-    chunk_dimensions: &[u32],
+    chunk_dimensions: &[u64],
    element_size: u32,
 ) -> Vec<ChunkInfo> {
    generate_implicit_chunks_in_grid(
@@ -1097,17 +1108,18 @@ pub fn generate_implicit_chunks_in_grid(
    base_address: u64,
    dataset_dims: &[u64],
    max_dims: &[u64],
-    chunk_dimensions: &[u32],
+    chunk_dimensions: &[u64],
    element_size: u32,
 ) -> Vec<ChunkInfo> {
    let rank = chunk_dimensions.len();
-    let chunk_byte_size: u64 =
+    let chunk_byte_size: u64 = chunk_dimensions
-        chunk_dimensions.iter().map(|&d| d as u64).product::<u64>() * element_size as u64;
+        .iter()
        .fold(u64::from(element_size), |acc, &d| acc.saturating_mul(d));
    let mut num_chunks_per_dim = Vec::with_capacity(rank);
    let mut grid_per_dim = Vec::with_capacity(rank);
    for d in 0..rank {
-        let ch = chunk_dimensions[d] as u64;
+        let ch = chunk_dimensions[d];
        let n = dataset_dims[d].div_ceil(ch);
        num_chunks_per_dim.push(n);
        grid_per_dim.push(max_dims.get(d).map_or(n, |m| m.div_ceil(ch)).max(n));
@@ -1125,7 +1137,7 @@ pub fn generate_implicit_chunks_in_grid(
            let nchunks = num_chunks_per_dim[d];
            let chunk_idx = remaining % nchunks;
            remaining /= nchunks;
-            offsets[d] = chunk_idx * chunk_dimensions[d] as u64;
+            offsets[d] = chunk_idx * chunk_dimensions[d];
            grid_idx = grid_idx.saturating_add(chunk_idx.saturating_mul(down));
            down = down.saturating_mul(grid_per_dim[d]);
        }
@@ -1350,7 +1362,7 @@ pub fn list_chunks_in<S: Storage + ?Sized>(
        }
        (4, Some(2)) => {
            // Implicit index — use spatial chunk dims only
-            let spatial_chunk_dims: &[u32] = &chunk_dimensions[..rank];
+            let spatial_chunk_dims: &[u64] = &chunk_dimensions[..rank];
            generate_implicit_chunks_in_grid(
                addr,
                &dataspace.dimensions,
@@ -1364,7 +1376,7 @@ pub fn list_chunks_in<S: Storage + ?Sized>(
        }
        (4, Some(3)) => {
            // Fixed Array — use spatial chunk dims only
-            let spatial_chunk_dims: &[u32] = &chunk_dimensions[..rank];
+            let spatial_chunk_dims: &[u64] = &chunk_dimensions[..rank];
            let header = FixedArrayHeader::parse_in(
                file_data,
                checked_addr(addr)?,
@@ -1384,7 +1396,7 @@ pub fn list_chunks_in<S: Storage + ?Sized>(
        }
        (4, Some(4)) => {
            // Extensible Array — use spatial chunk dims only
-            let spatial_chunk_dims: &[u32] = &chunk_dimensions[..rank];
+            let spatial_chunk_dims: &[u64] = &chunk_dimensions[..rank];
            let header = ExtensibleArrayHeader::parse_in(
                file_data,
                checked_addr(addr)?,
@@ -2435,12 +2447,12 @@ mod tests {
    /// A leaf whose final key is the one libhdf5 writes past the last chunk:
    /// each coordinate of the last chunk plus its chunk dimension (the
    /// element size last).
-    fn build_chunk_btree_leaf_dims(chunks: &[ChunkInfo], dims: &[u32], offset_size: u8) -> Vec<u8> {
+    fn build_chunk_btree_leaf_dims(chunks: &[ChunkInfo], dims: &[u64], offset_size: u8) -> Vec<u8> {
        let last = &chunks.last().expect("a chunk").offsets;
        let end: Vec<u64> = dims
            .iter()
            .enumerate()
-            .map(|(d, &c)| last.get(d).copied().unwrap_or(0) + u64::from(c))
+            .map(|(d, &c)| last.get(d).copied().unwrap_or(0) + c)
            .collect();
        build_chunk_btree_leaf_to(chunks, &end, offset_size)
    }
@@ -2771,7 +2783,7 @@ mod tests {
        }
        // Build B-tree at offset 0x100
-        let dims = [chunk_size_elems as u32, elem_size as u32];
+        let dims = [chunk_size_elems as u64, elem_size as u64];
        let btree = build_chunk_btree_leaf_dims(&chunk_infos, &dims, os);
        let btree_addr = 0x100usize;
        file_data[btree_addr..btree_addr + btree.len()].copy_from_slice(&btree);
@@ -3145,7 +3157,7 @@ mod tests {
            data_offset += compressed.len() + 16; // some padding
        }
-        let dims = [chunk_elems as u32, elem_size as u32];
+        let dims = [chunk_elems as u64, elem_size as u64];
        let btree = build_chunk_btree_leaf_dims(&chunk_infos, &dims, os);
        let btree_addr = 0x100usize;
        file_data[btree_addr..btree_addr + btree.len()].copy_from_slice(&btree);
@@ -3228,7 +3240,7 @@ mod tests {
            }
        }
-        let dims = [chunk_dims[0] as u32, chunk_dims[1] as u32, elem_size as u32];
+        let dims = [chunk_dims[0] as u64, chunk_dims[1] as u64, elem_size as u64];
        let btree = build_chunk_btree_leaf_dims(&chunk_infos, &dims, os);
        let btree_addr = 0x100usize;
        file_data[btree_addr..btree_addr + btree.len()].copy_from_slice(&btree);
@@ -3408,7 +3420,7 @@ mod tests {
        }
        let layout = DataLayout::Chunked {
-            chunk_dimensions: vec![chunk_elems as u32, elem_size as u32],
+            chunk_dimensions: vec![chunk_elems as u64, elem_size as u64],
            btree_address: Some(data_addr as u64),
            version: 4,
            chunk_index_type: Some(1),
@@ -5,12 +5,14 @@ extern crate alloc;
 use crate::addr::saturating_usize;
 #[cfg(not(feature = "std"))]
-use alloc::{format, vec, vec::Vec};
+use alloc::{borrow::Cow, format, vec, vec::Vec};
 #[cfg(feature = "std")]
 use std::borrow::Cow;
 use crate::btree_v1_write;
 use crate::btree_v2_write::{BTreeV2Params, build_btree_v2};
 use crate::checksum::jenkins_lookup3;
-use crate::chunk_cache::{CACHE_LINE_SIZE, align_to_cache_line};
+use crate::chunk_cache::CACHE_LINE_SIZE;
 use crate::chunk_grid::ChunkGrid;
 use crate::ea_writer;
 use crate::error::FormatError;
@@ -464,7 +466,9 @@ fn extract_chunk(
        let dst: usize = (0..rank).map(|d| idx[d] * chunk_strides[d]).sum::<usize>() * element_size;
        // Whole elements only, as far as `raw_data` reaches.
        let n = row.min(raw_data.len().saturating_sub(src)) / element_size * element_size;
        if n > 0 {
            chunk[dst..dst + n].copy_from_slice(&raw_data[src..src + n]);
        }
        // Next row: advance every dimension but the last.
        let mut d = rank - 1;
        loop {
@@ -481,6 +485,66 @@ fn extract_chunk(
    }
 }
 /// The bytes of the `linear_idx`-th chunk (see [`extract_chunk`]),
 /// borrowed from `raw_data` when the chunk is one contiguous run of it: a
 /// chunk inside the dataset (not past its edge) whose dimensions after the
 /// first one above 1 equal the dataset's. A dataset stored as one chunk of
 /// its own shape is such a run, so its chunk is never copied. Other chunks
 /// are copied out, as [`extract_chunk`] does.
 fn chunk_bytes_of<'a>(
    raw_data: &'a [u8],
    shape: &[u64],
    chunk_dims: &[u64],
    element_size: usize,
    linear_idx: u64,
 ) -> Cow<'a, [u8]> {
    if shape.is_empty() {
        return Cow::Borrowed(raw_data);
    }
    if let Some(range) = contiguous_chunk(shape, chunk_dims, element_size, linear_idx)
        && let (Ok(start), Ok(end)) = (usize::try_from(range.start), usize::try_from(range.end))
        && end <= raw_data.len()
    {
        return Cow::Borrowed(&raw_data[start..end]);
    }
    Cow::Owned(extract_chunk(raw_data, shape, chunk_dims, element_size, linear_idx).1)
 }
 /// The byte range of the `linear_idx`-th chunk in the row-major dataset,
 /// when the chunk is a contiguous run of it (see [`chunk_bytes_of`]).
 fn contiguous_chunk(
    shape: &[u64],
    chunk_dims: &[u64],
    element_size: usize,
    linear_idx: u64,
 ) -> Option<core::ops::Range<u64>> {
    let rank = shape.len();
    // Past the first chunk dimension above 1, every chunk dimension must
    // span the dataset's.
    let first_wide = chunk_dims.iter().position(|&c| c > 1).unwrap_or(rank);
    if (first_wide + 1..rank).any(|d| chunk_dims[d] != shape[d]) {
        return None;
    }
    let mut remaining = linear_idx;
    let mut start = 0u64;
    let mut stride = element_size as u64;
    for d in (0..rank).rev() {
        let n = shape[d].div_ceil(chunk_dims[d]);
        let offset = (remaining % n).checked_mul(chunk_dims[d])?;
        remaining /= n;
        // A chunk past the dataset's edge is padded: not a run of it.
        if offset.checked_add(chunk_dims[d])? > shape[d] {
            return None;
        }
        start = start.checked_add(offset.checked_mul(stride)?)?;
        stride = stride.checked_mul(shape[d])?;
    }
    let len = chunk_dims
        .iter()
        .try_fold(element_size as u64, |acc, &c| acc.checked_mul(c))?;
    Some(start..start.checked_add(len)?)
 }
 /// Parallel compression threshold: use rayon when chunk count exceeds this.
 ///
 /// Lowered to 2 to enable parallel compression for typical 4-chunk workloads
@@ -508,25 +572,21 @@ const PARALLEL_COMPRESS_MAX_CHUNK_BYTES: u64 = 64 << 20;
 /// at most [`PARALLEL_COMPRESS_MAX_CHUNK_BYTES`], compression runs across
 /// rayon threads; otherwise it is sequential. Output order matches chunk
 /// order, so per-chunk bytes are identical to the sequential path.
-fn compress_all_chunks(
+fn compress_all_chunks<'a>(
-    raw_data: &[u8],
+    raw_data: &'a [u8],
    shape: &[u64],
    chunk_dims: &[u64],
    element_size: usize,
    chunk_bytes: u64,
    pipeline: &Option<FilterPipeline>,
-) -> Result<Vec<(u64, Vec<u8>, u32)>, FormatError> {
+) -> Result<Vec<PreparedChunk<'a>>, FormatError> {
-    let one = |i: u64| -> Result<(u64, Vec<u8>, u32), FormatError> {
+    let one = |i: u64| -> Result<PreparedChunk<'a>, FormatError> {
-        let (_, raw) = if shape.is_empty() {
+        let raw = chunk_bytes_of(raw_data, shape, chunk_dims, element_size, i);
            (Vec::new(), raw_data.to_vec())
        } else {
            extract_chunk(raw_data, shape, chunk_dims, element_size, i)
        };
        let raw_size = raw.len() as u64;
        match pipeline {
            Some(pl) => {
                let (stored, mask) = compress_chunk_masked(&raw, pl, element_size as u32)?;
-                Ok((raw_size, stored, mask))
+                Ok((raw_size, Cow::Owned(stored), mask))
            }
            None => Ok((raw_size, raw, 0)),
        }
@@ -557,7 +617,7 @@ fn compress_all_chunks(
 /// layout/pipeline messages. `base_address` is where the blob will be placed in the file.
 /// Serialize a v4 single chunk layout message (public for OH size estimation).
 pub fn serialize_v4_single_chunk_pub(
-    chunk_dims: &[u32],
+    chunk_dims: &[u64],
    chunk_address: u64,
    filtered_size: Option<u64>,
    filter_mask: Option<u32>,
@@ -578,7 +638,7 @@ pub fn serialize_v4_single_chunk_pub(
 /// Serialize a v4 (or, for a chunk of 4 GiB or more, v5) single chunk
 /// layout message.
 fn serialize_v4_single_chunk(
-    chunk_dims: &[u32],
+    chunk_dims: &[u64],
    chunk_address: u64,
    filtered_size: Option<u64>,
    filter_mask: Option<u32>,
@@ -622,7 +682,7 @@ fn serialize_v4_single_chunk(
 /// Serialize a v4 Fixed Array layout message.
 fn serialize_v4_fixed_array(
-    chunk_dims: &[u32],
+    chunk_dims: &[u64],
    fixed_array_address: u64,
    offset_size: u8,
    element_size: u32,
@@ -653,7 +713,8 @@ fn serialize_v4_fixed_array(
 /// dimensions, then the element size). Each takes the fewest bytes that hold
 /// the largest, as libhdf5 computes it (`H5D__chunk_set_sizes`:
 /// `(log2(dim) + 8) / 8`); HDF5 2.0.0 refuses any other width.
-pub(crate) fn push_v4_chunk_dims(buf: &mut Vec<u8>, chunk_dims: &[u32], element_size: u32) {
+pub(crate) fn push_v4_chunk_dims(buf: &mut Vec<u8>, chunk_dims: &[u64], element_size: u32) {
    let element_size = u64::from(element_size);
    let max_dim = chunk_dims
        .iter()
        .copied()
@@ -661,14 +722,14 @@ pub(crate) fn push_v4_chunk_dims(buf: &mut Vec<u8>, chunk_dims: &[u32], element_
        .max()
        .unwrap_or(1)
        .max(1);
-    let width = (32 - max_dim.leading_zeros()).div_ceil(8) as usize;
+    let width = (64 - max_dim.leading_zeros()).div_ceil(8) as usize;
    buf.push(width as u8);
    for &d in chunk_dims.iter().chain(core::iter::once(&element_size)) {
        buf.extend_from_slice(&d.to_le_bytes()[..width]);
    }
 }
-fn layout_v4_chunked_prefix(chunk_dims: &[u32], element_size: u32, version: u8) -> Vec<u8> {
+fn layout_v4_chunked_prefix(chunk_dims: &[u64], element_size: u32, version: u8) -> Vec<u8> {
    let mut buf = Vec::new();
    buf.push(version);
    buf.push(2); // class = chunked
@@ -884,7 +945,66 @@ pub fn precompress_chunks(
    element_size: usize,
    options: &ChunkOptions,
 ) -> Result<PrecompressedChunks, FormatError> {
-    let (_, chunk_bytes) = checked_chunk_dims(chunk_dims, element_size)?;
+    let p = prepare_chunks(raw_data, shape, chunk_dims, element_size, options)?;
    Ok(PrecompressedChunks {
        chunks: p
            .chunks
            .into_iter()
            .map(|(raw, stored, mask)| (raw, stored.into_owned(), mask))
            .collect(),
        has_filters: p.has_filters,
        element_size: p.element_size,
        shape: p.shape,
        chunk_dims: p.chunk_dims,
        pipeline_message: p.pipeline_message,
    })
 }
 /// One chunk of [`PreparedChunks`]: its raw size, its stored bytes and its
 /// filter mask.
 pub(crate) type PreparedChunk<'a> = (u64, Cow<'a, [u8]>, u32);
 /// [`PrecompressedChunks`] whose stored bytes may borrow the dataset's data:
 /// an unfiltered chunk that is a contiguous run of it (a dataset stored as
 /// one chunk, say) is not copied. What the file writer lays out.
 pub(crate) struct PreparedChunks<'a> {
    pub(crate) chunks: Vec<PreparedChunk<'a>>,
    pub(crate) has_filters: bool,
    pub(crate) element_size: usize,
    pub(crate) shape: Vec<u64>,
    pub(crate) chunk_dims: Vec<u64>,
    pub(crate) pipeline_message: Option<Vec<u8>>,
 }
 impl PrecompressedChunks {
    /// A view of these chunks as [`PreparedChunks`] (their bytes borrowed).
    fn prepared(&self) -> PreparedChunks<'_> {
        PreparedChunks {
            chunks: self
                .chunks
                .iter()
                .map(|(raw, stored, mask)| (*raw, Cow::Borrowed(&stored[..]), *mask))
                .collect(),
            has_filters: self.has_filters,
            element_size: self.element_size,
            shape: self.shape.clone(),
            chunk_dims: self.chunk_dims.clone(),
            pipeline_message: self.pipeline_message.clone(),
        }
    }
 }
 /// [`precompress_chunks`], borrowing what it can (see [`PreparedChunks`]).
 /// A filtered chunk that is a contiguous run of `raw_data` is compressed
 /// straight from it, without a copy of the raw chunk.
 pub(crate) fn prepare_chunks<'a>(
    raw_data: &'a [u8],
    shape: &[u64],
    chunk_dims: &[u64],
    element_size: usize,
    options: &ChunkOptions,
 ) -> Result<PreparedChunks<'a>, FormatError> {
    let chunk_bytes = checked_chunk_dims(chunk_dims, element_size)?;
    if chunk_bytes > MAX_V4_CHUNK_BYTES {
        check_huge_chunk_filters(options, chunk_bytes)?;
    }
@@ -902,7 +1022,7 @@ pub fn precompress_chunks(
        &pipeline,
    )?;
-    Ok(PrecompressedChunks {
+    Ok(PreparedChunks {
        chunks,
        has_filters,
        element_size,
@@ -912,26 +1032,87 @@ pub fn precompress_chunks(
    })
 }
-/// The chunk dimensions as the layout message stores them (each below
+/// A piece of a chunked dataset's data blob as laid out by
-/// 2^32), and one chunk's size in bytes. A chunk dimension of 2^32 or more
+/// [`place_chunked_data`]: zero padding, a chunk's stored bytes (by index
-/// (which HDF5 2.0 can store, in wider fields) is refused, as it is when
+/// into [`PreparedChunks::chunks`]), or index structures.
-/// read: clawhdf5 holds chunk dimensions as `u32`. So is a chunk whose size
+#[derive(Debug)]
-/// overflows 64 bits, or that this platform cannot hold in memory (a chunk
+pub(crate) enum Piece {
-/// of 4 GiB or more on a 32-bit target).
+    Zeros(usize),
-fn checked_chunk_dims(
+    Chunk(usize),
-    chunk_dims: &[u64],
+    Bytes(Vec<u8>),
-    element_size: usize,
+}
-) -> Result<(Vec<u32>, u64), FormatError> {
+
-    let dims = chunk_dims
+/// A chunked dataset laid out at an address, without its chunk bytes
-        .iter()
+/// copied: the pieces of its data blob, in order, and its messages.
-        .map(|&d| {
+pub(crate) struct ChunkedPlacement {
-            u32::try_from(d).map_err(|_| {
+    pub(crate) pieces: Vec<Piece>,
-                FormatError::InvalidChunkDimensions(format!(
+    /// Length of the data blob in bytes.
-                    "chunk dimension {d} is 2^32 or more, which clawhdf5 does not support"
+    pub(crate) len: u64,
-                ))
+    pub(crate) layout_message: Vec<u8>,
-            })
+    pub(crate) pipeline_message: Option<Vec<u8>>,
-        })
+}
-        .collect::<Result<Vec<u32>, _>>()?;
+
 impl ChunkedPlacement {
    /// The data blob as one buffer (the chunks copied in).
    fn into_result(self, pre: &PreparedChunks<'_>) -> ChunkedDataResult {
        let mut data_bytes = Vec::with_capacity(saturating_usize(self.len));
        for piece in &self.pieces {
            match piece {
                Piece::Zeros(n) => data_bytes.resize(data_bytes.len() + n, 0),
                Piece::Chunk(i) => data_bytes.extend_from_slice(&pre.chunks[*i].1),
                Piece::Bytes(b) => data_bytes.extend_from_slice(b),
            }
        }
        ChunkedDataResult {
            data_bytes,
            layout_message: self.layout_message,
            pipeline_message: self.pipeline_message,
        }
    }
 }
 /// Builds the pieces of a data blob, tracking its length.
 #[derive(Default)]
 struct Pieces {
    pieces: Vec<Piece>,
    len: u64,
 }
 impl Pieces {
    /// Pad to the next cache line.
    fn align(&mut self) {
        let aligned = align_chunk_offset(self.len);
        if aligned > self.len {
            self.pieces
                .push(Piece::Zeros(saturating_usize(aligned - self.len)));
            self.len = aligned;
        }
    }
    fn chunk(&mut self, i: usize, len: usize) {
        self.pieces.push(Piece::Chunk(i));
        self.len += len as u64;
    }
    fn bytes(&mut self, b: Vec<u8>) {
        self.len += b.len() as u64;
        self.pieces.push(Piece::Bytes(b));
    }
 }
 /// One chunk's size in bytes. A chunk dimension of 0 is refused, and so is
 /// a chunk whose size overflows 64 bits, or that this platform cannot hold
 /// in memory (a chunk of 4 GiB or more on a 32-bit target). Chunk
 /// dimensions of 2^32 or more are allowed, as in libhdf5 2.x: such a chunk
 /// is over 4 GiB, so it takes layout message version 5, whose dimension
 /// fields are up to 8 bytes wide (a version-3 layout, with 4-byte fields,
 /// is never written for it).
 fn checked_chunk_dims(chunk_dims: &[u64], element_size: usize) -> Result<u64, FormatError> {
    if chunk_dims.contains(&0) {
        return Err(FormatError::InvalidChunkDimensions(format!(
            "chunk dimensions {chunk_dims:?} include 0"
        )));
    }
    let bytes = chunk_dims
        .iter()
        .try_fold(element_size as u64, |acc, &d| acc.checked_mul(d))
@@ -941,7 +1122,7 @@ fn checked_chunk_dims(
                "a chunk of {chunk_dims:?} x {element_size} bytes exceeds this platform's address space"
            ))
        })?;
-    Ok((dims, bytes))
+    Ok(bytes)
 }
 /// The filters clawhdf5 can apply to a chunk of more than `u32::MAX` bytes:
@@ -1010,7 +1191,22 @@ pub fn build_chunked_data_from_precompressed_libver(
    low: LibVer,
    high: LibVer,
 ) -> Result<ChunkedDataResult, FormatError> {
-    let (_, chunk_bytes) = checked_chunk_dims(&pre.chunk_dims, pre.element_size)?;
+    let pre = pre.prepared();
    let placement = place_chunked_data(&pre, base_address, maxshape, low, high)?;
    Ok(placement.into_result(&pre))
 }
 /// [`build_chunked_data_from_precompressed_libver`] without copying the
 /// chunks into one buffer: the data blob as [`Piece`]s referring to
 /// `pre`'s chunks. The file writer writes the pieces out one by one.
 pub(crate) fn place_chunked_data(
    pre: &PreparedChunks<'_>,
    base_address: u64,
    maxshape: Option<&[u64]>,
    low: LibVer,
    high: LibVer,
 ) -> Result<ChunkedPlacement, FormatError> {
    let chunk_bytes = checked_chunk_dims(&pre.chunk_dims, pre.element_size)?;
    if chunk_bytes > MAX_V4_CHUNK_BYTES {
        if high < LibVer::V200 {
            return Err(FormatError::LibverBound {
@@ -1020,7 +1216,7 @@ pub fn build_chunked_data_from_precompressed_libver(
            });
        }
    } else if low < LibVer::V110 {
-        return build_btree_v1_chunked_data(pre, base_address, maxshape);
+        return place_btree_v1_chunked_data(pre, base_address, maxshape);
    }
    let index = ChunkIndexPlan::new(&pre.shape, maxshape, &pre.chunk_dims)?;
    let offset_size: u8 = 8;
@@ -1028,17 +1224,14 @@ pub fn build_chunked_data_from_precompressed_libver(
    let num_chunks = pre.chunks.len();
    let element_size = pre.element_size;
-    let mut data_buf = Vec::new();
+    let mut blob = Pieces::default();
    let mut written_chunks = Vec::with_capacity(num_chunks);
-    for (raw_size, compressed, filter_mask) in &pre.chunks {
+    for (i, (raw_size, compressed, filter_mask)) in pre.chunks.iter().enumerate() {
-        let aligned_offset = align_to_cache_line(data_buf.len());
+        blob.align();
-        if aligned_offset > data_buf.len() {
+        let address = base_address + blob.len;
            data_buf.resize(aligned_offset, 0u8);
        }
        let address = base_address + data_buf.len() as u64;
        let compressed_size = compressed.len() as u64;
-        data_buf.extend_from_slice(compressed);
+        blob.chunk(i, compressed.len());
        written_chunks.push(WrittenChunk {
            address,
            compressed_size,
@@ -1047,28 +1240,22 @@ pub fn build_chunked_data_from_precompressed_libver(
        });
    }
    let (chunk_dims_u32, chunk_bytes) = checked_chunk_dims(&pre.chunk_dims, element_size)?;
    let version = layout_version_for(chunk_bytes);
-
+    blob.align();
    let aligned_idx = align_to_cache_line(data_buf.len());
    if aligned_idx > data_buf.len() {
        data_buf.resize(aligned_idx, 0u8);
    }
    let layout_message = match &index {
        ChunkIndexPlan::ExtensibleArray(grid) => {
-            let ea_address = base_address + data_buf.len() as u64;
+            let ea_address = base_address + blob.len;
            let slots = index_slots(grid, &pre.shape, &pre.chunk_dims, &written_chunks, None)?;
-            let ea_bytes = ea_writer::build_extensible_array_at(
+            blob.bytes(ea_writer::build_extensible_array_at(
                &slots,
                offset_size,
                length_size,
                pre.has_filters,
                ea_address,
-            );
+            ));
            data_buf.extend_from_slice(&ea_bytes);
            ea_writer::serialize_v4_extensible_array(
-                &chunk_dims_u32,
+                &pre.chunk_dims,
                ea_address,
                offset_size,
                element_size as u32,
@@ -1084,7 +1271,7 @@ pub fn build_chunked_data_from_precompressed_libver(
            };
            let filter_mask = pre.has_filters.then_some(written_chunks[0].filter_mask);
            serialize_v4_single_chunk(
-                &chunk_dims_u32,
+                &pre.chunk_dims,
                chunk_addr,
                filtered_size,
                filter_mask,
@@ -1094,7 +1281,7 @@ pub fn build_chunked_data_from_precompressed_libver(
            )
        }
        ChunkIndexPlan::FixedArray(grid, nslots) => {
-            let fa_address = base_address + data_buf.len() as u64;
+            let fa_address = base_address + blob.len;
            let slots = index_slots(
                grid,
                &pre.shape,
@@ -1102,16 +1289,15 @@ pub fn build_chunked_data_from_precompressed_libver(
                &written_chunks,
                Some(*nslots),
            )?;
-            let fa_bytes = build_fixed_array_at(
+            blob.bytes(build_fixed_array_at(
                &slots,
                offset_size,
                length_size,
                pre.has_filters,
                fa_address,
-            );
+            ));
            data_buf.extend_from_slice(&fa_bytes);
            serialize_v4_fixed_array(
-                &chunk_dims_u32,
+                &pre.chunk_dims,
                fa_address,
                offset_size,
                element_size as u32,
@@ -1120,7 +1306,7 @@ pub fn build_chunked_data_from_precompressed_libver(
            )
        }
        ChunkIndexPlan::BTreeV2 => {
-            let bt_address = base_address + data_buf.len() as u64;
+            let bt_address = base_address + blob.len;
            let records: Vec<(Vec<u64>, &WrittenChunk)> = written_chunks
                .iter()
                .enumerate()
@@ -1134,9 +1320,9 @@ pub fn build_chunked_data_from_precompressed_libver(
                pre.has_filters,
                bt_address,
            )?;
-            data_buf.extend_from_slice(&bt_bytes);
+            blob.bytes(bt_bytes);
            serialize_v4_btree_v2(
-                &chunk_dims_u32,
+                &pre.chunk_dims,
                bt_address,
                offset_size,
                element_size as u32,
@@ -1146,8 +1332,9 @@ pub fn build_chunked_data_from_precompressed_libver(
        }
    };
-    Ok(ChunkedDataResult {
+    Ok(ChunkedPlacement {
-        data_bytes: data_buf,
+        pieces: blob.pieces,
        len: blob.len,
        layout_message,
        pipeline_message: pre.pipeline_message.clone(),
    })
@@ -1156,11 +1343,11 @@ pub fn build_chunked_data_from_precompressed_libver(
 /// Lay out precompressed chunks at `base_address` followed by a version-1
 /// B-tree chunk index, with a version-3 layout message: what libhdf5 writes
 /// for a chunked dataset under a low bound of 1.8.
-fn build_btree_v1_chunked_data(
+fn place_btree_v1_chunked_data(
-    pre: &PrecompressedChunks,
+    pre: &PreparedChunks<'_>,
    base_address: u64,
    maxshape: Option<&[u64]>,
-) -> Result<ChunkedDataResult, FormatError> {
+) -> Result<ChunkedPlacement, FormatError> {
    if let Some(ms) = maxshape {
        let bad = |what: &str| FormatError::ChunkedReadError(format!("maxshape: {what}"));
        if ms.len() != pre.shape.len() {
@@ -1171,20 +1358,17 @@ fn build_btree_v1_chunked_data(
        }
    }
    let offset_size: u8 = 8;
-    let mut data_buf = Vec::new();
+    let mut blob = Pieces::default();
    let mut entries = Vec::with_capacity(pre.chunks.len());
    for (i, (_raw_size, stored, filter_mask)) in pre.chunks.iter().enumerate() {
-        let aligned_offset = align_to_cache_line(data_buf.len());
+        blob.align();
        if aligned_offset > data_buf.len() {
            data_buf.resize(aligned_offset, 0u8);
        }
        entries.push(btree_v1_write::ChunkEntry {
            scaled: scaled_coords(&pre.shape, &pre.chunk_dims, i),
            nbytes: stored.len() as u64,
            filter_mask: *filter_mask,
-            address: base_address + data_buf.len() as u64,
+            address: base_address + blob.len,
        });
-        data_buf.extend_from_slice(stored);
+        blob.chunk(i, stored.len());
    }
    let element_size = u32::try_from(pre.element_size)
        .map_err(|_| FormatError::Overflow("element size".into()))?;
@@ -1193,11 +1377,8 @@ fn build_btree_v1_chunked_data(
    let btree_address = if entries.is_empty() {
        u64::MAX
    } else {
-        let aligned_idx = align_to_cache_line(data_buf.len());
+        blob.align();
-        if aligned_idx > data_buf.len() {
+        let addr = base_address + blob.len;
            data_buf.resize(aligned_idx, 0u8);
        }
        let addr = base_address + data_buf.len() as u64;
        let tree = btree_v1_write::build_chunk_btree_v1_at(
            &entries,
            &pre.chunk_dims,
@@ -1205,13 +1386,14 @@ fn build_btree_v1_chunked_data(
            addr,
            offset_size,
        )?;
-        data_buf.extend_from_slice(&tree);
+        blob.bytes(tree);
        addr
    };
    let layout_message =
        serialize_v3_chunked(&pre.chunk_dims, btree_address, offset_size, element_size)?;
-    Ok(ChunkedDataResult {
+    Ok(ChunkedPlacement {
-        data_bytes: data_buf,
+        pieces: blob.pieces,
        len: blob.len,
        layout_message,
        pipeline_message: pre.pipeline_message.clone(),
    })
@@ -1424,7 +1606,7 @@ fn build_btree_v2_chunk_index_at(
 /// Serialize a v4 layout message for a version-2 B-tree chunk index.
 fn serialize_v4_btree_v2(
-    chunk_dims: &[u32],
+    chunk_dims: &[u64],
    btree_address: u64,
    offset_size: u8,
    element_size: u32,
@@ -2184,7 +2366,7 @@ mod tests {
    /// filtered index element stores the chunk's size in 8 bytes.
    #[test]
    fn huge_chunk_layout_messages_and_index_elements() {
-        let dims = [HUGE_DIM as u32];
+        let dims = [HUGE_DIM];
        let parsed = |msg: &[u8]| {
            assert_eq!(msg[0], 5, "layout message version");
            // Class chunked, then (after the flags) 2 dimensions of 4 bytes.
@@ -2197,7 +2379,7 @@ mod tests {
                    chunk_index_type,
                    ..
                } => {
-                    assert_eq!(chunk_dimensions, vec![HUGE_DIM as u32, 8]);
+                    assert_eq!(chunk_dimensions, vec![HUGE_DIM, 8]);
                    chunk_index_type.unwrap()
                }
                other => panic!("{other:?}"),
@@ -2245,22 +2427,89 @@ mod tests {
        assert_eq!(u16::from_le_bytes([bt[10], bt[11]]), 28);
    }
    /// A chunk dimension of 2^32 or more (a 1-byte element, 2^32 + 7 per
    /// chunk): the dimensions take 5 bytes each, as libhdf5 2.x writes
    /// them (`(log2(dim) + 8) / 8`), and parse back whole.
    #[test]
    fn chunk_dimensions_of_2_pow_32_or_more_take_5_bytes() {
        const DIM: u64 = (1 << 32) + 7;
        let msg = serialize_v4_single_chunk(&[DIM], 0x800, None, None, 8, 1, 5);
        assert_eq!(&msg[..5], &[5, 2, 0, 2, 5]);
        assert_eq!(&msg[5..10], &[7, 0, 0, 0, 1]);
        assert_eq!(&msg[10..15], &[1, 0, 0, 0, 0]);
        assert_eq!(msg[15], 1, "Single Chunk index");
        match DataLayout::parse(&msg, 8, 8).unwrap() {
            DataLayout::Chunked {
                chunk_dimensions, ..
            } => assert_eq!(chunk_dimensions, [DIM, 1]),
            other => panic!("{other:?}"),
        }
        // The largest dimension takes all 8 bytes.
        let mut buf = Vec::new();
        push_v4_chunk_dims(&mut buf, &[u64::MAX, 3], 4);
        assert_eq!(buf[0], 8);
        assert_eq!(buf.len(), 1 + 3 * 8);
        // A version-3 layout has 4-byte dimensions: refused, not truncated.
        assert!(serialize_v3_chunked(&[DIM], 0x800, 8, 1).is_err());
    }
    /// Which chunks are contiguous runs of the dataset's data (borrowed,
    /// not copied, when unfiltered).
    #[test]
    fn contiguous_chunks_are_borrowed() {
        // Rows of a 2-D dataset: chunk (2, 5) of shape (5, 5).
        assert_eq!(contiguous_chunk(&[5, 5], &[2, 5], 4, 0), Some(0..40));
        assert_eq!(contiguous_chunk(&[5, 5], &[2, 5], 4, 1), Some(40..80));
        // The last row chunk runs past the edge: padded, so copied.
        assert_eq!(contiguous_chunk(&[5, 5], &[2, 5], 4, 2), None);
        // Columns split: not contiguous.
        assert_eq!(contiguous_chunk(&[4, 6], &[2, 3], 1, 0), None);
        // Leading dimensions of 1, then a partial row: contiguous.
        assert_eq!(contiguous_chunk(&[2, 3, 8], &[1, 1, 4], 2, 5), Some(40..48));
        // The whole dataset as one chunk.
        assert_eq!(contiguous_chunk(&[3, 4], &[3, 4], 8, 0), Some(0..96));
        let raw: Vec<u8> = (0..100).collect();
        let whole = chunk_bytes_of(&raw, &[10, 10], &[10, 10], 1, 0);
        assert!(matches!(whole, Cow::Borrowed(b) if b.as_ptr() == raw.as_ptr()));
        let rows = chunk_bytes_of(&raw, &[10, 10], &[3, 10], 1, 1);
        assert!(matches!(rows, Cow::Borrowed(b) if b == &raw[30..60]));
        // Data shorter than the shape is copied (and zero-padded) as before.
        let short = chunk_bytes_of(&raw[..50], &[10, 10], &[10, 10], 1, 0);
        assert!(matches!(&short, Cow::Owned(v) if v.len() == 100));
        // Borrowed or copied, the bytes are what extract_chunk gives.
        for (shape, chunk) in [
            (&[10u64, 10][..], &[3u64, 10][..]),
            (&[10, 10], &[3, 4]),
            (&[4, 5, 5], &[1, 5, 5]),
            (&[4, 5, 5], &[1, 2, 5]),
        ] {
            for i in 0..chunk_count(shape, chunk) {
                let got = chunk_bytes_of(&raw, shape, chunk, 1, i);
                assert_eq!(&got[..], &extract_chunk(&raw, shape, chunk, 1, i).1[..]);
            }
        }
    }
    #[test]
    fn huge_chunks_refused_where_unsupported() {
-        // A chunk dimension of 2^32 or more.
+        // A chunk dimension of 0.
        assert!(matches!(
-            checked_chunk_dims(&[1 << 32], 1),
+            checked_chunk_dims(&[4, 0], 1),
-            Err(FormatError::InvalidChunkDimensions(m)) if m.contains("2^32")
+            Err(FormatError::InvalidChunkDimensions(_))
        ));
        // A chunk dimension of 2^32 or more is fine (on a 64-bit target).
        #[cfg(target_pointer_width = "64")]
        assert_eq!(
            checked_chunk_dims(&[(1 << 32) + 5], 1).unwrap(),
            (1 << 32) + 5
        );
        // A chunk size that overflows u64.
        assert!(matches!(
            checked_chunk_dims(&[u32::MAX.into(), u32::MAX.into(), 2], 8),
            Err(FormatError::Overflow(_))
        ));
-        assert_eq!(
+        assert_eq!(checked_chunk_dims(&[HUGE_DIM], 8).unwrap(), HUGE_BYTES);
            checked_chunk_dims(&[HUGE_DIM], 8).unwrap(),
            (vec![HUGE_DIM as u32], HUGE_BYTES)
        );
        // Filters that cannot take a chunk that large.
        for plugin in [
            PluginFilter::Lzf,
@@ -34,7 +34,7 @@ const MAX_LAYOUT_NDIMS: usize = 33;
 /// (`H5O__layout_decode`): at most [`MAX_LAYOUT_NDIMS`], no dimension 0, and
 /// before version 4 at least one dataspace dimension plus the element size.
 /// A zero chunk dimension used to read the dataset as all fill values.
-fn check_chunk_dims(dims: Vec<u32>, layout_version: u8) -> Result<Vec<u32>, FormatError> {
+fn check_chunk_dims(dims: Vec<u64>, layout_version: u8) -> Result<Vec<u64>, FormatError> {
    if dims.len() > MAX_LAYOUT_NDIMS {
        return Err(FormatError::InvalidChunkDimensions(
            "dimensionality is too large".into(),
@@ -72,7 +72,7 @@ pub enum DataLayout {
    /// Chunked: data stored in chunks via a B-tree.
    Chunked {
        /// Chunk dimension sizes.
-        chunk_dimensions: Vec<u32>,
+        chunk_dimensions: Vec<u64>,
        /// B-tree address, or `None` if undefined.
        btree_address: Option<u64>,
        /// Layout version (3 or 4). Version 1/2 messages (HDF5 1.4/1.6-era)
@@ -404,11 +404,11 @@ impl DataLayout {
            _ => return Err(FormatError::InvalidLayoutClass(layout_class)),
        };
        ensure_len(data, p, dimensionality * 4)?;
-        let dims: Vec<u32> = data[p..p + dimensionality * 4]
+        let dims: Vec<u64> = data[p..p + dimensionality * 4]
            .as_chunks::<4>()
            .0
            .iter()
-            .map(|c| u32::from_le_bytes(*c))
+            .map(|c| u64::from(u32::from_le_bytes(*c)))
            .collect();
        p += dimensionality * 4;
        match layout_class {
@@ -424,7 +424,7 @@ impl DataLayout {
            1 => {
                let size = dims
                    .iter()
-                    .try_fold(1u64, |acc, &d| acc.checked_mul(d as u64))
+                    .try_fold(1u64, |acc, &d| acc.checked_mul(d))
                    .ok_or_else(|| {
                        FormatError::Overflow(format!("contiguous layout size {dims:?}"))
                    })?;
@@ -489,7 +489,12 @@ impl DataLayout {
                ensure_len(data, p, dimensionality * 4)?;
                let mut chunk_dimensions = Vec::with_capacity(dimensionality);
                for _ in 0..dimensionality {
-                    let dim = u32::from_le_bytes([data[p], data[p + 1], data[p + 2], data[p + 3]]);
+                    let dim = u64::from(u32::from_le_bytes([
                        data[p],
                        data[p + 1],
                        data[p + 2],
                        data[p + 3],
                    ]));
                    chunk_dimensions.push(dim);
                    p += 4;
                }
@@ -565,14 +570,8 @@ impl DataLayout {
                        .iter()
                        .rev()
                        .fold(0u64, |acc, &b| (acc << 8) | u64::from(b));
-                    // Chunk dimensions are held as u32; HDF5 2.0 can write
+                    // HDF5 2.0 writes dimensions of 2^32 or more (layout
-                    // larger ones (layout version 5), which are refused
+                    // version 5, chunks over 4 GiB) in 5 to 8 bytes.
                    // rather than truncated.
                    let val = u32::try_from(val).map_err(|_| {
                        FormatError::InvalidChunkDimensions(format!(
                            "chunk dimension {val} is larger than 2^32 - 1, which is not supported"
                        ))
                    })?;
                    chunk_dimensions.push(val);
                    p += dim_size_encoded_length;
                }
@@ -858,7 +857,7 @@ mod tests {
                .unwrap_or_else(|e| panic!("width {width}: {e:?}"));
            assert!(
                matches!(&layout, DataLayout::Chunked { chunk_dimensions, .. }
-                    if chunk_dimensions.iter().map(|&d| u64::from(d)).eq(dims)),
+                    if chunk_dimensions[..] == dims),
                "width {width}: {layout:?}"
            );
        }
@@ -871,13 +870,14 @@ mod tests {
                )
            );
        }
-        // A dimension past u32 cannot be represented and is refused, not
+        // HDF5 2.0 writes dimensions of 2^32 or more (up to 8 bytes each).
-        // truncated.
+        for (width, dim) in [(5u8, (1u64 << 32) + 3), (8, u64::MAX)] {
            assert!(matches!(
-            DataLayout::parse(&v4_chunked_msg(5, &[1 << 32, 8]), 8, 8),
+                DataLayout::parse(&v4_chunked_msg(width, &[dim, 1]), 8, 8),
-            Err(FormatError::InvalidChunkDimensions(m)) if m.contains("2^32")
+                Ok(DataLayout::Chunked { chunk_dimensions, .. }) if chunk_dimensions == [dim, 1]
            ));
        }
    }
    #[test]
    fn v1v2_rejects_bad_class_dimensionality_and_truncation() {
@@ -14,7 +14,7 @@ use crate::chunked_write::{
 /// Serialize a v4 Extensible Array layout message.
 pub(crate) fn serialize_v4_extensible_array(
-    chunk_dims: &[u32],
+    chunk_dims: &[u64],
    ea_address: u64,
    offset_size: u8,
    element_size: u32,
@@ -454,7 +454,7 @@ pub fn read_extensible_array_chunks(
    header: &ExtensibleArrayHeader,
    dataset_dims: &[u64],
    max_dims: Option<&[u64]>,
-    chunk_dimensions: &[u32],
+    chunk_dimensions: &[u64],
    element_size: u32,
    offset_size: u8,
    length_size: u8,
@@ -480,7 +480,7 @@ pub fn read_extensible_array_chunks_in<S: Storage + ?Sized>(
    header: &ExtensibleArrayHeader,
    dataset_dims: &[u64],
    max_dims: Option<&[u64]>,
-    chunk_dimensions: &[u32],
+    chunk_dimensions: &[u64],
    element_size: u32,
    offset_size: u8,
    _length_size: u8,
@@ -489,12 +489,12 @@ pub fn read_extensible_array_chunks_in<S: Storage + ?Sized>(
    // Linear indexes follow the maximum dimensions, with the unlimited
    // dimension swizzled to the slowest position (see `chunk_grid`).
-    let dims_u64: Vec<u64> = chunk_dimensions.iter().map(|&d| d as u64).collect();
+    let grid = ChunkGrid::extensible_array(dataset_dims, max_dims, chunk_dimensions)?;
    let grid = ChunkGrid::extensible_array(dataset_dims, max_dims, &dims_u64)?;
    let grid = &grid;
-    let chunk_byte_size: u64 =
+    let chunk_byte_size: u64 = chunk_dimensions
-        chunk_dimensions.iter().map(|&d| d as u64).product::<u64>() * element_size as u64;
+        .iter()
        .fold(u64::from(element_size), |acc, &d| acc.saturating_mul(d));
    // Parse index block (EAIB): signature(4) + version(1) + client_id(1)
    // + header address(offset_size), then the inline elements, then the
@@ -930,7 +930,7 @@ mod tests {
        let header = ExtensibleArrayHeader::parse(&file_data, aehd_offset, os, ls).unwrap();
        let ds_dims = vec![40u64]; // 2 chunks × 20 elements
-        let chunk_dims = vec![20u32];
+        let chunk_dims = vec![20u64];
        let chunks = read_extensible_array_chunks(
            &file_data,
            &header,
@@ -1057,7 +1057,7 @@ mod tests {
        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 chunk_dims = vec![10u64];
        let chunks = read_extensible_array_chunks(
            &file_data,
            &header,
@@ -3,15 +3,14 @@
 //! Produces valid HDF5 files with v3 superblock, v2 object headers,
 //! link messages, contiguous datasets, inline and dense attributes.
-use crate::addr::saturating_usize;
+use crate::addr::{saturating_usize, to_usize};
 #[cfg(not(feature = "std"))]
 use alloc::{format, string::String, vec, vec::Vec};
 use crate::attribute::AttributeMessage;
 use crate::btree_v2_write::{BTreeV2Params, build_btree_v2};
 use crate::chunked_write::{
-    ChunkOptions, PrecompressedChunks, build_chunked_data_from_precompressed_libver,
+    ChunkOptions, Piece, PreparedChunks, place_chunked_data, prepare_chunks,
    precompress_chunks,
 };
 use crate::data_layout::VdsMapping;
 use crate::dataspace::{Dataspace, DataspaceType};
@@ -1612,7 +1611,30 @@ impl FileWriter {
        self
    }
    /// The file, in memory. Its datasets' data is copied into the buffer
    /// once (a dataset stored as one chunk of its own shape, or unfiltered
    /// chunks that are contiguous runs of its data, are not copied before);
    /// to write a file without holding it all, use [`Self::finish_with`].
    pub fn finish(self) -> Result<Vec<u8>, FormatError> {
        let mut buf = Vec::new();
        self.finish_into(&mut buf)?;
        Ok(buf)
    }
    /// The file, handed to `put` in order, a piece at a time, without
    /// assembling it in memory: an unfiltered chunk or contiguous dataset
    /// is passed straight from the data the dataset was given. Writing a
    /// dataset stored as one unfiltered chunk this way needs little memory
    /// beyond the dataset's own data. `put`'s first error stops the write
    /// and is returned.
    pub fn finish_with<E: From<FormatError>>(
        self,
        put: impl FnMut(&[u8]) -> Result<(), E>,
    ) -> Result<(), E> {
        self.finish_into(&mut FnSink(put))
    }
    fn finish_into<S: Sink>(self, sink: &mut S) -> Result<(), S::Error> {
        let page_size = self.page_size;
        if let Some(ps) = page_size
            && !(MIN_FILE_SPACE_PAGE_SIZE..=MAX_FILE_SPACE_PAGE_SIZE).contains(&ps)
@@ -1620,7 +1642,8 @@ impl FileWriter {
            return Err(FormatError::SerializationError(format!(
                "file space page size {ps} is outside libhdf5's \
                 {MIN_FILE_SPACE_PAGE_SIZE}..={MAX_FILE_SPACE_PAGE_SIZE} bytes"
-            )));
+            ))
            .into());
        }
        let (low, high) = (self.low, self.high);
@@ -1774,15 +1797,16 @@ impl FileWriter {
            })
            .collect::<Result<_, _>>()?;
-        struct DataBlob {
+        struct DataBlob<'a> {
            data: Vec<u8>,
            oh_bytes: Vec<u8>,
            /// Cached compressed chunks for chunked datasets; reused in Pass 2
-            /// to avoid re-compressing the same data.
+            /// to avoid re-compressing the same data. Unfiltered chunks
-            precompressed: Option<PrecompressedChunks>,
+            /// borrow the dataset's data where they can.
            precompressed: Option<PreparedChunks<'a>>,
        }
-        let mut dummy_blobs: Vec<DataBlob> = Vec::new();
+        let mut dummy_blobs: Vec<DataBlob<'_>> = Vec::new();
        let mut dummy_cursor = 0u64;
        for (i, d) in all_ds.iter().enumerate() {
            let dense_blob = ds_dense[i]
@@ -1820,21 +1844,16 @@ impl FileWriter {
                    .resolve_chunk_dims_for(&d.ds.dimensions, elem_size);
                // Compress once in Pass 1; cache the result so Pass 2 can skip
                // re-compression and just rebuild the index with real addresses.
-                let pre = precompress_chunks(
+                let pre = prepare_chunks(
                    &d.raw,
                    &d.ds.dimensions,
                    &chunk_dims,
                    elem_size,
                    &d.chunk_options,
                )?;
-                let result = build_chunked_data_from_precompressed_libver(
+                let result =
-                    &pre,
+                    place_chunked_data(&pre, dummy_cursor, d.maxshape.as_deref(), low, high)?;
-                    dummy_cursor,
+                dummy_cursor += result.len;
                    d.maxshape.as_deref(),
                    low,
                    high,
                )?;
                dummy_cursor += result.data_bytes.len() as u64;
                let oh = build_chunked_dataset_oh(
                    &d.dt,
                    &d.ds,
@@ -1849,7 +1868,7 @@ impl FileWriter {
                    d.refcount,
                )?;
                dummy_blobs.push(DataBlob {
-                    data: result.data_bytes,
+                    data: Vec::new(),
                    oh_bytes: oh,
                    precompressed: Some(pre),
                });
@@ -1955,7 +1974,7 @@ impl FileWriter {
            })
            .collect::<Result<_, FormatError>>()?;
-        let mut ds_blobs2: Vec<DataBlob> = Vec::new();
+        let mut ds_blobs2: Vec<OutBlob<'_>> = Vec::new();
        let global_align_threshold = self.alignment_threshold;
        let global_align_bytes = self.alignment_bytes;
        for (i, d) in all_ds.iter().enumerate() {
@@ -1977,26 +1996,21 @@ impl FileWriter {
                    &d.fill_message,
                    d.refcount,
                )?;
-                ds_blobs2.push(DataBlob {
+                ds_blobs2.push(OutBlob {
-                    data: gcol_bytes.clone(),
+                    data: vec![Out::Slice(gcol_bytes)],
                    oh_bytes: oh,
                    precompressed: None,
                });
            } else if is_chunked[i] {
                let base_address = cursor2 as u64;
                // Reuse precompressed chunks from Pass 1 — avoids re-compressing
                // the same data a second time.
-                let result = build_chunked_data_from_precompressed_libver(
+                let pre = dummy_blobs[i]
                    dummy_blobs[i]
                    .precompressed
                    .as_ref()
-                        .expect("chunked dataset missing precompressed cache"),
+                    .expect("chunked dataset missing precompressed cache");
-                    base_address,
+                let result =
-                    d.maxshape.as_deref(),
+                    place_chunked_data(pre, base_address, d.maxshape.as_deref(), low, high)?;
-                    low,
+                cursor2 += to_usize(result.len)?;
                    high,
                )?;
                cursor2 += result.data_bytes.len();
                let oh = build_chunked_dataset_oh(
                    &d.dt,
                    &d.ds,
@@ -2010,11 +2024,16 @@ impl FileWriter {
                    &d.fill_message,
                    d.refcount,
                )?;
-                ds_blobs2.push(DataBlob {
+                let data = result
-                    data: result.data_bytes,
+                    .pieces
-                    oh_bytes: oh,
+                    .into_iter()
-                    precompressed: None,
+                    .map(|p| match p {
-                });
+                        Piece::Zeros(n) => Out::Zeros(n),
                        Piece::Chunk(c) => Out::Slice(&pre.chunks[c].1),
                        Piece::Bytes(b) => Out::Bytes(b),
                    })
                    .collect();
                ds_blobs2.push(OutBlob { data, oh_bytes: oh });
            } else if is_compact[i] {
                // Compact: data is inline in the object header, no external blob
                let oh = build_compact_dataset_oh(
@@ -2030,10 +2049,9 @@ impl FileWriter {
                    d.refcount,
                    layout_version,
                )?;
-                ds_blobs2.push(DataBlob {
+                ds_blobs2.push(OutBlob {
                    data: vec![],
                    oh_bytes: oh,
                    precompressed: None,
                });
            } else {
                // Determine alignment: per-dataset overrides global
@@ -2060,13 +2078,10 @@ impl FileWriter {
                    d.refcount,
                    layout_version,
                )?;
                let mut data = vec![0u8; padding];
                data.extend_from_slice(&d.raw);
                cursor2 += d.raw.len();
-                ds_blobs2.push(DataBlob {
+                ds_blobs2.push(OutBlob {
-                    data,
+                    data: vec![Out::Zeros(padding), Out::Slice(&d.raw)],
                    oh_bytes: oh,
                    precompressed: None,
                });
            }
        }
@@ -2080,7 +2095,8 @@ impl FileWriter {
            cursor2 = cursor2.next_multiple_of(ps as usize);
        }
        let eof_addr2 = cursor2 as u64;
-        let mut buf = Vec::with_capacity(cursor2);
+        sink.reserve(cursor2);
        let mut out = Counted { sink, written: 0 };
        let sb = Superblock {
            version: superblock_version,
@@ -2103,9 +2119,9 @@ impl FileWriter {
            checksum: None,
            page_size: None,
        };
-        buf.extend_from_slice(&sb.serialize());
+        out.put(&sb.serialize())?;
        if let Some(ref ext) = sb_ext {
-            buf.extend_from_slice(ext);
+            out.put(ext)?;
        }
        // Group OHs + dense blobs (link blob, then attr blob, matching pass 2)
@@ -2132,32 +2148,120 @@ impl FileWriter {
                g.refcount,
            )?;
            debug_assert_eq!(oh.len(), group_oh_sizes[gi]);
-            debug_assert_eq!(buf.len() as u64, group_addrs2[gi]);
+            debug_assert_eq!(out.written as u64, group_addrs2[gi]);
-            buf.extend_from_slice(&oh);
+            out.put(&oh)?;
            if let Some(ref b) = link_blob {
-                buf.extend_from_slice(&b.blob);
+                out.put(&b.blob)?;
            }
            if let Some(ref blob) = group_dense_blobs[gi] {
-                buf.extend_from_slice(&blob.blob);
+                out.put(&blob.blob)?;
            }
        }
        // Dataset OHs + dense blobs
        for (i, blob) in ds_blobs2.iter().enumerate() {
-            buf.extend_from_slice(&blob.oh_bytes);
+            out.put(&blob.oh_bytes)?;
            if let Some(ref dense) = ds_dense_blobs[i] {
-                buf.extend_from_slice(&dense.blob);
+                out.put(&dense.blob)?;
            }
        }
        // Data
        for blob in &ds_blobs2 {
-            buf.extend_from_slice(&blob.data);
+            for piece in &blob.data {
                match piece {
                    Out::Bytes(b) => out.put(b)?,
                    Out::Slice(s) => out.put(s)?,
                    Out::Zeros(n) => out.zeros(*n)?,
                }
            }
        }
-        debug_assert_eq!(buf.len(), data_end);
+        debug_assert_eq!(out.written, data_end);
-        buf.resize(cursor2, 0);
+        out.zeros(cursor2 - data_end)?;
-        Ok(buf)
+        Ok(())
    }
 }
 /// A piece of a dataset's data as [`FileWriter`] writes it out.
 enum Out<'a> {
    Bytes(Vec<u8>),
    /// Borrowed from the dataset's data or its prepared chunks.
    Slice(&'a [u8]),
    Zeros(usize),
 }
 /// A dataset's object header and the pieces of its data.
 struct OutBlob<'a> {
    oh_bytes: Vec<u8>,
    data: Vec<Out<'a>>,
 }
 /// Where [`FileWriter`] puts a file's bytes, in order.
 trait Sink {
    type Error: From<FormatError>;
    /// The file will be `_len` bytes long.
    fn reserve(&mut self, _len: usize) {}
    fn put(&mut self, bytes: &[u8]) -> Result<(), Self::Error>;
    fn zeros(&mut self, n: usize) -> Result<(), Self::Error> {
        const ZEROS: [u8; 4096] = [0; 4096];
        let mut left = n;
        while left > 0 {
            let k = left.min(ZEROS.len());
            self.put(&ZEROS[..k])?;
            left -= k;
        }
        Ok(())
    }
 }
 impl Sink for Vec<u8> {
    type Error = FormatError;
    fn reserve(&mut self, len: usize) {
        self.reserve_exact(len.saturating_sub(self.len()));
    }
    fn put(&mut self, bytes: &[u8]) -> Result<(), FormatError> {
        self.extend_from_slice(bytes);
        Ok(())
    }
    fn zeros(&mut self, n: usize) -> Result<(), FormatError> {
        self.resize(self.len() + n, 0);
        Ok(())
    }
 }
 /// A [`Sink`] calling a function ([`FileWriter::finish_with`]).
 struct FnSink<F>(F);
 impl<E: From<FormatError>, F: FnMut(&[u8]) -> Result<(), E>> Sink for FnSink<F> {
    type Error = E;
    fn put(&mut self, bytes: &[u8]) -> Result<(), E> {
        (self.0)(bytes)
    }
 }
 /// A [`Sink`] and the number of bytes put into it.
 struct Counted<'s, S> {
    sink: &'s mut S,
    written: usize,
 }
 impl<S: Sink> Counted<'_, S> {
    fn put(&mut self, bytes: &[u8]) -> Result<(), S::Error> {
        self.written += bytes.len();
        self.sink.put(bytes)
    }
    fn zeros(&mut self, n: usize) -> Result<(), S::Error> {
        self.written += n;
        self.sink.zeros(n)
    }
 }
@@ -3015,4 +3119,75 @@ mod tests {
        assert_eq!(Superblock::parse(&bytes, 0).unwrap().version, 3);
        assert_eq!(layout_of(&bytes, "x")[0], 3);
    }
    /// `finish_with` hands out exactly the bytes `finish` returns, for every
    /// storage (contiguous, compact, chunked filtered and not, chunks
    /// borrowed and copied, a version-1 B-tree, a paged file), and stops at
    /// the sink's first error.
    #[test]
    fn finish_with_matches_finish() {
        let build = |low: LibVer, paged: bool| {
            let mut fw = FileWriter::new();
            fw.libver_bounds(low, LibVer::Latest);
            if paged {
                fw.with_page_size(4096);
            }
            let v: Vec<f64> = (0..300).map(f64::from).collect();
            fw.create_dataset("contig").with_f64_data(&v);
            fw.create_dataset("compact")
                .with_f64_data(&[1.0, 2.0])
                .compact();
            fw.create_dataset("one_chunk")
                .with_f64_data(&v)
                .with_shape(&[30, 10])
                .with_chunks(&[30, 10]);
            fw.create_dataset("rows")
                .with_f64_data(&v)
                .with_shape(&[30, 10])
                .with_chunks(&[7, 10]);
            fw.create_dataset("tiles")
                .with_f64_data(&v)
                .with_shape(&[30, 10])
                .with_chunks(&[7, 3]);
            fw.create_dataset("deflated")
                .with_f64_data(&v)
                .with_chunks(&[64])
                .with_shuffle()
                .with_deflate(4);
            fw.create_dataset("grows")
                .with_u8_data_owned((0..=255).collect())
                .with_chunks(&[100])
                .with_maxshape(&[u64::MAX]);
            fw
        };
        for (low, paged) in [
            (LibVer::V18, false),
            (LibVer::V110, false),
            (LibVer::Latest, true),
        ] {
            let bytes = build(low, paged).finish().unwrap();
            let mut streamed = Vec::new();
            build(low, paged)
                .finish_with(|b: &[u8]| {
                    streamed.extend_from_slice(b);
                    Ok::<(), FormatError>(())
                })
                .unwrap();
            assert_eq!(streamed, bytes, "{low:?} paged {paged}");
        }
        let mut calls = 0;
        let err = build(LibVer::V110, false)
            .finish_with(|_| {
                calls += 1;
                if calls == 3 {
                    Err(FormatError::SerializationError("full".into()))
                } else {
                    Ok(())
                }
            })
            .unwrap_err();
        assert_eq!(err, FormatError::SerializationError("full".into()));
        assert_eq!(calls, 3);
    }
 }
@@ -155,7 +155,7 @@ pub fn read_fixed_array_chunks(
    header: &FixedArrayHeader,
    dataset_dims: &[u64],
    max_dims: Option<&[u64]>,
-    chunk_dimensions: &[u32],
+    chunk_dimensions: &[u64],
    element_size: u32,
    offset_size: u8,
    length_size: u8,
@@ -180,7 +180,7 @@ pub fn read_fixed_array_chunks_in<S: Storage + ?Sized>(
    header: &FixedArrayHeader,
    dataset_dims: &[u64],
    max_dims: Option<&[u64]>,
-    chunk_dimensions: &[u32],
+    chunk_dimensions: &[u64],
    element_size: u32,
    offset_size: u8,
    _length_size: u8,
@@ -227,11 +227,11 @@ pub fn read_fixed_array_chunks_in<S: Storage + ?Sized>(
    // The index is laid out over the chunk grid of the *maximum* dimensions
    // (row-major), so a dataset smaller than its maxshape has gaps.
-    let dims_u64: Vec<u64> = chunk_dimensions.iter().map(|&d| d as u64).collect();
+    let grid = ChunkGrid::fixed_array(dataset_dims, max_dims, chunk_dimensions)?;
    let grid = ChunkGrid::fixed_array(dataset_dims, max_dims, &dims_u64)?;
-    let chunk_byte_size: u64 =
+    let chunk_byte_size: u64 = chunk_dimensions
-        chunk_dimensions.iter().map(|&d| d as u64).product::<u64>() * element_size as u64;
+        .iter()
        .fold(u64::from(element_size), |acc, &d| acc.saturating_mul(d));
    let mut chunks = Vec::new();
    // `rel` is relative to the data block, whose bytes are in `w`.
@@ -670,7 +670,7 @@ mod tests {
        let header =
            FixedArrayHeader::parse(&file_data, fahd_offset, offset_size, length_size).unwrap();
        let ds_dims = vec![100u64];
-        let chunk_dims = vec![20u32];
+        let chunk_dims = vec![20u64];
        let chunks = read_fixed_array_chunks(
            &file_data,
            &header,
@@ -747,7 +747,7 @@ mod tests {
        let header =
            FixedArrayHeader::parse(&file_data, fahd_offset, offset_size, length_size).unwrap();
        let ds_dims = vec![60u64];
-        let chunk_dims = vec![20u32];
+        let chunk_dims = vec![20u64];
        let chunks = read_fixed_array_chunks(
            &file_data,
            &header,
@@ -848,7 +848,7 @@ mod tests {
        assert_eq!(header.num_elements, 11);
        let ds_dims = vec![11u64 * 20];
-        let chunk_dims = vec![20u32];
+        let chunk_dims = vec![20u64];
        let chunks = read_fixed_array_chunks(
            &file_data,
            &header,
@@ -663,11 +663,17 @@ impl DatasetBuilder {
    }
    pub fn with_u8_data(&mut self, data: &[u8]) -> &mut Self {
        self.with_u8_data_owned(data.to_vec())
    }
    /// [`Self::with_u8_data`] without the copy: the builder takes the
    /// vector. For large data this halves the memory a write needs.
    pub fn with_u8_data_owned(&mut self, data: Vec<u8>) -> &mut Self {
        self.datatype = Some(make_u8_type());
        self.data = Some(data.to_vec());
        if self.shape.is_none() {
            self.shape = Some(vec![data.len() as u64]);
        }
        self.data = Some(data);
        self
    }
@@ -189,12 +189,7 @@ pub(crate) fn chunk_shape(file: &File, hdr: &ObjectHeader, rank: usize) -> Optio
    {
        clawhdf5_format::data_layout::DataLayout::Chunked {
            chunk_dimensions, ..
-        } if chunk_dimensions.len() >= rank => Some(
+        } if chunk_dimensions.len() >= rank => Some(chunk_dimensions[..rank].to_vec()),
            chunk_dimensions[..rank]
                .iter()
                .map(|&d| u64::from(d))
                .collect(),
        ),
        _ => None,
    }
 }
@@ -873,9 +873,9 @@ impl Checker<'_> {
            self.counts.chunk_index_checksummed += 1;
        }
        let filtered = matches!(&info.filters, Ok(Some(p)) if !p.filters.is_empty());
-        let chunk_bytes = cdims.iter().try_fold(u64::from(dt.type_size()), |a, &d| {
+        let chunk_bytes = cdims
-            a.checked_mul(u64::from(d))
+            .iter()
-        });
+            .try_fold(u64::from(dt.type_size()), |a, &d| a.checked_mul(d));
        let max = ds.max_dimensions.clone();
        let mut seen: HashSet<Vec<u64>> = HashSet::with_capacity(chunks.len().min(1 << 20));
        let mut reported = 0usize;
@@ -893,7 +893,7 @@ impl Checker<'_> {
                ));
            } else {
                for (i, (&o, &cd)) in c.offsets.iter().zip(cdims).enumerate() {
-                    if o % u64::from(cd) != 0 {
+                    if o % cd != 0 {
                        bad.push(format!(
                            "offset {o} in dimension {i} is not a multiple of the chunk size {cd}"
                        ));
@@ -292,7 +292,7 @@ impl Ls<'_> {
                    .unwrap_or(0);
                let bytes = dims
                    .iter()
-                    .try_fold(esize, |a, &d| a.checked_mul(u64::from(d)))
+                    .try_fold(esize, |a, &d| a.checked_mul(d))
                    .map(|b| b.to_string())
                    .unwrap_or_else(|| "?".into());
                writeln!(
@@ -0,0 +1,46 @@
 //! Write one dataset of `u8` stored as a single chunk, for measuring the
 //! writer's peak memory (`/usr/bin/time -v`):
 //!
 //! ```text
 //! cargo run --release -p clawhdf5 --example write_one_chunk -- BYTES OUT.h5 [slice|owned] [deflate|lz4|zstd]
 //! ```
 //!
 //! `slice` passes the data by reference (`with_u8_data`, which copies it),
 //! `owned` hands the builder the vector (`with_u8_data_owned`). The data is
 //! `i % 251`, one byte per element; with no filter the chunk is stored raw.
 fn main() {
    let args: Vec<String> = std::env::args().collect();
    let n: u64 = args[1].parse().expect("BYTES");
    let out = &args[2];
    let mode = args.get(3).map_or("slice", String::as_str);
    let filter = args.get(4).map(String::as_str);
    let data: Vec<u8> = (0..n).map(|i| (i % 251) as u8).collect();
    let mut b = clawhdf5::FileBuilder::new();
    let ds = b.create_dataset("x");
    match mode {
        "slice" => {
            ds.with_u8_data(&data);
        }
        "owned" => {
            ds.with_u8_data_owned(data);
        }
        m => panic!("mode {m}"),
    }
    ds.with_chunks(&[n]);
    match filter {
        None => {}
        Some("deflate") => {
            ds.with_deflate(1);
        }
        #[cfg(feature = "lz4")]
        Some("lz4") => {
            ds.with_lz4();
        }
        #[cfg(feature = "zstd")]
        Some("zstd") => {
            ds.with_zstd(1);
        }
        Some(f) => panic!("filter {f} (not built in?)"),
    }
    b.write(out).unwrap();
 }
@@ -408,10 +408,7 @@ impl<'t> ChunkedEdit<'t> {
                "chunk rank differs from the dataspace".into(),
            ));
        }
-        let cd: Vec<u64> = chunk_dimensions[..rank]
+        let cd: Vec<u64> = chunk_dimensions[..rank].to_vec();
            .iter()
            .map(|&c| u64::from(c))
            .collect();
        // Chunks of 4 GiB or more (HDF5 2.0 writes them with layout message
        // version 5) are read, but not rewritten: the editor holds a chunk
        // it rewrites in memory, compressed and not. Writing values, and a
@@ -137,10 +137,15 @@ impl FileBuilder {
        Ok(self.writer.finish()?)
    }
-    /// Serialize and write the file to the given path.
+    /// Serialize and write the file to the given path. The file is written
    /// as it is serialized, not assembled in memory first: unfiltered
    /// chunks and contiguous data go straight from the datasets' data.
    pub fn write<P: AsRef<std::path::Path>>(self, path: P) -> Result<(), Error> {
-        let bytes = self.finish()?;
+        use std::io::Write;
-        write_file_atomically(path.as_ref(), &bytes).map_err(Error::Io)
+        write_file_atomically_with(path.as_ref(), |f| {
            self.writer
                .finish_with(|bytes| f.write_all(bytes).map_err(Error::Io))
        })
    }
 }
@@ -259,8 +264,19 @@ pub fn create_datasets_parallel(specs: Vec<DatasetSpec>) -> Result<Vec<u8>, Erro
 /// file or the complete new one — never a truncated mix. `std::fs::write`
 /// truncates the destination first, so dying mid-write used to destroy the
 /// existing file.
 #[cfg(test)]
 fn write_file_atomically(path: &std::path::Path, bytes: &[u8]) -> std::io::Result<()> {
    use std::io::Write;
    write_file_atomically_with(path, |f| f.write_all(bytes))
 }
 /// [`write_file_atomically`] with the contents written by `write` (into a
 /// buffered writer over the temporary file).
 fn write_file_atomically_with<E: From<std::io::Error>>(
    path: &std::path::Path,
    write: impl FnOnce(&mut std::io::BufWriter<std::fs::File>) -> Result<(), E>,
 ) -> Result<(), E> {
    use std::io::Write;
    // Same directory as the target, so the rename stays on one filesystem.
    let mut tmp_name = path
@@ -272,11 +288,13 @@ fn write_file_atomically(path: &std::path::Path, bytes: &[u8]) -> std::io::Resul
    tmp_name.push(format!(".tmp-{}", std::process::id()));
    let tmp_path = path.with_file_name(tmp_name);
-    let result = (|| {
+    let result = (|| -> Result<(), E> {
-        let mut f = std::fs::File::create(&tmp_path)?;
+        let mut f = std::io::BufWriter::with_capacity(1 << 20, std::fs::File::create(&tmp_path)?);
-        f.write_all(bytes)?;
+        write(&mut f)?;
-        f.sync_all()?;
+        f.flush()?;
-        std::fs::rename(&tmp_path, path)
+        f.get_ref().sync_all()?;
        std::fs::rename(&tmp_path, path)?;
        Ok(())
    })();
    if result.is_err() {
        let _ = std::fs::remove_file(&tmp_path);
@@ -2,6 +2,12 @@
    python gen_huge_chunks.py filtered OUT.h5     # huge_chunks_filtered.h5
    python gen_huge_chunks.py unfiltered OUT.h5   # generated at test time
    python gen_huge_chunks.py dims OUT.h5         # huge_chunk_dims.h5
    python gen_huge_chunks.py dims-unfiltered OUT.h5   # generated at test time
 `dims` and `dims-unfiltered` (see `make_dims`) hold `u1` datasets whose
 chunk dimension is N8 = 2**32 + 7: a chunk dimension of 2**32 or more,
 which a layout message of version 5 stores in 5 bytes.
 libhdf5 2.x writes a chunk of more than 0xFFFFFFFF bytes with layout message
 version 5 (`H5D__chunk_construct`: "chunk size > 4GB requires
@@ -38,7 +44,8 @@ allocated early (see `make`).
 libhdf5 holds a whole filtered chunk in memory while it writes it, so the
 `filtered` run needs about 4 GiB of memory and 100 s (tank).
-Generated 2026-09-28 with h5py 3.16.0 (libhdf5 2.0.0).
+huge_chunks_filtered.h5 was generated 2026-09-28 and huge_chunk_dims.h5
 2026-09-29, both with h5py 3.16.0 (libhdf5 2.0.0).
 """
 import sys
@@ -92,12 +99,60 @@ def make(fid, name, index, filtered):
    dsid.close()
 N8 = 2**32 + 7
 def make_dims(fid, name, index, filtered):
    """A `u1` dataset whose chunks are N8 elements (a chunk dimension of
    2**32 or more). `d[0:10] = 0..9` and `d[N8-10:N8] = 100..109`; where
    there is a second chunk (`farray`, `earray`: shape N8 + 10),
    `d[N8:N8+10] = 200..209`.
    filtered (`dims`, committed): deflate level 9 twice, fill value 7;
    `single` (Single Chunk) and `earray` (Extensible Array, maxshape None).
    Writing it holds a 4 GiB chunk: 4.1 GiB peak resident memory and about
    a minute (tank, 2026-09-29).
    unfiltered (`dims-unfiltered`): fill time "never", sparse; `single`
    (allocated early, as in `make`) and `farray` (Fixed Array).
    """
    dcpl = h5py.h5p.create(h5py.h5p.DATASET_CREATE)
    if index == "single":
        shape, maxshape = (N8,), (N8,)
    elif index == "farray":
        shape, maxshape = (N8 + 10,), (N8 + 10,)
    elif index == "earray":
        shape, maxshape = (N8 + 10,), (UNLIM,)
    dcpl.set_chunk((N8,))
    if filtered:
        dcpl.set_deflate(9)
        dcpl.set_deflate(9)
        dcpl.set_fill_value(np.array(7, dtype="u1"))
    else:
        dcpl.set_fill_time(h5py.h5d.FILL_TIME_NEVER)
        if index == "single":
            dcpl.set_alloc_time(h5py.h5d.ALLOC_TIME_EARLY)
    space = h5py.h5s.create_simple(shape, maxshape)
    dsid = h5py.h5d.create(fid, name.encode(), h5py.h5t.STD_U8LE, space, dcpl=dcpl)
    ds = h5py.Dataset(dsid)
    ds[0:10] = np.arange(10, dtype="u1")
    ds[N8 - 10 : N8] = np.arange(100, 110, dtype="u1")
    if shape[0] > N8:
        ds[N8 : N8 + 10] = np.arange(200, 210, dtype="u1")
    dsid.close()
 def main():
    mode, out = sys.argv[1], sys.argv[2]
    filtered = {"filtered": True, "unfiltered": False}[mode]
    fapl = h5py.h5p.create(h5py.h5p.FILE_ACCESS)
    fapl.set_libver_bounds(h5py.h5f.LIBVER_EARLIEST, h5py.h5f.LIBVER_V200)
    fid = h5py.h5f.create(out.encode(), h5py.h5f.ACC_TRUNC, fapl=fapl)
    if mode in ("dims", "dims-unfiltered"):
        filtered = mode == "dims"
        for index in ["single", "earray"] if filtered else ["single", "farray"]:
            make_dims(fid, index, index, filtered)
        fid.close()
        return
    filtered = {"filtered": True, "unfiltered": False}[mode]
    indexes = ["single", "farray", "earray", "btree2"]
    if not filtered:
        indexes.insert(1, "implicit")
@@ -81,8 +81,15 @@ fn layout_of(
    let lm = msg(MessageType::DataLayout);
    let layout = DataLayout::parse(&lm.data, sb.offset_size, sb.length_size).unwrap();
    let space = Dataspace::parse(&msg(MessageType::Dataspace).data, sb.length_size).unwrap();
    // The element size is the layout's last dimension.
    let elem = match &layout {
        DataLayout::Chunked {
            chunk_dimensions, ..
        } => *chunk_dimensions.last().unwrap() as usize,
        _ => 8,
    };
    let (chunks, _) =
-        list_chunks(data, &layout, &space, 8, sb.offset_size, sb.length_size).unwrap();
+        list_chunks(data, &layout, &space, elem, sb.offset_size, sb.length_size).unwrap();
    (lm.data[0], layout, space, chunks)
 }
@@ -516,3 +523,343 @@ print("ok")
        }
    }
 }
 // ---- Chunk dimensions of 2^32 or more ----
 /// Elements per chunk of the `u8` datasets below: a chunk dimension of
 /// 2^32 or more (4 GiB + 7 bytes), which a version-5 layout message stores
 /// in 5 bytes.
 const N8: u64 = (1 << 32) + 7;
 fn dims_fixture() -> PathBuf {
    Path::new(env!("CARGO_MANIFEST_DIR")).join("tests/fixtures/huge_chunk_dims.h5")
 }
 /// `u8` values of `sel` in dataset `name` of `file`.
 fn sel_u8(file: &File, name: &str, start: u64, count: u64) -> Vec<u8> {
    let ds = file.dataset(name).unwrap();
    let s = Selection::Hyperslab {
        start: vec![start],
        stride: vec![1],
        count: vec![count],
        block: vec![1],
    };
    ds.read_selection(&s).unwrap()
 }
 fn u8s(range: std::ops::Range<u8>) -> Vec<u8> {
    range.collect()
 }
 /// `fixtures/huge_chunk_dims.h5` (libhdf5 2.0.0 through h5py 3.16,
 /// `gen_huge_chunks.py dims`): `u8` datasets chunked by N8 = 2^32 + 7, in a
 /// Single Chunk and an Extensible Array index. Their layouts parse to the
 /// whole dimension (it was refused: chunk dimensions were `u32`) and their
 /// chunks list at the right origins.
 #[test]
 fn huge_chunk_dims_list() {
    let data = std::fs::read(dims_fixture()).unwrap();
    for (name, index, shape, origins) in [
        ("single", 1u8, N8, &[0u64][..]),
        ("earray", 4, N8 + 10, &[0, N8]),
    ] {
        let (version, layout, space, mut chunks) = layout_of(&data, name);
        assert_eq!(version, 5, "{name}");
        let DataLayout::Chunked {
            chunk_dimensions,
            chunk_index_type,
            ..
        } = &layout
        else {
            panic!("{name}: {layout:?}");
        };
        assert_eq!(chunk_dimensions, &[N8, 1], "{name}");
        assert_eq!(*chunk_index_type, Some(index), "{name}");
        assert_eq!(space.dimensions, [shape], "{name}");
        chunks.sort_by_key(|c| c.offsets[0]);
        let got: Vec<u64> = chunks.iter().map(|c| c.offsets[0]).collect();
        assert_eq!(got, origins, "{name}");
    }
 }
 /// Decoding the fixture's chunks (4 GiB each, one at a time): written
 /// values, the fill value (7) around them, and the second chunk.
 #[test]
 fn huge_chunk_dims_read() {
    if !heavy() {
        return;
    }
    let file = File::open(dims_fixture()).unwrap();
    let mut first = u8s(0..10);
    first.extend([7, 7]);
    let mut last = vec![7, 7];
    last.extend(u8s(100..110));
    for name in ["single", "earray"] {
        assert_eq!(sel_u8(&file, name, 0, 12), first, "{name}");
        assert_eq!(sel_u8(&file, name, N8 - 12, 12), last, "{name}");
    }
    let mut across = u8s(108..110);
    across.extend(u8s(200..210));
    assert_eq!(sel_u8(&file, "earray", N8 - 2, 12), across);
 }
 /// Unfiltered chunks of N8 bytes that libhdf5 2.0.0 wrote (a sparse file,
 /// `gen_huge_chunks.py dims-unfiltered`), in a Single Chunk and a Fixed
 /// Array index: read through a memory map, and through positioned reads
 /// that fetch only the rows selected.
 #[test]
 fn huge_chunk_dims_unfiltered_read() {
    if !heavy() {
        return;
    }
    let dir = scratch();
    let Some(path) = generate(dir.path(), "dims-unfiltered") else {
        return;
    };
    let check = |file: &File| {
        let mut first = u8s(0..10);
        first.extend([0, 0]);
        let mut last = vec![0, 0];
        last.extend(u8s(100..110));
        for name in ["single", "farray"] {
            assert_eq!(sel_u8(file, name, 0, 12), first, "{name}");
            assert_eq!(sel_u8(file, name, N8 - 12, 12), last, "{name}");
        }
        let mut across = u8s(108..110);
        across.extend(u8s(200..210));
        assert_eq!(sel_u8(file, "farray", N8 - 2, 12), across);
    };
    check(&File::open(&path).unwrap());
    let file = std::fs::File::open(&path).unwrap();
    let len = file.metadata().unwrap().len();
    let storage = std::sync::Arc::new(Counting {
        file,
        len,
        read: 0.into(),
    });
    check(&File::open_storage(storage.clone()).unwrap());
    let read = storage.read.load(std::sync::atomic::Ordering::Relaxed);
    assert!(read < 1 << 20, "{read} bytes read");
 }
 /// Run `script` under h5py with `path` as `sys.argv[1]`; skipped without
 /// h5py (a failure under `CLAWHDF5_REQUIRE_INTEROP=1`).
 fn h5py_check(script: &str, path: &Path) {
    if !python_available() {
        assert!(!interop_required(), "h5py not available");
        eprintln!("SKIP: python3 with h5py not available");
        return;
    }
    let out = Command::new(python())
        .arg("-c")
        .arg(script)
        .arg(path)
        .output()
        .unwrap();
    assert!(
        out.status.success(),
        "h5py:\n{}",
        String::from_utf8_lossy(&out.stderr)
    );
 }
 /// `h5dump` 2.x's output for `args` on `path`, checked to succeed without
 /// errors; `None` without `CLAWHDF5_H5DUMP2`.
 fn h5dump2_text(args: &[&str], path: &Path) -> Option<String> {
    let h5dump = h5dump2()?;
    let out = Command::new(&h5dump).args(args).arg(path).output().unwrap();
    let text = format!(
        "{}{}",
        String::from_utf8_lossy(&out.stdout),
        String::from_utf8_lossy(&out.stderr)
    );
    assert!(out.status.success(), "h5dump {args:?}:\n{text}");
    assert!(!text.contains("rror"), "h5dump {args:?}:\n{text}");
    Some(text)
 }
 /// clawhdf5 writes deflated `u8` chunks of N8 elements (a chunk dimension
 /// of 2^32 or more) in a Single Chunk and an Extensible Array index; it,
 /// h5py (libhdf5 2.0.0) and h5dump 2.2.0 (layouts only: it cannot inflate
 /// chunks over 4 GiB, see known-issues) read them.
 #[test]
 fn writer_huge_chunk_dims_round_trip() {
    if !heavy() {
        return;
    }
    let dir = scratch();
    let path = dir.path().join("dims.h5");
    let values = u8s(0..10);
    let mut b = clawhdf5::FileBuilder::new();
    for (name, max) in [("single", N8), ("earray", u64::MAX)] {
        b.create_dataset(name)
            .with_u8_data(&values)
            .with_maxshape(&[max])
            .with_chunks(&[N8])
            .with_deflate(1);
    }
    b.write(&path).unwrap();
    let data = std::fs::read(&path).unwrap();
    // Deflate level 1 leaves about 40 MiB of each 4 GiB chunk of zeros.
    assert!(data.len() < 256 << 20, "{} bytes", data.len());
    for (name, index) in [("single", 1), ("earray", 4)] {
        let (version, layout, _, chunks) = layout_of(&data, name);
        assert_eq!(version, 5, "{name}");
        assert!(
            matches!(&layout, DataLayout::Chunked { chunk_dimensions, chunk_index_type: Some(t), .. }
                if *t == index && chunk_dimensions == &[N8, 1]),
            "{name}: {layout:?}"
        );
        assert_eq!(chunks.len(), 1, "{name}");
    }
    let file = File::open(&path).unwrap();
    for name in ["single", "earray"] {
        assert_eq!(sel_u8(&file, name, 0, 10), values, "{name}");
    }
    drop(file);
    h5py_check(
        r#"
 import sys, h5py
 with h5py.File(sys.argv[1], "r") as f:
    for name in ("single", "earray"):
        d = f[name]
        assert d.chunks == (2**32 + 7,), (name, d.chunks)
        assert list(d[:]) == list(range(10)), (name, d[:])
 "#,
        &path,
    );
    for name in ["single", "earray"] {
        if let Some(text) = h5dump2_text(&["-H", "-p", "-d", name], &path) {
            assert!(text.contains("CHUNKED ( 4294967303 )"), "{name}:\n{text}");
        }
    }
 }
 /// An unfiltered chunk of N8 bytes (a chunk dimension of 2^32 or more)
 /// written by clawhdf5 from data it was handed (`with_u8_data_owned`):
 /// `FileBuilder::write` streams it to the file without copying it, so the
 /// write needs about the data's own 4 GiB. Read back by clawhdf5, h5py
 /// (a few elements) and h5dump 2.2.0.
 #[test]
 fn writer_unfiltered_huge_chunk_round_trip() {
    if !heavy() {
        return;
    }
    let dir = scratch();
    let path = dir.path().join("raw.h5");
    let value = |i: u64| (i % 251) as u8;
    let mut b = clawhdf5::FileBuilder::new();
    b.create_dataset("single")
        .with_u8_data_owned((0..N8).map(value).collect())
        .with_chunks(&[N8]);
    b.write(&path).unwrap();
    let file = File::open(&path).unwrap();
    let data = file.as_bytes();
    let (version, layout, space, chunks) = layout_of(data, "single");
    assert_eq!(version, 5);
    assert!(
        matches!(&layout, DataLayout::Chunked { chunk_dimensions, chunk_index_type: Some(1), .. }
            if chunk_dimensions == &[N8, 1]),
        "{layout:?}"
    );
    assert_eq!(space.dimensions, [N8]);
    assert_eq!(chunks.len(), 1);
    assert_eq!(chunks[0].chunk_size, N8);
    for start in [0, (1 << 32) - 3, N8 - 6] {
        let want: Vec<u8> = (start..start + 6).map(value).collect();
        assert_eq!(sel_u8(&file, "single", start, 6), want, "at {start}");
    }
    drop(file);
    h5py_check(
        r#"
 import sys, h5py, numpy as np
 n = 2**32 + 7
 with h5py.File(sys.argv[1], "r") as f:
    d = f["single"]
    assert d.chunks == (n,) and d.shape == (n,) and d.compression is None
    for start in (0, 2**32 - 3, n - 6):
        want = [i % 251 for i in range(start, start + 6)]
        assert list(d[start:start + 6]) == want, (start, d[start:start + 6])
 "#,
        &path,
    );
    let start = (1u64 << 32) - 3;
    if let Some(text) = h5dump2_text(
        &["-d", "single", "-s", &start.to_string(), "-c", "6"],
        &path,
    ) {
        let want: Vec<String> = (start..start + 6).map(|i| value(i).to_string()).collect();
        assert!(text.contains(&want.join(", ")), "{text}");
    }
 }
 /// LZ4 and Zstandard chunks of 4 GiB or more: written by clawhdf5, read
 /// back by it and by h5py (through `hdf5plugin`'s LZ4 and Zstandard
 /// filters when installed).
 #[cfg(any(feature = "lz4", feature = "zstd"))]
 #[test]
 fn writer_huge_chunks_lz4_zstd_round_trip() {
    if !heavy() {
        return;
    }
    let dir = scratch();
    let path = dir.path().join("codecs.h5");
    let values = u8s(0..10);
    let mut b = clawhdf5::FileBuilder::new();
    let mut names = Vec::new();
    #[cfg(feature = "lz4")]
    {
        b.create_dataset("lz4")
            .with_u8_data(&values)
            .with_maxshape(&[N8])
            .with_chunks(&[N8])
            .with_lz4();
        names.push("lz4");
    }
    #[cfg(feature = "zstd")]
    {
        b.create_dataset("zstd")
            .with_u8_data(&values)
            .with_maxshape(&[N8])
            .with_chunks(&[N8])
            .with_zstd(3);
        names.push("zstd");
    }
    b.write(&path).unwrap();
    let data = std::fs::read(&path).unwrap();
    assert!(data.len() < 256 << 20, "{} bytes", data.len());
    for name in &names {
        let (version, _, _, chunks) = layout_of(&data, name);
        assert_eq!(version, 5, "{name}");
        assert_eq!(chunks.len(), 1, "{name}");
    }
    drop(data);
    let file = File::open(&path).unwrap();
    for name in &names {
        assert_eq!(sel_u8(&file, name, 0, 10), values, "{name}");
    }
    drop(file);
    h5py_check(
        &format!(
            r#"
 import sys, h5py
 try:
    import hdf5plugin
 except ImportError:
    print("SKIP: no hdf5plugin")
    sys.exit(0)
 with h5py.File(sys.argv[1], "r") as f:
    for name in {names:?}:
        d = f[name]
        assert d.chunks == (2**32 + 7,), (name, d.chunks)
        assert list(d[0:10]) == list(range(10)), (name, d[0:10])
 "#,
            names = names
        ),
        &path,
    );
 }