Listing a large group over openUrl still took 6-11 passes (network round
trips) for the reviewer's 3000-dataset h5py file: each pass only found
the structures the walk reached before its first miss.
- The v1 and v2 B-tree collectors descend into every child of a node
after one fails (they only read the siblings before, so a sibling's
subtree came a pass later), then return the first error: results and
errors unchanged. The v2 walk stops once its record budget is spent,
so a shared-subtree tree still cannot multiply the work.
- Hints (`Storage::hint`, a no-op for every backend but the lazy one):
a group B-tree node's and a symbol table node's body (read once their
header gives a length, a round trip later when the body is in the
next block), an object header's first chunk and its continuation
chunks, the symbol table nodes a B-tree leaf names, a dense group's
name index header and the heap's root block (both read right after
the heap header). A listing also hints every child's object header as
its entry is read, even after a failure, and every direct block of a
dense group's heap (reading the indirect blocks, at most 4096 entries
and 4 levels deep); a lookup does not.
- The fractal heap's indirect-block layout (entry sizes, where the first
n entries end) is one helper used by the object reads and the hints.
Measured with tests/lazy.rs listing_cost_of_a_given_file on an h5py file
like the reviewer's (3000 datasets of 64 KiB, 198 MB), list('/'),
passes/requests/bytes, before -> after:
earliest, 1 MiB: 6/73/192.5 MB -> 4/68/192.5 MB
earliest, 64 KiB: 8/531/35.2 MB -> 5/530/35.3 MB
latest, 1 MiB: 9/98/196.5 MB -> 5/86/196.5 MB
latest, 64 KiB: 11/452/29.6 MB -> 6/454/30.5 MB
listing_a_large_group_takes_a_few_passes (512-byte blocks), budgets
tightened to the new counts: FileBuilder 600 children 5 -> 4 passes,
h5py 2000 children earliest 8 -> 5, latest 11 -> 6.
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
