MEASURED on a production-shaped store (14,056 nouns / 72,679 verbs, an 11 GB
generation history), measured solo under an exclusive lock: the generation-store phase cost 55,538 ms of a WARM
REOPEN after a clean close — with the fold correctly skipped, so nothing in
that phase's name explained it.
This is what it was doing. Discovering which generations exist on disk called
listRawObjects('_generations'), which RECURSES the whole tree and returns
every file in every generation directory — to extract a set of integers that
the top-level directory NAMES already spell out. The cost scales with the
entire history, is paid on every open, warm or cold, and grows for the life of
the store.
A one-level door — listRawPrefixes(prefix), the immediate child directory
names — is added to the storage seam. The filesystem adapter answers it with a
single readdir; BaseStorage derives it from the recursive listing, so an
adapter without a cheap implementation is never wrong, only never faster; and
the generation store falls back to the old listing when the door is absent.
One behavioural difference, stated: an EMPTY generation directory is now
discovered where the file listing could not see it. Above the committed
watermark that is a crash scar, and recovery already has an explicit branch
for it ("indeterminate partial dir" — dropped, narrated). Below it, it becomes
a resolvable generation holding no records, which is what an empty generation
means.
Suites: the durability kill matrix (15), db-mvcc (30), history repacking (4),
rollback trapdoor (3), entity-tree stamp (4) and the full unit suite (2,105)
all green.
David Snelling
9dd399216b