Brainstormed 2026-08-28. The iteration is split: part A batches, part B
(io_uring submission) is deferred until A's measurement says whether the
blocking boundary still dominates.
The story's premise needed correcting first:
- it says "replace fsync-per-commit with io_uring group-commit", but the
engine commits per STATEMENT — db.c calls wo_wal_commit right after
every append, all six sites, so each row change is one pwrite + one
fdatasync
- so two independent wins were being carried as one, and only the second
needs io_uring. The staging buffer already holds any number of records;
today it never holds more than one. Part A is mostly deleting calls
- iteration 22's numbers say A is where the payoff is: durable writes
4460 ops/s, mixwrite 1023 ops/s p99 664us, against 1.28M ops/s reads
Forks settled:
- batch boundary is QUEUE-DRAIN, not the tick this story had recorded: a
tick adds latency to a lone writer, taxing an idle system to serve a
busy one. Queue-drain self-tunes and needs no knob
- shard 0 holds each reply envelope instead of sending it, commits once
when the queue empties, then releases all — so a writer is acked after
the barrier carrying ITS record, which today is true only because
every batch has one member
- a failure between "RAM mutated" and "record durable" is a FATAL,
diagnosed abort. This replaces uneven behaviour that already exists:
insert rolls back, update and delete do not and say so in a comment
("RAM ahead of disk"). Batching would have multiplied that
- consequence stated, not slipped in: WO_T_IO leaves the write path
- no batch cap initially; peak staged bytes is measured so the question
is settled by a number
One gap disclosed rather than hidden: forcing a real fdatasync failure
needs mount privileges, so the unit test proves the error is DETECTED and
the abort itself stays covered by inspection.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
shoney.arickathil
75aedf1216