Re: Skip prefetch for block references that follow a FPW or WILL_INIT of the same block

Hi,

We tested v3 on the shape from Jeremy Schneider's October 20 post: a
CHECKPOINT, then UPDATE pgbench_accounts SET abalance = abalance + 1
WHERE aid % 20 = 0 at scale 200. Each run replays the same archived
WAL by archive recovery from a cold page cache, in a container with 2
CPUs and 1 GiB, on a local HDD, with default settings
(shared_buffers 128MB) apart from recovery_prefetch. We built 18.6
from the release tarball twice in one image, with and without v3,
same configure flags. "Read IOs" are the container's read requests
on the data disk.

full_page_writes = on (3.36 GB of WAL), no IO cap:

redo (s) read IOs prefetch
off, Debian 18.6 126.6, 129.9 13.4k 0
try, unpatched build 159.9, 151.6 395.9k 437,160
try, v3 181.9, 133.6 68.0k 54,644

Same WAL, 300 read IOPS cap on the container (docker
--device-read-iops):

off, Debian 18.6 145.6, 137.0 13.4k
try, Debian 18.6 1410.8, 1453.4 395.9k
try, v3 284.9 (1 run) 68.0k

So v3 removes 86% of the read IOs above recovery_prefetch = off, and
under the cap it makes replay 5x faster than unpatched try. It is
still 2.0x slower than off there. Without the cap we saw no wall-time
difference on this disk. The unpatched source build matched the
Debian 18.6 package, whose try runs took 151.9 and 179.7 s with
395.9k read IOs.

The remaining 54,644 prefetches are within 0.4% of the size of
pgbench_accounts_pkey after the update (54,840 pages). We have not
tested why.

With full_page_writes = off (0.30 GB of WAL), v3 changes nothing:
437,163 prefetches and 383.9k read IOs, as unpatched. try and off
read the same 3.47 GB, but off does it in about 25.4k reads averaging
134 kB, and try in 383.9k reads of 8.8 kB. Under the same 300 IOPS cap
that was 1284.7 s for try against 100.1 s for off (one run each). This
UPDATE walks the heap in key order, so off gets kernel read-ahead; a
random-key update would not, and we did not test one. This is outside
what v3 sets out to do. We did not run v3 under the cap with
full_page_writes = off, but with the same read IOs we would expect
the same result there.

19rc1 showed the same read IOs and prefetch counts as 18.6 (one run
per setting, no cap).

Limits: one host and one HDD; archive recovery, not streaming; we
added the capped runs after seeing the uncapped times. Scripts, every
run's server log, IO counters and pg_stat_recovery_prefetch samples:
https://github.com/CyberNative-AI/.github/tree/main/notes/postgres-recovery-prefetch

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Corrections: hello(at)cybernative(dot)ai