Re: Don't keep closed WAL segment in page cache after replay

On 2025/07/02 19:10, Anthonin Bonnefoy wrote:
> Hi,
>
> I've been looking at page cache usage as some of our replicas were
> under memory pressure (no inactive pages available) which led to WAL
> replay lag as the recovery process had to read from disk. One thing
> I've noticed was that the last WAL files are in the pagecache even
> after having been replayed.
>
> This can be checked with vmtouch:
> vmtouch pg_wal/*
> Files: 141
> Directories: 2
> Resident Pages: 290816/290816 1G/1G 100%
>
> And page-types shows a replayed WAL file in the active LRU:
> page-types -Cl -f 000000010000001B00000076
> page-count MB long-symbolic-flags
> 4096 16 referenced,uptodate,lru,active
>
> From my understanding, once replayed on a replica, WAL segment files
> won't be re-read. So keeping it in the pagecache seems like an
> unnecessary strain on the memory (more so that they appear to be in
> the active LRU).

WAL files that have already been replayed can still be read again
for WAL archiving (if archive_mode = always) or for replication
(if the standby is acting as a streaming replication sender or
a logical replication publisher). No?

> This patch adds a POSIX_FADV_DONTNEED before closing a WAL segment,
> immediately releasing cached pages.

Maybe we should do this only on a standby where WAL archiving
isn't working and it isn't acting as a sender or publisher.

Regards,

--
Fujii Masao
NTT DATA Japan Corporation