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

On 2025/07/02 22:24, Fujii Masao wrote:
>
>
> 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?

Also, the WAL summarizer might read those WAL files as well.

Regards,

--
Fujii Masao
NTT DATA Japan Corporation