Message-ID: <b231adfe-1e80-4c30-9059-1dd7752a26a9@postgrespro.ru>
Date: Thu, 12 Mar 2026 14:25:17 +0300
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Subject: Re: Why clearing the VM doesn't require registering vm buffer in wal
 record
To: Andres Freund <andres@anarazel.de>,
 Matthias van de Meent <boekewurm+postgres@gmail.com>
Cc: Melanie Plageman <melanieplageman@gmail.com>,
 PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
 Robert Haas <robertmhaas@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>,
 "Andrey M. Borodin" <x4mmm@yandex-team.ru>
References: 
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 <oqcsevg35xjan2327x5kdfth6q4fgeqboxfo3v3imeyih2uiny@6sez5dzxl6nt>
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From: Yura Sokolov <y.sokolov@postgrespro.ru>
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06.03.2026 00:01, Andres Freund пишет:
> Hi,
> 
> On 2026-03-05 15:38:24 -0500, Andres Freund wrote:
>>> On Thu, 5 Mar 2026 at 21:16, Andres Freund <andres@anarazel.de> wrote:
>>>>
>>>> But it does seem like it could be a problem for incremental backup /
>>>> walsummarizer?
>>>
>>> I don't think it is, because that doesn't do calculations for non-main
>>> forks, it considers those forks always changed and includes them in
>>> full. Or at least, that was the response I got when I raised concerns
>>> about the FSM back when the incremental backup feature was being
>>> developed [0].
>>
>> There's explicit code for ignoring the FSM, but I don't see the same for the
>> VM. And that makes sense: VM changes are mostly WAL logged, just not
>> completely / generically (i.e. this complaint), whereas FSM changes are not
>> WAL logged at all.
> 
> Unfortunately I can confirm that incremental backups end up with an outdated
> VM.

That is why pg_probackup still archive VM at whole in incremental (WAL
parsing) backup.
That is why WAL-G's incremental backup in WAL-parsing mode is (was?)
considered unstable.

I know the problem for couple of years. Excuse me I didn't write about.

I didn't recognize fix could be as simple as registering VM buffers.
My bad. I fill so stupid :-(

It would be great if it will be fixed in all supported versions.

-- 
regards
Yura Sokolov aka funny-falcon