Re: WAL usage calculation patch

From: Julien Rouhaud <rjuju123(at)gmail(dot)com>
To: Kirill Bychik <kirill(dot)bychik(at)gmail(dot)com>
Cc: PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Subject: Re: WAL usage calculation patch
Date: 2020-03-18 17:19:16
Message-ID: 20200318171916.GA69354@nol
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On Wed, Mar 18, 2020 at 09:02:58AM +0300, Kirill Bychik wrote:
> There is a higher-level Instrumentation API that can be used with
> INSTRUMENT_WAL flag to collect the wal usage information. I believe
> the instrumentation is widely used in the executor code, so it should
> not be a problem to colelct instrumentation information on autovacuum
> worker level.
> Just a recommendation/chat, though. I am happy with the way the data
> is collected now. If you commit this variant, please add a TODO to
> rework wal usage to common instr API.

The instrumentation is somewhat intended to be used with executor nodes, not
backend commands. I don't see real technical reason that would prevent that,
but I prefer to keep things as-is for now, as it sound less controversial.
This is for the 3rd patch, which may not even be considered for this CF anyway.

> > > As for the tests, please get somebody else to review this. I strongly
> > > believe checking full page writes here could be a source of
> > > instability.
> >
> >
> > I'm also a little bit dubious about it. The initial checkpoint should make
> > things stable (of course unless full_page_writes is disabled), and Cfbot also
> > seems happy about it. At least keeping it for the temporary tables test
> > shouldn't be a problem.
> Temp tables should show zero FPI WAL records, true :)
> I have no objections to the patch.

I'm attaching a v5 with fp records only for temp tables, so there's no risk of
instability. As I previously said I'm fine with your two patches, so unless
you have objections on the fpi test for temp tables or the documentation
changes, I believe those should be ready for committer.

Attachment Content-Type Size
v5-0001-Track-WAL-usage.patch text/plain 13.3 KB
v5-0002-Keep-track-of-WAL-usage-in-pg_stat_statements.patch text/plain 22.2 KB
v5-0003-Keep-track-of-auto-vacuum-WAL-usage-in-pg_stat_da.patch text/plain 13.1 KB

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