Date: Thu, 13 Mar 2025 06:42:57 +0000
From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: Jim Nasby <jnasby@upgrade.com>
Cc: Alena Rybakina <a.rybakina@postgrespro.ru>,
	Ilia Evdokimov <ilya.evdokimov@tantorlabs.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>,
	Alexander Korotkov <aekorotkov@gmail.com>,
	Kirill Reshke <reshkekirill@gmail.com>,
	Andrei Zubkov <zubkov@moonset.ru>,
	Masahiko Sawada <sawada.mshk@gmail.com>,
	Melanie Plageman <melanieplageman@gmail.com>,
	jian he <jian.universality@gmail.com>, a.lepikhov@postgrespro.ru,
	Sami Imseih <samimseih@gmail.com>
Subject: Re: Vacuum statistics
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Hi,

On Wed, Mar 12, 2025 at 05:15:53PM -0500, Jim Nasby wrote:
> The usecase I can see here is that we don't want autovac creating so much
> WAL traffic that it starts forcing other backends to have to write WAL out.
> But tracking how many times autovac writes WAL buffers won't help with that

Right, because the one that increments the wal_buffers_full metric could "just"
be a victim (i.e the one that happens to trigger the WAL buffers disk flush,
even though other backends contributed most of the buffer usage).

> (though we also don't want any WAL buffers written by autovac to be counted
> in the system-wide wal_buffers_full:

why? Or do you mean that it would be good to have 2 kinds of metrics: one
generated by "maintenance" activity and one by "regular" backends?

> What would be helpful would be a way to determine if autovac was causing
> enough traffic to force other backends to write WAL. Offhand I'm not sure
> how practical that actually is though.

a051e71e28a could help to see how much WAL has by written by the autovac workers.

> BTW, there's also an argument to be made that autovac should throttle
> itself if we're close to running out of available WAL buffers...

hmm, yeah I think that's an interesting idea OTOH that would mean to "delegate"
the WAL buffers flush to another backend.

Regards,

-- 
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com