Re: pg_stat_bgwriter.buffers_backend is pretty meaningless (and more?)

From: Andres Freund <andres(at)anarazel(dot)de>
To: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Cc: Justin Pryzby <pryzby(at)telsasoft(dot)com>, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>, Magnus Hagander <magnus(at)hagander(dot)net>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>, Lukas Fittl <lukas(at)fittl(dot)com>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Subject: Re: pg_stat_bgwriter.buffers_backend is pretty meaningless (and more?)
Date: 2022-07-12 17:18:10
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On 2022-07-11 22:22:28 -0400, Melanie Plageman wrote:
> Yes, per an off list suggestion by you, I have changed the tests to use a
> sum of writes. I've also added a test for IOPATH_LOCAL and fixed some of
> the missing calls to count IO Operations for IOPATH_LOCAL and
> I struggled to come up with a way to test writes for a particular
> type of backend are counted correctly since a dirty buffer could be
> written out by another type of backend before the target BackendType has
> a chance to write it out.

I guess temp file writes would be reliably done by one backend... Don't have a
good idea otherwise.

> I also struggled to come up with a way to test IO operations for
> background workers. I'm not sure of a way to deterministically have a
> background worker do a particular kind of IO in a test scenario.

I think it's perfectly fine to not test that - for it to be broken we'd have
to somehow screw up setting the backend type. Everything else is the same as
other types of backends anyway.

If you *do* want to test it, you probably could use
SET parallel_leader_participation = false;
SET force_parallel_mode = 'regress';
SELECT something_triggering_io;

> I'm not sure how to cause a strategy "extend" for testing.

COPY into a table should work. But might be unattractive due to the size of of
the COPY ringbuffer.

> > Would be nice to have something testing that the ringbuffer stats stuff
> > does something sensible - that feels not entirely trivial.
> >
> >
> I've added a test to test that reused strategy buffers are counted as
> allocs. I would like to add a test which checks that if a buffer in the
> ring is pinned and thus not reused, that it is not counted as a strategy
> alloc, but I found it challenging without a way to pause vacuuming, pin
> a buffer, then resume vacuuming.

Yea, that's probably too hard to make reliable to be worth it.


Andres Freund

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