Date: Fri, 10 Mar 2023 14:19:04 -0600
From: Justin Pryzby <pryzby@telsasoft.com>
To: Andres Freund <andres@anarazel.de>
Cc: Melanie Plageman <melanieplageman@gmail.com>,
	Kyotaro Horiguchi <horikyota.ntt@gmail.com>, tgl@sss.pgh.pa.us,
	vignesh21@gmail.com, lukas@fittl.com, alvherre@alvh.no-ip.org,
	magnus@hagander.net, pgsql-hackers@postgresql.org,
	thomas.munro@gmail.com, m.sakrejda@gmail.com
Subject: Re: pg_stat_bgwriter.buffers_backend is pretty meaningless (and
 more?)
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On Thu, Mar 09, 2023 at 11:43:01AM -0800, Andres Freund wrote:
> On 2023-03-09 06:51:31 -0600, Justin Pryzby wrote:
> > On Tue, Mar 07, 2023 at 10:18:44AM -0800, Andres Freund wrote:
> > > Hi,
> > > 
> > > On 2023-03-06 15:21:14 -0500, Melanie Plageman wrote:
> > > > Good point. Attached is what you suggested. I committed the transaction
> > > > before the drop table so that the statistics would be visible when we
> > > > queried pg_stat_io.
> > > 
> > > Pushed, thanks for the report, analysis and fix, Tom, Horiguchi-san, Melanie.
> > 
> > There's a 2nd portion of the test that's still flapping, at least on
> > cirrusci.
> > 
> > The issue that Tom mentioned is at:
> >  SELECT :io_sum_shared_after_writes > :io_sum_shared_before_writes;
> > 
> > But what I've seen on cirrusci is at:
> >  SELECT :io_sum_shared_after_writes > :io_sum_shared_before_writes;
> 
> Seems you meant to copy a different line for Tom's (s/writes/redas/)?

Seems so

> > https://api.cirrus-ci.com/v1/artifact/task/6701069548388352/log/src/test/recovery/tmp_check/regression.diffs
> 
> Hm. I guess the explanation here is that the buffers were already all written
> out by another backend. Which is made more likely by your patch.

FYI: that patch would've made it more likely for each backend to write
out its *own* dirty pages of TOAST ... but the two other failures that I
mentioned were for patches which wouldn't have affected this at all.

-- 
Justin