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

Hi,

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/)?


> 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.


I found a few more occurances and chatted with Melanie. Melanie will come up
with a fix I think.

Greetings,

Andres Freund