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

On Mon, Mar 6, 2023 at 2:34 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> Hi,
>
> On 2023-03-06 14:24:09 -0500, Melanie Plageman wrote:
> > On Mon, Mar 06, 2023 at 11:09:19AM -0800, Andres Freund wrote:
> > > On 2023-03-06 10:09:24 -0500, Melanie Plageman wrote:
> > > > On Mon, Mar 6, 2023 at 1:48 AM Kyotaro Horiguchi
> > > > <horikyota(dot)ntt(at)gmail(dot)com> wrote:
> > > > >
> > > > > At Mon, 06 Mar 2023 15:24:25 +0900 (JST), Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com> wrote in
> > > > > > In any case, I think we need to avoid such concurrent autovacuum/analyze.
> > > > >
> > > > > If it is correct, I believe the attached fix works.
> > > >
> > > > Thanks for investigating this!
> > > >
> > > > Yes, this fix looks correct and makes sense to me.
> > >
> > > Wouldn't it be better to just perform the section from the ALTER TABLE till
> > > the DROP TABLE in a transaction? Then there couldn't be any other accesses in
> > > just that section. I'm not convinced it's good to disallow all concurrent
> > > activity in other parts of the test.
> >
> > You mean for test coverage reasons? Because the table in question only
> > exists for a few operations in this test file.
>
> That, but also because it's simply more reliable. autovacuum=off doesn't
> protect against a anti-wraparound vacuum or such. Or a concurrent test somehow
> triggering a read. Or ...

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.

- Melanie