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

On Fri, Mar 10, 2023 at 3:19 PM Justin Pryzby <pryzby@telsasoft.com> wrote:
>
> On Thu, Mar 09, 2023 at 11:43:01AM -0800, Andres Freund wrote:
> > > 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.

I think your patch made it more likely that a backend needing to flush a
buffer in order to fit its own data would be doing so in a buffer access
strategy IO context.

Your patch makes it so those toast table writes are using a
BAS_BULKWRITE (see GetBulkInsertState()) and when they are looking for
buffers to put their data in, they have to evict other data (theirs and
others) but all of this is tracked in io_context = 'bulkwrite' -- and
the test only counted writes done in io_context 'normal'. But it is good
that your patch did that! It helped us to see that this test is not
reliable.

The other times this test failed in cfbot were for a patch that had many
failures and might have something wrong with its code, IIRC.

Thanks again for the report!

- Melanie