Re: Show WAL write and fsync stats in pg_stat_io

Hi,

Thanks for the feedback.

On Mon, 20 Nov 2023 at 10:47, Michael Paquier <michael(at)paquier(dot)xyz> wrote:
>
> On Thu, Nov 09, 2023 at 02:39:26PM +0300, Nazir Bilal Yavuz wrote:
> > There are some differences between pg_stat_wal and pg_stat_io while
> > collecting WAL stats. For example in the XLogWrite() function in the
> > xlog.c file, pg_stat_wal counts wal_writes as write system calls. This
> > is not something we want for pg_stat_io since pg_stat_io counts the
> > number of blocks rather than the system calls, so instead incremented
> > pg_stat_io by npages.
> >
> > Could that cause a problem since pg_stat_wal's behaviour will be
> > changed? Of course, as an alternative we could change pg_stat_io's
> > behaviour but in the end either pg_stat_wal's or pg_stat_io's
> > behaviour will be changed.
>
> Yep, that could be confusing for existing applications that track the
> information of pg_stat_wal. The number of writes is not something
> that can be correctly shared between both. The timings for the writes
> and the syncs could be shared at least, right?

Yes, the timings for the writes and the syncs should work. Another
question I have in mind is the pg_stat_reset_shared() function. When
we call it with 'io' it will reset pg_stat_wal's timings and when we
call it with 'wal' it won't reset them, right?

>
> This slightly relates to pgstat_count_io_op_n() in your latest patch,
> where it feels a bit weird to see an update of
> PendingWalStats.wal_sync sit in the middle of a routine dedicated to
> pg_stat_io.. I am not completely sure what's the right balance here,
> but I would try to implement things so as pg_stat_io paths does not
> need to know about PendingWalStats.

Write has block vs system calls differentiation but it is the same for
sync. Because of that I put PendingWalStats.wal_sync to pg_stat_io but
I agree that it looks a bit weird.

--
Regards,
Nazir Bilal Yavuz
Microsoft