Re: Show WAL write and fsync stats in pg_stat_io

Hi,

On Wed, 8 Nov 2023 at 04:19, Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> Hi,
>
> On 2023-11-08 09:52:16 +0900, Michael Paquier wrote:
> > By the way, if the write/sync quantities and times begin to be tracked
> > by pg_stat_io, I'd see a pretty good argument in removing the
> > equivalent columns in pg_stat_wal. It looks like this would reduce
> > the confusion related to the handling of PendingWalStats added in
> > pgstat_io.c, for one.
>
> Another approach would be to fetch the relevant columns from pg_stat_io in the
> pg_stat_wal view. That'd avoid double accounting and breaking existing
> monitoring.

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.

Regards,
Nazir Bilal Yavuz
Microsoft