Re: per backend WAL statistics

Hi,

On Tue, Feb 18, 2025 at 08:34:32AM +0900, Michael Paquier wrote:
> On Mon, Feb 17, 2025 at 03:14:59PM +0000, Bertrand Drouvot wrote:
> > PFA the whole picture. 0001 is implementing the fields removal in pg_stat_wal
> > (and also PendingWalStats). I think that's ok given the backend's type for which
> > pgstat_tracks_io_bktype() returns false. But now you make me doubt about 0001.
>
> Double-checking the code now and my doubts are wrong.

Thanks for double checking.

> I think that I would vote for a removal of the fields from pg_stat_wal
> rather than a replacement in pg_stat_wal, for the following reasons:
> - pg_stat_stat.wal_write is the same value as "select sum(writes)
> from pg_stat_io where object = 'wal' and context = 'normal'" as these
> are incremented in XLogWrite().
> - Same argument about pg_stat_wal.wal_write_time with
> pg_stat_io.write_time.
> - issue_xlog_fsync() tells that pg_stat_wal.wal_sync_time and
> sum(pg_stat_io.fsync_time) under object=wal and context=normal are the
> same values.
> - Same argument with the fsync counters pg_stat_wal.wal_sync and
> pg_stat_io.fsyncs.
> - Encourage monitoring pull to move to pg_stat_io, where there is much
> more context and granularity of the stats data.

Agree with all of the above + pgstat_tracks_io_bktype() returns false for backend's
that do not generate WAL (so that we don't lose WAL information in pg_stat_io).

> Regarding the GUC track_wal_io_timing, my take is that we'll live
> better if we just let it go. It loses its meaning once pg_stat_wal
> does not track the write and sync timings.

Yeah, done that way in the dedicated thread ([1]).

> > Anyway, it's probably better to move the 0001 discussion to a dedicated thread,
> > thoughts?
>
> Yes. And we cannot really move forward with what we have here without
> deciding about this part. The simplifications I can read from
> v7-0002~v7-0004 are really nice. These make the implementation of WAL
> stats at backend-level really simpler to think about.

yup.

> The doc additions of v7-0001 about the description of what the 'wal'
> object does in pg_stat_io are actually worth a change of their own?
> We already track them in pg_stat_io.

Agree, done that way in the dedicated thread ([1]).

[1]: https://www.postgresql.org/message-id/flat/Z7RkQ0EfYaqqjgz/%40ip-10-97-1-34.eu-west-3.compute.internal

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com