Re: per backend WAL statistics

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.

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.

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.

> 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.

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.
--
Michael