Re: per backend WAL statistics

On Tue, Jan 21, 2025 at 07:19:55AM +0000, Bertrand Drouvot wrote:
> PFA v6 that now relies on the new PendingBackendStats variable introduced in
> 4feba03d8b9.
>
> Remark: I moved PendingBackendStats back to pgstat.h because I think that the
> "simple" pending stats increment that we are adding in xlog.c are not worth
> an extra function call overhead (while it made more sense for the more complex IO
> stats handling). So PendingBackendStats is now visible to the outside world like
> PendingWalStats and friends.

You are re-doing here a pattern I was trying to avoid so as we don't
copy-paste more checks based on pgstat_tracks_backend_bktype more than
necessary. I am wondering if we should think harder about the
interface used to register WAL stats, and make it more consistent with
the way pg_stat_io is handled, avoiding the hardcoded attribute
numbers if we have an enum to control which field to update in some
input routine.

As we have only five counters in PgStat_PendingWalStats, the result
you have is not that invasive, true.

Are you sure that the interactions between pgWalUsage, prevWalUsage
and prevBackendWalUsage are correct? As far I got it from a code
read, prevWalUsage, prevBackendWalUsage and their local trackings in
pgstat_backend.c and pgstat_wal.c rely on instrument.c as the primary
source, as pgWalUsage can never be reset. Is that right?
--
Michael