Horton

Re: per backend WAL statistics

Bertrand Drouvot <bertranddrouvot.pg@gmail.com>

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From: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
To: Michael Paquier <michael@paquier.xyz>
Cc: Andres Freund <andres@anarazel.de>, Xuneng Zhou <xunengzhou@gmail.com>, pgsql-hackers@lists.postgresql.org
Date: 2025-03-07T08:33:04Z
Lists: pgsql-hackers 
Hi,

On Fri, Mar 07, 2025 at 02:42:13PM +0900, Michael Paquier wrote:
> On Thu, Mar 06, 2025 at 10:33:52AM +0000, Bertrand Drouvot wrote:
> > Indeed, there is no reason for pgstat_backend_have_pending_cb() to return true if 
> > pgstat_tracks_backend_bktype() is not satisfied.
> > 
> > So it deserves a dedicated patch to fix this already existing issue:
> > 0001 attached.
> 
>  pgstat_backend_have_pending_cb(void)
>  {
> -    return (!pg_memory_is_all_zeros(&PendingBackendStats,
> -                                    sizeof(struct PgStat_BackendPending)));
> +    if (!pgstat_tracks_backend_bktype(MyBackendType))
> +        return false;
> +    else
> +        return (!pg_memory_is_all_zeros(&PendingBackendStats,
> +                                        sizeof(struct PgStat_BackendPending)));
> 
> So, if I understand your point correctly, it is not a problem on HEAD
> because we are never going to update PendingBackendStats in the
> checkpointer as pgstat_count_backend_io_op[_time]() blocks any attempt
> to do so.

I think this is wrong on HEAD because we initialize PendingBackendStats to 
zeros in pgstat_create_backend() based on the backend type (pgstat_tracks_backend_bktype()).

But when it's time to flush, then pgstat_backend_have_pending_cb() checks
for zeros in PendingBackendStats *without* any check on the backend type.

I think the issue is "masked" on HEAD because PendingBackendStats is
probably automatically initialized with zeros (as being a static variable at
file scope).

Regards,

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

Commits

  1. Fix use-after-free in pgstat_fetch_stat_backend_by_pid()

  2. Remove initialization from PendingBackendStats

  3. Add WAL data to backend statistics

  4. Improve check for detection of pending data in backend statistics

  5. Fix some gaps in pg_stat_io with WAL receiver and WAL summarizer

  6. Handle auxiliary processes in SQL functions of backend statistics

  7. Invent pgstat_fetch_stat_backend_by_pid()

  8. Refactor code of pg_stat_get_wal() building result tuple

  9. Adding new PgStat_WalCounters structure in pgstat.h

  10. Remove pgstat_flush_wal()

  11. Refactor some code related to backend statistics

  12. Add backend-level statistics to pgstats