Horton

Re: per backend WAL statistics

Michael Paquier <michael@paquier.xyz>

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From: Michael Paquier <michael@paquier.xyz>
To: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, pgsql-hackers@lists.postgresql.org
Date: 2025-02-05T02:16:15Z
Lists: pgsql-hackers 
On Tue, Feb 04, 2025 at 08:49:41AM +0000, Bertrand Drouvot wrote:
> I think that: 
> 
> wal_write (and wal_write_time)
> wal_sync (and wal_sync_time)

Right.  We are not able to get this data from XLogWrite() and
issue_xlog_fsync(), so there is no need to duplicate that anymore in
your patch.

> can be extracted from pg_stat_get_backend_io(), so there is no need to duplicate
> this information. The same comment could be done for pg_stat_wal and pg_stat_io
> though, but pg_stat_wal already exists so removing fields has not the same
> effect.
> 
> What are you thoughts about keeping in pg_stat_get_backend_wal() only the
> 4 stats mentioned above?

wal_buffers_full is incremented in AdvanceXLInsertBuffer(), part of
PendingWalStats.  wal_records, wal_fpi and wal_bytes are part of the
instrumentation field.  It looks to me that if you discard the
wal_buffers_full part, the implementation of the data in the backend
could just be tied to the fields coming from WalUsage.

Actually, could it actually be useful to have wal_buffers_full be
available in WalUsage, so as it would show up in EXPLAIN in a
per-query basis with show_wal_usage()?  Consolidating that would make
what you are trying it a bit easier, because we would have the
WalUsage and the pg_stat_io parts without any of the PendingWalStats
part.  And it is just a counter not that expensive to handle, like the
data for records, fpis and bytes.  This extra information could be
useful to have in the context of an EXPLAIN.
--
Michael

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