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>, pgsql-hackers@lists.postgresql.org
Date: 2025-02-04T08:49:41Z
Lists: pgsql-hackers 
Hi,

On Thu, Jan 23, 2025 at 09:57:50AM +0000, Bertrand Drouvot wrote:
> On Thu, Jan 23, 2025 at 05:05:30PM +0900, Michael Paquier wrote:
> > 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?
> 
> yeah, IIUC pgWalUsage acts as the primary source that both prevWalUsage and
> prevBackendWalUsage diff against to calculate incremental stats.
> 

Now that a051e71e28a is in, I think that we can reduce the scope of this patch
(i.e reduce the number of stats provided by pg_stat_get_backend_wal()).

I think we can keep:

wal_records
wal_fpi 
wal_bytes (because it differs from write_bytes in pg_stat_get_backend_io())
wal_buffers_full

The first 3 are in the WalUsage struct.

I think that: 

wal_write (and wal_write_time)
wal_sync (and wal_sync_time)

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?

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