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: pgsql-hackers@lists.postgresql.org
Date: 2025-01-09T04:03:15Z
Lists: pgsql-hackers

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On Wed, Jan 08, 2025 at 11:11:59AM +0000, Bertrand Drouvot wrote:
> Yeah, that's more elegant as it also means that the main callback will not change
> (should we add even more stats in the future). Done that way in v2 attached.

I've put my hands on v2-0002 to begin with something.

+/* flag bits for different types of statistics to flush */
+#define PGSTAT_FLUSH_IO    (1 << 0) /* Flush I/O statistics */
+#define PGSTAT_FLUSH_ALL   (PGSTAT_FLUSH_IO)

These are located and used only in pgstat_backend.c.  It seems to me
that we'd better declare them in pgstat_internal.h and extend the
existing pgstat_flush_backend() with an argument so as callers can do
what they want.

+	/* Get our own entry_ref if not provided */
+	if (!entry_ref)
+		entry_ref = pgstat_get_entry_ref(PGSTAT_KIND_BACKEND, InvalidOid,
+										 MyProcNumber, false, NULL);

This relates to the previous remark, actually, where I think that it
is cleaner to have pgstat_flush_backend() do pgstat_get_entry_ref(),
same way as HEAD, and just pass down the flags.  pgstat_flush_backend
cannot call directly pgstat_backend_flush_cb(), of course, so I've
settled down to a new pgstat_flush_backend_entry() that handles the
entry locking.  This comes at the cost of pgstat_flush_backend_entry()
requiring an extra pgstat_tracks_backend_bktype(), which is not a big
issue, and the patch gets a bit shorter.
--
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