Horton

Re: Remove wal_[sync|write][_time] from pg_stat_wal and track_wal_io_timing

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Michael Paquier <michael@paquier.xyz>
Cc: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, pgsql-hackers@lists.postgresql.org
Date: 2025-02-24T09:41:36Z
Lists: pgsql-hackers 
Hi,

On 2025-02-20 14:37:18 +0900, Michael Paquier wrote:
> a051e71e28a added this information into pg_stat_io (with more details and
> granularity), so there is no need to keep it in pg_stat_wal. This also
> allows to remove PendingWalStats and simplifies up coming commits related
> to per backend WAL statistics. Once done, track_wal_io_timing becomes useless
> so it is also removed.
> 
> In passing remove the pgstat_prepare_io_time() parameter now that
> track_wal_io_timing is gone.

I don't think this is a good idea - there was a good reason for
track_wal_io_timing to exist, namely that it happens while holding one of the
two most contended locks in postgres.  On many systems it'll be an ok constant
overhead to enable track_io_timing, but enabling track_wal_io_timing will
cause scalability issues.  Now you made it impossible to separate those two
situations, forcing disabling of all IO timing.

Greetings,

Andres Freund

Commits

  1. Re-add GUC track_wal_io_timing

  2. Remove read/sync fields from pg_stat_wal and GUC track_wal_io_timing

  3. doc: Fix typo in section "WAL configuration"

  4. doc: Add details about object "wal" in pg_stat_io

  5. doc: Recommend pg_stat_io rather than pg_stat_wal in WAL configuration