Horton

Re: Show WAL write and fsync stats in pg_stat_io

Nazir Bilal Yavuz <byavuz81@gmail.com>

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From: Nazir Bilal Yavuz <byavuz81@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Michael Paquier <michael@paquier.xyz>, pgsql-hackers <pgsql-hackers@postgresql.org>, "bharath.rupireddyforpostgres@gmail.com" <bharath.rupireddyforpostgres@gmail.com>
Date: 2023-11-09T11:39:26Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Fix copy-paste error related to the autovacuum launcher in pgstat_io.c

  2. Move SQL tests of pg_stat_io for WAL data to recovery test 029_stats_restart

  3. Add data for WAL in pg_stat_io and backend statistics

  4. Improve comment on top of pgstat_count_io_op_time()

  5. Refactor pgstat_prepare_io_time() with an input argument instead of a GUC

Hi,

On Wed, 8 Nov 2023 at 04:19, Andres Freund <andres@anarazel.de> wrote:
>
> Hi,
>
> On 2023-11-08 09:52:16 +0900, Michael Paquier wrote:
> > By the way, if the write/sync quantities and times begin to be tracked
> > by pg_stat_io, I'd see a pretty good argument in removing the
> > equivalent columns in pg_stat_wal.  It looks like this would reduce
> > the confusion related to the handling of PendingWalStats added in
> > pgstat_io.c, for one.
>
> Another approach would be to fetch the relevant columns from pg_stat_io in the
> pg_stat_wal view. That'd avoid double accounting and breaking existing
> monitoring.

There are some differences between pg_stat_wal and pg_stat_io while
collecting WAL stats. For example in the XLogWrite() function in the
xlog.c file, pg_stat_wal counts wal_writes as write system calls. This
is not something we want for pg_stat_io since pg_stat_io counts the
number of blocks rather than the system calls, so instead incremented
pg_stat_io by npages.

Could that cause a problem since pg_stat_wal's behaviour will be
changed? Of course, as an alternative we could change pg_stat_io's
behaviour but in the end either pg_stat_wal's or pg_stat_io's
behaviour will be changed.

Regards,
Nazir Bilal Yavuz
Microsoft