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From: Nazir Bilal Yavuz <byavuz81@gmail.com>
Date: Fri, 13 Sep 2024 16:45:08 +0300
Message-ID: <CAN55FZ0mnVe4rra2WXca=j+7fabVRY8uhRFPXFmkdRW9J4dKbQ@mail.gmail.com>
Subject: Re: per backend I/O statistics
To: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>, Kyotaro Horiguchi <horikyota.ntt@gmail.com>, 
	pgsql-hackers@lists.postgresql.org
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Hi,

Thanks for working on this!

Your patch applies and builds cleanly.

On Fri, 6 Sept 2024 at 18:03, Bertrand Drouvot
<bertranddrouvot.pg@gmail.com> wrote:
>
> - As stated up-thread, the pg_stat_get_backend_io() behaves as if
> stats_fetch_consistency is set to none (each execution re-fetches counters
> from shared memory). Indeed, the snapshot is not build in each backend to copy
> all the others backends stats, as 1/ there is no use case (there is no need to
> get others backends I/O statistics while taking care of the stats_fetch_consistency)
> and 2/ that could be memory expensive depending of the number of max connections.

I believe this is the correct approach.

I manually tested your patches, and they work as expected. Here is
some feedback:

- The stats_reset column is NULL in both pg_my_stat_io and
pg_stat_get_backend_io() until the first call to reset io statistics.
While I'm not sure if it's necessary, it might be useful to display
the more recent of the two times in the stats_reset column: the
statistics reset time or the backend creation time.

- The pgstat_reset_io_counter_internal() is called in the
pgstat_shutdown_hook(). This causes the stats_reset column showing the
termination time of the old backend when its proc num is reassigned to
a new backend.

--
Regards,
Nazir Bilal Yavuz
Microsoft