From: | "Jonathan S(dot) Katz" <jkatz(at)postgresql(dot)org> |
---|---|
To: | Andres Freund <andres(at)anarazel(dot)de>, pgsql-hackers(at)postgresql(dot)org, Melanie Plageman <melanieplageman(at)gmail(dot)com> |
Cc: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> |
Subject: | Re: pg_stat_io not tracking smgrwriteback() is confusing |
Date: | 2023-04-22 19:25:11 |
Message-ID: | e30c4211-cd36-d882-355d-084f392b6cf4@postgresql.org |
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On 4/19/23 1:23 PM, Andres Freund wrote:
> Hi,
>
> I noticed that the numbers in pg_stat_io dont't quite add up to what I
> expected in write heavy workloads. Particularly for checkpointer, the numbers
> for "write" in log_checkpoints output are larger than what is visible in
> pg_stat_io.
>
> That partially is because log_checkpoints' "write" covers way too many things,
> but there's an issue with pg_stat_io as well:
>
> Checkpoints, and some other sources of writes, will often end up doing a lot
> of smgrwriteback() calls - which pg_stat_io doesn't track. Nor do any
> pre-existing forms of IO statistics.
>
> It seems pretty clear that we should track writeback as well. I wonder if it's
> worth doing so for 16? It'd give a more complete picture that way. The
> counter-argument I see is that we didn't track the time for it in existing
> stats either, and that nobody complained - but I suspect that's mostly because
> nobody knew to look.
[RMT hat]
(sorry for slow reply on this, I've been out for a few days).
It does sound generally helpful to track writeback to ensure anyone
building around pg_stat_io can see tthe more granular picture. How big
of an effort is this? Do you think this helps to complete the feature
for v16?
Thanks,
Jonathan
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