From: | Andres Freund <andres(at)anarazel(dot)de> |
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To: | Melanie Plageman <melanieplageman(at)gmail(dot)com> |
Cc: | "Drouvot, Bertrand" <bertranddrouvot(dot)pg(at)gmail(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>, Maciek Sakrejda <m(dot)sakrejda(at)gmail(dot)com>, Justin Pryzby <pryzby(at)telsasoft(dot)com>, smilingsamay(at)gmail(dot)com, Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com>, Lukas Fittl <lukas(at)fittl(dot)com>, Magnus Hagander <magnus(at)hagander(dot)net>, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org> |
Subject: | Re: Track IO times in pg_stat_io |
Date: | 2023-03-07 18:47:51 |
Message-ID: | 20230307184751.jdokdy2xmx3xpaua@awork3.anarazel.de |
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Lists: | pgsql-hackers |
On 2023-03-07 13:43:28 -0500, Melanie Plageman wrote:
> > Now I've a second thought: what do you think about resetting the related number
> > of operations and *_time fields when enabling/disabling track_io_timing? (And mention it in the doc).
> >
> > That way it'd prevent bad interpretation (at least as far the time per operation metrics are concerned).
> >
> > Thinking that way as we'd loose some (most?) benefits of the new *_time columns
> > if one can't "trust" their related operations and/or one is not sampling pg_stat_io frequently enough (to discard the samples
> > where the track_io_timing changes occur).
> >
> > But well, resetting the operations could also lead to bad interpretation about the operations...
> >
> > Not sure about which approach I like the most yet, what do you think?
>
> Oh, this is an interesting idea. I think you are right about the
> synchronization issues making the statistics untrustworthy and, thus,
> unuseable.
No, I don't think we can do that. It can be enabled on a per-session basis.
I think we simply shouldn't do anything here. This is a pre-existing issue. I
also think that loosing stats when turning track_io_timing on/off would not be
helpful.
Greetings,
Andres Freund
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