From: Imseih (AWS), Sami <simseih@amazon.com>
To: Drouvot, Bertrand <bertranddrouvot.pg@gmail.com>
To: Andres Freund <andres@anarazel.de>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>
Cc: Maciek Sakrejda <m.sakrejda@gmail.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>
Cc: smilingsamay@gmail.com <smilingsamay@gmail.com>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Cc: Lukas Fittl <lukas@fittl.com>
Cc: Magnus Hagander <magnus@hagander.net>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>
Subject: Re: Track IO times in pg_stat_io
Date: Thu, 9 Mar 2023 14:28:38 +0000
Message-ID: <3BCD168B-CFE2-453C-B3C1-4CC9EE7AB293@amazon.com> (raw)
In-Reply-To: <7b450eef-8717-ce4a-a9c8-ccab24ecd89a@gmail.com>
References: <CAAKRu_ay5iKmnbXZ3DsauViF3eMxu4m1oNnJXqV_HyqYeg55Ww@mail.gmail.com>
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<b5486aa0-e7bc-ada6-3ba0-03d3d29d5a8a@gmail.com>
<CAAKRu_bV70cbMiGTw2drMmgvu4UJcioQ1=H4eAQRCmAamVNCJw@mail.gmail.com>
<20230307184751.jdokdy2xmx3xpaua@awork3.anarazel.de>
<7b450eef-8717-ce4a-a9c8-ccab24ecd89a@gmail.com>
> >>> 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.
> Oh right. So it's even less clear to me to get how one would make use of those new *_time fields, given that:
> - pg_stat_io is "global" across all sessions. So, even if one session is doing some "testing" and needs to turn track_io_timing on, then it
> is even not sure it's only reflecting its own testing (as other sessions may have turned it on too).
> - There is the risk mentioned above of bad interpretations for the "time per operation" metrics.
> - Even if there is frequent enough sampling of it pg_stat_io, one does not know which samples contain track_io_timing changes (at the cluster or session level).
As long as track_io_timing can be toggled, blk_write_time could lead to wrong conclusions.
I think it may be helpful to track the blks_read when track_io_timing is enabled
Separately.
blks_read will be as is and give the overall blks_read, while a new column
blks_read_with_timing will only report on blks_read with track_io_timing enabled.
blks_read_with_timing should never be larger than blks_read.
This will then make the blks_read_time valuable if it's looked at with
the blks_read_with_timing column.
Regards,
--
Sami Imseih
Amazon Web Services (AWS)
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To: pgsql-hackers@postgresql.org
Cc: simseih@amazon.com, bertranddrouvot.pg@gmail.com, andres@anarazel.de, melanieplageman@gmail.com, m.sakrejda@gmail.com, pryzby@telsasoft.com, smilingsamay@gmail.com, horikyota.ntt@gmail.com, lukas@fittl.com, magnus@hagander.net, alvherre@alvh.no-ip.org
Subject: Re: Track IO times in pg_stat_io
In-Reply-To: <3BCD168B-CFE2-453C-B3C1-4CC9EE7AB293@amazon.com>
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