From: 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, 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: Wed, 8 Mar 2023 12:55:34 +0100
Message-ID: <7b450eef-8717-ce4a-a9c8-ccab24ecd89a@gmail.com> (raw)
In-Reply-To: <20230307184751.jdokdy2xmx3xpaua@awork3.anarazel.de>
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<20230307184751.jdokdy2xmx3xpaua@awork3.anarazel.de>
Hi,
On 3/7/23 7:47 PM, Andres Freund wrote:
> 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.
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).
> I think we simply shouldn't do anything here. This is a pre-existing issue.
Oh, never thought about it. You mean like for pg_stat_database.blks_read and pg_stat_database.blk_read_time for example?
> I also think that loosing stats when turning track_io_timing on/off would not be
> helpful.
>
Yeah not 100% sure too as that would lead to other possible bad interpretations.
Regards,
--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com
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To: pgsql-hackers@postgresql.org
Cc: bertranddrouvot.pg@gmail.com, andres@anarazel.de, melanieplageman@gmail.com, m.sakrejda@gmail.com, pryzby@telsasoft.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: <7b450eef-8717-ce4a-a9c8-ccab24ecd89a@gmail.com>
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