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Date: Wed, 8 Mar 2023 12:55:34 +0100
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Subject: Re: Track IO times in pg_stat_io
To: Andres Freund <andres@anarazel.de>,
 Melanie Plageman <melanieplageman@gmail.com>
Cc: Pg Hackers <pgsql-hackers@postgresql.org>,
 Maciek Sakrejda <m.sakrejda@gmail.com>, Justin Pryzby
 <pryzby@telsasoft.com>, smilingsamay@gmail.com,
 Kyotaro Horiguchi <horikyota.ntt@gmail.com>, Lukas Fittl <lukas@fittl.com>,
 Magnus Hagander <magnus@hagander.net>,
 Alvaro Herrera <alvherre@alvh.no-ip.org>
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From: "Drouvot, Bertrand" <bertranddrouvot.pg@gmail.com>
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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