Re: Change default of checkpoint_completion_target

From: Andres Freund <andres(at)anarazel(dot)de>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: "Bossart, Nathan" <bossartn(at)amazon(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>, Peter Eisentraut <peter(dot)eisentraut(at)enterprisedb(dot)com>, Michael Paquier <michael(at)paquier(dot)xyz>, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>, Bernd Helmle <mailings(at)oopsware(dot)de>, "tsunakawa(dot)takay(at)fujitsu(dot)com" <tsunakawa(dot)takay(at)fujitsu(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Change default of checkpoint_completion_target
Date: 2021-01-15 21:51:56
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On 2020-12-08 12:41:35 -0500, Tom Lane wrote:
> FWIW, I kind of like the idea of getting rid of it completely.
> Is there really ever a good reason to set it to something different
> than that? If not, well, we have too many GUCs already, and each
> of them carries nonzero performance, documentation, and maintenance
> overhead.

I like the idea of getting rid of it too, but I think we should consider
evaluating the concrete hard-coded value a bit more careful than just
going for 0.9 based on some old recommendations in the docs. It not
being changeable afterwards...

I think it might be a good idea to immediately change the default to
0.9, and concurrently try to evaluate whether it's really the best value
(vs 0.95, 1 or ...).

FWIW I have seen a few cases in the past where setting the target to
something very small helped, but I think that was mostly because we
didn't yet tell the kernel to flush dirty data more aggressively.


Andres Freund

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