Re: Change default of checkpoint_completion_target

From: Stephen Frost <sfrost(at)snowman(dot)net>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Peter Eisentraut <peter(dot)eisentraut(at)enterprisedb(dot)com>, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>, "Bossart, Nathan" <bossartn(at)amazon(dot)com>, Michael Paquier <michael(at)paquier(dot)xyz>, 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: 2021-01-19 19:47:48
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* Tom Lane (tgl(at)sss(dot)pgh(dot)pa(dot)us) wrote:
> Stephen Frost <sfrost(at)snowman(dot)net> writes:
> > Any further comments or thoughts on this one?
> This:
> + total time between checkpoints. The default is 0.9, which spreads the
> + checkpoint across the entire checkpoint timeout period of time,
> is confusing because 0.9 is obviously not 1.0; people will wonder
> whether the scale is something strange or the text is just wrong.
> They will also wonder why not use 1.0 instead. So perhaps more like
> ... The default is 0.9, which spreads the checkpoint across almost
> all the available interval, providing fairly consistent I/O load
> while also leaving some slop for checkpoint completion overhead.
> The other chunk of text seems accurate, but there's no reason to let
> this one be misleading.

Good point, updated along those lines.

In passing, I noticed that we have a lot of documentation like:

This parameter can only be set in the postgresql.conf file or on the
server command line.

... which hasn't been true since the introduction of ALTER SYSTEM. I
don't really think it's this patch's job to clean that up but it doesn't
seem quite right that we don't include ALTER SYSTEM in that list either.
If this was C code, maybe we could get away with just changing such
references as we find them, but I don't think we'd want the
documentation to be in an inconsistent state regarding that.

Anyone want to opine about what to do with that? Should we consider
changing those to mention ALTER SYSTEM? Or perhaps have a way of saying
"at server start" that then links to "how to set options at server
start", perhaps..



Attachment Content-Type Size
cct_def_v4.patch text/x-diff 6.9 KB

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