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Subject: Re: allow spread checkpoints when changing checksums online
From: Daniel Gustafsson <daniel@yesql.se>
In-Reply-To: <7985b839-8c0c-4194-bbf8-ee07ae5ef13b@vondra.me>
Date: Tue, 7 Jul 2026 23:00:27 +0200
Cc: PostgreSQL Hackers <pgsql-hackers@postgresql.org>
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Message-Id: <C2F1C022-DA80-4F05-AF03-4811CC02F406@yesql.se>
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 <7985b839-8c0c-4194-bbf8-ee07ae5ef13b@vondra.me>
To: Tomas Vondra <tomas@vondra.me>
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> On 6 Jul 2026, at 13:43, Tomas Vondra <tomas@vondra.me> wrote:

> Here's a rebased patch, addressing the minor bitrot, and with some =
very
> minor adjustments.

Thanks for picking up this!

> 3) I've noticed the SGML docs don't have default values for the =
existing
> parameters (cost_delay, cost_limit), so I added those. Maybe =
backpatch?

+1, that sounds like a good idea.

> 4) I've added the new 'fast' parameter to the DataChecksums module, =
but
> mostly just for completeness. The TAP tests still use the default =
value,
> so that it doesn't take longer.

+        spread (throttled), which may increase the time needed to =
enable checksums,

I think we should stick to the "data checksums" terminology for =
consistency.

@@ -1676,6 +1686,7 @@ DataChecksumsWorkerMain(Datum arg)
=20
 			DataChecksumState->cost_delay =3D =
DataChecksumState->launch_cost_delay;
 			DataChecksumState->cost_limit =3D =
DataChecksumState->launch_cost_limit;
+			DataChecksumState->fast_checkpoint =3D =
DataChecksumState->launch_fast_checkpoint;
 		}
 		else
 			costs_updated =3D false;

This update will only change the fast parameter if cost_limit or =
cost_delay
were changed, shouldn't the fast parameter also be checked for runtime =
update?

> I've spent some time thinking about the default value. For a while I
> thought maybe it should default to 'false' (in which case we use =
spread
> checkpoints, which is the less disruptive option).
>=20
> But that's actually futile, because the cost_delay still defaults to =
0,
> which means the "data page rewrite" phase won't be paced. It seems
> surprising the first phase would not be throttled by default, while =
the
> checkpoints are ...
>=20
> So I ended up setting the 'fast=3Dtrue' default.

I think fast=3Dtrue is the right default.

> This is not a very complicated patch, and it's been part of the old
> patch series for quite a while. So unless there's some objections I'll
> get this committed in a couple days.

+1

--
Daniel Gustafsson