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From: Bossart, Nathan <[email protected]>
To: Euler Taveira <[email protected]>
To: Andres Freund <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: O(n) tasks cause lengthy startups and checkpoints
Date: Thu, 2 Dec 2021 21:19:16 +0000
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>
	<[email protected]>
	<[email protected]>

On 12/1/21, 6:06 PM, "Euler Taveira" <[email protected]> wrote:
> Saying that a certain task is O(n) doesn't mean it needs a separate process to
> handle it. Did you have a use case or even better numbers (% of checkpoint /
> startup time) that makes your proposal worthwhile?

I don't have specific numbers on hand, but each of the four functions
I listed is something I routinely see impacting customers.

> For (3), there is already a GUC that would avoid the slowdown during startup.
> Use it if you think the startup time is more important that disk space occupied
> by useless files.

Setting remove_temp_files_after_crash to false only prevents temp file
cleanup during restart after a backend crash.  It is always called for
other startups.

> For (4), you are forgetting that the on-disk state of replication slots is
> stored in the pg_replslot/SLOTNAME/state. It seems you cannot just rename the
> replication slot directory and copy the state file. What happen if there is a
> crash before copying the state file?

Good point.  I think it's possible to deal with this, though.  Perhaps
the files that should be deleted on startup should go in a separate
directory, or maybe we could devise a way to ensure the state file is
copied even if there is a crash at an inconvenient time.

Nathan

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