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From: Aleksander Alekseev <aleksander@timescale.com>
Date: Thu, 12 Sep 2024 12:33:14 +0300
Message-ID: <CAJ7c6TNs8RY4Yvng4m375mAvsS4wHeUcqR8t4b2_i2j=PE6iNQ@mail.gmail.com>
Subject: Re: [PATCH] Refactor SLRU to always use long file names
To: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Cc: Michael Paquier <michael@paquier.xyz>
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Hi Michael,

> On Wed, Sep 11, 2024 at 04:07:06PM +0300, Aleksander Alekseev wrote:
> > Commit 4ed8f0913bfd introduced long SLRU file names. The proposed
> > patch removes SlruCtl->long_segment_names flag and makes SLRU always
> > use long file names. This simplifies both the code and the API.
> > Corresponding changes to pg_upgrade are included.
>
> That's leaner, indeed.
>
> > One drawback I see is that technically SLRU is an exposed API and
> > changing it may affect third-party code. I'm not sure if we should
> > seriously worry about this. Firstly, the change is trivial and
> > secondly, it's not clear whether such third-party code even exists (we
> > broke this API just recently in 4ed8f0913bfd and no one complained).
>
> Any third-party code using custom SLRUs would need to take care of
> handling their upgrade path outside pg_upgrade.  Not sure there are
> any of them, TBH, but let's see.
>
> > I didn't include any tests for the new pg_upgrade code. To my
> > knowledge we test it manually, with buildfarm members and during
> > alpha- and beta-testing periods. Please let me know if you think there
> > should be a corresponding TAP test.
>
> Removing the old API means that it is impossible to test a move from
> short to long file names.  That's OK by me to rely on the pg_upgrade
> paths in the buildfarm code.  We have a few of them.

Thanks for the feedback.

> There is one thing I am wondering, here, though, which is to think
> harder about a validity check at the end of 002_pg_upgrade.pl to make
> sure that all the SLRU use long file names after running the tests.
> That would mean thinking about a mechanism to list all of them from a
> backend, rather than hardcode a list of them.  Perhaps that's not
> worth it, just dropping an idea in the bucket of ideas.  I would guess
> in the shape of a catalog that's able to represent at SQL level all
> the SLRUs that exist in a backend.

Hmm... IMO it would be a rather niche facility to maintain in PG core.
At least I'm not aware of cases when a DBA wanted to list initialized
SLRUs. Would it be convenient for core / extensions developers?
Creating a breakpoint on SimpleLruInit() or adding a temporary elog()
sounds simpler to me.

It wouldn't hurt re-checking the segment file names in the TAP test
but this would mean hardcoding catalog names which as I understand you
want to avoid. With high probability PG wouldn't start if the
corresponding piece of pg_upgrade is wrong (I checked more than once
:). So I'm not entirely sure if it's worth the effort, but let's see
what others think.

-- 
Best regards,
Aleksander Alekseev