Subject: Re: [PATCH] Refactor SLRU to always use long file names

From: Michael Paquier <[email-masked]>
Date: Thu, 12 Sep 2024 08:55:35 +0900

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.

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.
--
Michael

---

From: Aleksander Alekseev <[email-masked]>
Date: Thu, 12 Sep 2024 12:33:14 +0300

Hi Michael,

> paths in the buildfarm code.  We have a few of them.

Thanks for the feedback.

> 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