From: Ronan Dunklau <ronan.dunklau@aiven.io>
To: Noah Misch <noah@leadboat.com>
Cc: Michael Paquier <michael@paquier.xyz>
Cc: pgsql-bugs <pgsql-bugs@lists.postgresql.org>
Subject: Re: FSM Corruption (was: Could not read block at end of the relation)
Date: Thu, 11 Apr 2024 09:36:50 +0200
Message-ID: <5959995.31r3eYUQgx@aivenlaptop> (raw)
In-Reply-To: <20240406223037.c5@rfd.leadboat.com>
References: <VtPLBUerbMM8BSDLQwIMV9Co_38iHS1TXE4V09x2ZVFhPkZMNE8LwgPSIzB8PHkVFXykGx04QNAzZVjJFAnU1w==@protonmail.internalid>
<6055942.lOV4Wx5bFT@aivenlaptop>
<20240406223037.c5@rfd.leadboat.com>
Le dimanche 7 avril 2024, 00:30:37 CEST Noah Misch a écrit :
> Your v3 has the right functionality. As further confirmation of the fix, I
> tried reverting the non-test parts of commit 917dc7d "Fix WAL-logging of FSM
> and VM truncation". That commit's 008_fsm_truncation.pl fails with 917dc7d
> reverted from master, and adding this patch makes it pass again. I ran
> pgindent and edited comments. I think the attached version is ready to go.
>
Thank you Noah, the updated comments are much better. I think it should be
backported at least to 16 since the chances of tripping on that behaviour are
quite high here, but what about previous versions ?
> While updating comments in FreeSpaceMapPrepareTruncateRel(), I entered a
> rabbit hole about the comments 917dc7d left about torn pages. I'm sharing
> these findings just in case it helps a reader of the $SUBJECT patch avoid
> the same rabbit hole. Both fsm and vm read with RBM_ZERO_ON_ERROR, so I
> think they're fine with torn pages. Per the README sentences I'm adding,
> FSM could stop writing WAL. I'm not proposing that, but I do bet it's the
> right thing. visibilitymap_prepare_truncate() has mirrored fsm truncate
> since 917dc7d. The case for removing WAL there is clearer still, because
> parallel function visibilitymap_clear() does not write WAL. I'm attaching
> a WIP patch to remove visibilitymap_prepare_truncate() WAL. I'll abandon
> that or pursue it for v18, in a different thread.
That's an interesting finding.
> If I were continuing the benchmark study, I would try SSD, a newer kernel,
> and/or shared_buffers=48GB. Instead, since your perf results show only
> +0.01% CPU from new lseek() calls, I'm going to stop there and say it's
> worth taking the remaining risk that some realistic scenario gets a
> material regression from those new lseek() calls.
Agree with you here.
Many thanks,
--
Ronan Dunklau
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To: pgsql-bugs@postgresql.org
Cc: ronan.dunklau@aiven.io, noah@leadboat.com, michael@paquier.xyz, pgsql-bugs@lists.postgresql.org
Subject: Re: FSM Corruption (was: Could not read block at end of the relation)
In-Reply-To: <5959995.31r3eYUQgx@aivenlaptop>
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