Date: Fri, 15 Mar 2024 20:03:39 -0700
From: Noah Misch <noah@leadboat.com>
To: Ronan Dunklau <ronan.dunklau@aiven.io>
Cc: Michael Paquier <michael@paquier.xyz>,
	pgsql-bugs <pgsql-bugs@lists.postgresql.org>
Subject: Re: FSM Corruption (was: Could not read block at end of the relation)
Message-ID: <20240316030339.03.nmisch@google.com>
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On Fri, Mar 15, 2024 at 07:23:46AM +0100, Ronan Dunklau wrote:
> Le mercredi 13 mars 2024, 17:55:23 CET Noah Misch a écrit :
> > On Wed, Mar 13, 2024 at 10:37:21AM +0100, Ronan Dunklau wrote:
> > > I'm a bit worried about triggering additional lseeks
> > > when we in fact have other free pages in the map, that would pass the
> > > test. I'm not sure how relevant it is given the way we search the FSM
> > > with fp_next_slot though...
> > 
> > That's a reasonable thing to worry about.  We could do wrong by trying too
> > hard to use an FSM slot, and we could do wrong by not trying hard enough.
> > 
> > > To address that, I've given a bit of thought about enabling / disabling
> > > the
> > > auto-repair behaviour with a flag in GetPageWithFreeSpace to distinguish
> > > the cases where we know we have a somewhat up-to-date value compared to
> > > the case where we don't (as in, for heap, try without repair, then get an
> > > uptodate value to try the last block, and if we need to look at the FSM
> > > again then ask for it to be repaired) but it brings way too much
> > > complexity and would need careful thought for each AM.
> > > 
> > > So please find attached a patch with the change you propose.
> > > 
> > > Do you have a link to the benchmark you mention though to evaluate the
> > > patch against it ?
> > 
> > Here is one from the thread that created commit 719c84c:
> > https://www.postgresql.org/message-id/flat/CA%2BTgmob7xED4AhoqLspSOF0wCMYEom
> > gHfuVdzNJnwWVoE_c60g%40mail.gmail.com
> > 
> > There may be other benchmarks earlier in that thread.
> 
> Thank you. I tried to run that benchmark, with 4 clients all copying 10M 
> pgbench_accounts row concurrently. With and without the patch, I ran the 
> benchmark 5 times, and took the average. 

Does "that benchmark" refer to "unlogged tables, 4 parallel copies", "logged
tables, 4 parallel copies", or something else?

> master: 15.05s
> patched: 15.24s (+1.25%)
> 
> If I remove the best and worst run for each of those, the difference falls at 
> +0.7%.

To get some additional perspective on the benchmark, how hard would it be to
run one or both of the following?  Feel free to decline if difficult.

- Make GetPageWithFreeSpace() just "return InvalidBlockNumber", then rerun the
  benchmark.  This should be a lot slower.  If not, the bottleneck is
  somewhere unexpected, and we'd need a different benchmark.

- Get profiles with both master and patched.  (lseek or freespace.c functions
  rising by 0.1%-1% would fit what we know.)

Distinguishing a 1% change from a 0% change would need different techniques
still.  If we're genuinely slowing bulk loads by ~1% to account for the
possibly of a flawed post-recovery FSM, that's sad, but I'm inclined to accept
the loss.  A persistent error in an innocent INSERT is unacceptable, and the
alternatives we discussed upthread have their own substantial trouble.  Other
opinions?

Thanks,
nm