Date: Mon, 15 Mar 2021 23:09:13 -0700
From: Noah Misch <noah@leadboat.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Mark Dilger <mark.dilger@enterprisedb.com>,
	Robert Haas <robertmhaas@gmail.com>, Peter Geoghegan <pg@bowt.ie>,
	Thomas Munro <thomas.munro@gmail.com>,
	"Andrey M. Borodin" <x4mmm@yandex-team.ru>,
	Stephen Frost <sfrost@snowman.net>,
	Michael Paquier <michael@paquier.xyz>, Amul Sul <sulamul@gmail.com>,
	Dilip Kumar <dilipbalaut@gmail.com>,
	Andres Freund <andres@anarazel.de>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: pg_amcheck contrib application
Message-ID: <20210316060913.GA3323238@rfd.leadboat.com>
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On Mon, Mar 15, 2021 at 02:57:20PM -0400, Tom Lane wrote:
> Mark Dilger <mark.dilger@enterprisedb.com> writes:
> > On Mar 15, 2021, at 10:04 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >> These animals have somewhat weird alignment properties: MAXALIGN is 8
> >> but ALIGNOF_DOUBLE is only 4.  I speculate that that is affecting their
> >> choices about whether an out-of-line TOAST value is needed, breaking
> >> this test case.

That machine also has awful performance for filesystem metadata operations,
like open(O_CREAT).  Its CPU and read()/write() performance are normal.

> > The logic in verify_heapam only looks for a value in the toast table if
> > the tuple it gets from the main table (in this case, from pg_statistic)
> > has an attribute that claims to be toasted.  The error message we're
> > seeing that you quoted above simply means that no entry exists in the
> > toast table.
> 
> Yeah, that could be phrased better.  Do we have a strong enough lock on
> the table under examination to be sure that autovacuum couldn't remove
> a dead toast entry before we reach it?  But this would only be an
> issue if we are trying to check validity of toasted fields within
> known-dead tuples, which I would argue we shouldn't, since lock or
> no lock there's no guarantee the toast entry is still there.
> 
> Not sure that I believe the theory that this is from bad luck of
> concurrent autovacuum timing, though.

With autovacuum_naptime=1s, on hornet, the failure reproduced twice in twelve
runs.  With v6-0001-Turning-off-autovacuum-during-corruption-tests.patch
applied, 196 runs all succeeded.

> The fact that we're seeing
> this on just those two animals suggests strongly to me that it's
> architecture-correlated, instead.

That is possible.