From: Tom Lane <tgl@sss.pgh.pa.us>
To: Thomas Munro <thomas.munro@gmail.com>
cc: Tomas Vondra <tomas.vondra@enterprisedb.com>,
        Daniel Gustafsson <daniel@yesql.se>,
        Stephen Frost <sfrost@snowman.net>,
        Alvaro Herrera <alvherre@2ndquadrant.com>,
        Andres Freund <andres@anarazel.de>,
 David Steele <david@pgmasters.net>,
        Dmitry Dolgov <9erthalion6@gmail.com>,
        Jakub Wartak <Jakub.Wartak@tomtom.com>,
        Tomas Vondra <tomas.vondra@2ndquadrant.com>,
        pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: WIP: WAL prefetch (another approach)
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Comments: In-reply-to Thomas Munro <thomas.munro@gmail.com>
	message dated "Sat, 27 Nov 2021 14:47:01 +1300"
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Date: Fri, 26 Nov 2021 21:46:55 -0500
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Thomas Munro <thomas.munro@gmail.com> writes:
> On Sat, Nov 27, 2021 at 12:34 PM Tomas Vondra
> <tomas.vondra@enterprisedb.com> wrote:
>> One thing that's not clear to me is what happened to the reasons why
>> this feature was reverted in the PG14 cycle?

> 3.  A wild goose chase for bugs on Tom Lane's antique 32 bit PPC
> machine.  Tom eventually reproduced it with the patches reverted,
> which seemed to exonerate them but didn't leave a good feeling: what
> was happening, and why did the patches hugely increase the likelihood
> of the failure mode?  I have no new information on that, but I know
> that several people spent a huge amount of time and effort trying to
> reproduce it on various types of systems, as did I, so despite not
> reaching a conclusion of a bug, this certainly contributed to a
> feeling that the patch had run out of steam for the 14 cycle.

Yeah ... on the one hand, that machine has shown signs of
hard-to-reproduce flakiness, so it's easy to write off the failures
I saw as hardware issues.  On the other hand, the flakiness I've
seen has otherwise manifested as kernel crashes, which is nothing
like the consistent test failures I was seeing with the patch.

Andres speculated that maybe we were seeing a kernel bug that
affects consistency of concurrent reads and writes.  That could
be an explanation; but it's just evidence-free speculation so far,
so I don't feel real convinced by that idea either.

Anyway, I hope to find time to see if the issue still reproduces
with Thomas' new patch set.

			regards, tom lane