Date: Tue, 2 Aug 2022 07:37:27 -0700
From: Noah Misch <noah@leadboat.com>
To: David Steele <david@pgmasters.net>
Cc: pgsql-hackers@postgresql.org,
	Kyotaro Horiguchi <horikyota.ntt@gmail.com>,
	Don Seiler <don@seiler.us>
Subject: Re: Race between KeepFileRestoredFromArchive() and restartpoint
Message-ID: <20220802143727.GA3796043@rfd.leadboat.com>
References: <20210202151416.GB3304930@rfd.leadboat.com>
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 <20220731061747.GA3692882@rfd.leadboat.com>
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On Tue, Aug 02, 2022 at 10:14:22AM -0400, David Steele wrote:
> On 7/31/22 02:17, Noah Misch wrote:
> >On Tue, Jul 26, 2022 at 07:21:29AM -0400, David Steele wrote:
> >>On 6/19/21 16:39, Noah Misch wrote:
> >>>On Tue, Feb 02, 2021 at 07:14:16AM -0800, Noah Misch wrote:
> >>>>Recycling and preallocation are wasteful during archive recovery, because
> >>>>KeepFileRestoredFromArchive() unlinks every entry in its path.  I propose to
> >>>>fix the race by adding an XLogCtl flag indicating which regime currently owns
> >>>>the right to add long-term pg_wal directory entries.  In the archive recovery
> >>>>regime, the checkpointer will not preallocate and will unlink old segments
> >>>>instead of recycling them (like wal_recycle=off).  XLogFileInit() will fail.
> >>>
> >>>Here's the implementation.  Patches 1-4 suffice to stop the user-visible
> >>>ERROR.  Patch 5 avoids a spurious LOG-level message and wasted filesystem
> >>>writes, and it provides some future-proofing.
> >>>
> >>>I was tempted to (but did not) just remove preallocation.  Creating one file
> >>>per checkpoint seems tiny relative to the max_wal_size=1GB default, so I
> >>>expect it's hard to isolate any benefit.  Under the old checkpoint_segments=3
> >>>default, a preallocated segment covered a respectable third of the next
> >>>checkpoint.  Before commit 63653f7 (2002), preallocation created more files.
> >>
> >>This also seems like it would fix the link issues we are seeing in [1].
> >>
> >>I wonder if that would make it worth a back patch?
> >
> >Perhaps.  It's sad to have multiple people deep-diving into something fixed on
> >HEAD.  On the other hand, I'm not eager to spend risk-of-backpatch points on
> >this.  One alternative would be adding an errhint like "This is known to
> >happen occasionally during archive recovery, where it is harmless."  That has
> >an unpolished look, but it's low-risk and may avoid deep-dive efforts.
> 
> I think in this case a HINT might be sufficient to at least keep people from
> wasting time tracking down a problem that has already been fixed.
> 
> However, there is another issue [1] that might argue for a back patch if
> this patch (as I believe) would fix the issue.

> [1] https://www.postgresql.org/message-id/CAHJZqBDxWfcd53jm0bFttuqpK3jV2YKWx%3D4W7KxNB4zzt%2B%2BqFg%40mail.gmail.com

That makes sense.  Each iteration of the restartpoint recycle loop has a 1/N
chance of failing.  Recovery adds >N files between restartpoints.  Hence, the
WAL directory grows without bound.  Is that roughly the theory in mind?