Date: Fri, 5 Jun 2020 22:04:14 +0200
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Thomas Munro <thomas.munro@gmail.com>
Cc: Dmitry Dolgov <9erthalion6@gmail.com>,
	David Steele <david@pgmasters.net>,
	Andres Freund <andres@anarazel.de>,
	Alvaro Herrera <alvherre@2ndquadrant.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: WIP: WAL prefetch (another approach)
Message-ID: <20200605200414.kp24q5yznzln74hl@development>
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On Fri, Jun 05, 2020 at 05:20:52PM +0200, Tomas Vondra wrote:
>
> ...
>
>which is not particularly great, I guess. There however seems to be
>something wrong, because with the prefetching I see this in the log:
>
>prefetch:
>2020-06-05 02:47:25.970 CEST 1591318045.970 [22961] LOG:  recovery no
>longer prefetching: unexpected pageaddr 108/E8000000 in log segment
>0000000100000108000000FF, offset 0
>
>prefetch2:
>2020-06-05 15:29:23.895 CEST 1591363763.895 [26676] LOG:  recovery no
>longer prefetching: unexpected pageaddr 108/E8000000 in log segment
>000000010000010900000001, offset 0
>
>Which seems pretty suspicious, but I have no idea what's wrong. I admit
>the archive/restore commands are a bit hacky, but I've only seen this
>with prefetching on the SATA storage, while all other cases seem to be
>just fine. I haven't seen in on NVME (which processes much more WAL).
>And the SATA baseline (no prefetching) also worked fine.
>
>Moreover, the pageaddr value is the same in both cases, but the WAL
>segments are different (but just one segment apart). Seems strange.
>

I suspected it might be due to a somewhat hackish restore_command that
prefetches some of the WAL segments,  so I tried again with a much
simpler restore_command - essentially just:

   restore_command = 'cp /archive/%f %p.tmp && mv %p.tmp %p'

which I think should be fine for testing purposes. And I got this:

   LOG:  recovery no longer prefetching: unexpected pageaddr 108/57000000
         in log segment 0000000100000108000000FF, offset 0
   LOG:  restored log file "0000000100000108000000FF" from archive

which is the same segment as in the earlier examples, but with a
different pageaddr value. Of course, there's no such pageaddr in the WAL
segment (and recovery of that segment succeeds).

So I think there's something broken ...


regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services