Date: Fri, 5 Jun 2020 22:40:57 +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: <20200605204057.anopw4rntkg6hsag@development>
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On Fri, Jun 05, 2020 at 10:04:14PM +0200, Tomas Vondra wrote:
>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 ...
>

BTW in all three cases it happens right after the first restart point in
the WAL stream:

    LOG:  restored log file "0000000100000108000000FD" from archive
    LOG:  restartpoint starting: time
    LOG:  restored log file "0000000100000108000000FE" from archive
    LOG:  restartpoint complete: wrote 236092 buffers (22.5%); 0 WAL ...
    LOG:  recovery restart point at 108/FC000028
    DETAIL:  Last completed transaction was at log time 2020-06-04
             15:27:00.95139+02.
    LOG:  recovery no longer prefetching: unexpected pageaddr
          108/57000000 in log segment 0000000100000108000000FF, offset 0
    LOG:  restored log file "0000000100000108000000FF" from archive

It looks exactly like this in case of all 3 failures ...

regards

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