Date: Sun, 4 Aug 2019 19:30:04 +0200
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Petr Jelinek <petr@2ndquadrant.com>
Cc: Andres Freund <andres@anarazel.de>,
	Andrey Borodin <x4mmm@yandex-team.ru>,
	Konstantin Knizhnik <k.knizhnik@postgrespro.ru>,
	Michael Paquier <michael@paquier.xyz>,
	pgsql-hackers <pgsql-hackers@postgresql.org>,
	Vladimir Leskov <vladimirlesk@yandex-team.ru>
Subject: Re: pglz performance
Message-ID: <20190804173004.ovg6tsf336teyy57@development>
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On Sun, Aug 04, 2019 at 05:53:26PM +0200, Petr Jelinek wrote:
>
> ...
>
>>
>>4) I did a simple test with physical replication, with lz4 enabled on
>>both sides (well, can't build without lz4 anyway, per previous point).
>>It immediately failed like this:
>>
>>FATAL:  failed to restore block image
>>CONTEXT:  WAL redo at 0/5000A40 for Btree/INSERT_LEAF: off 138
>>LOG:  startup process (PID 15937) exited with exit code 1
>>
>>This is a simple UPDATE on a trivial table:
>>
>>create table t (a int primary key);
>>insert into t select i from generate_series(1,1000) s(i);
>>update t set a = a - 100000 where random () < 0.1;
>>
>>with some checkpoints to force FPW (and wal_compression=on, of course).
>>
>>I haven't tried `make check-world` but I suppose some of the TAP tests
>>should fail because of this. And if not, we need to improve coverage.
>>
>
>FWIW I did run check-world without problems, will have to look into this.
>

Not sure if we bother to set wal_compression=on for check-world (I don't
think we do, but I may be missing something), so maybe check-world does
not really test wal compression.

IMO the issue is that RestoreBlockImage() still calls pglz_decompress
directly, instead of going through pg_decompress().


regards

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