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Re: [HACKERS] Full page writes improvement, code update

From: "Zeugswetter Andreas ADI SD" <ZeugswetterA(at)spardat(dot)at>
To: "Koichi Suzuki" <suzuki(dot)koichi(at)oss(dot)ntt(dot)co(dot)jp>,"Josh Berkus" <josh(at)agliodbs(dot)com>
Cc: <pgsql-hackers(at)postgresql(dot)org>,<pgsql-patches(at)postgresql(dot)org>
Subject: Re: [HACKERS] Full page writes improvement, code update
Date: 2007-04-24 08:08:44
Message-ID: E1539E0ED7043848906A8FF995BDA57901F3FFCC@m0143.s-mxs.net (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
> 3) To maintain crash recovery chance and reduce the amount of 
> archive log, removal of  unnecessary full page writes from 
> archive logs is a good choice.

Definitely, yes. pg_compresslog could even move the full pages written
during backup out of WAL and put them in a different file that needs to
be applied before replay of the corresponding WAL after a physical
restore. This would further help reduce log shipping volume.

> To do this, we need both logical log and full page writes in WAL.

This is only true in the sense, that it allows a less complex
implementation of pg_compresslog.

Basically a WAL record consists of info about what happened and
currently eighter per tuple new data or a full page image. The info of
"what happened" together with the full page image is sufficient to
reconstruct the "per tuple new data". There might be a few WAL record
types (e.g. in btree split ?) where this is not so, but we could eighter
fix those or not compress those.

This is why I don't like Josh's suggested name of wal_compressable
eighter.
WAL is compressable eighter way, only pg_compresslog would need to be
more complex if you don't turn off the full page optimization. I think a
good name would tell that you are turning off an optimization.
(thus my wal_fullpage_optimization on/off)

Andreas


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