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

From: Richard Huxton <dev(at)archonet(dot)com>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Zeugswetter Andreas ADI SD <ZeugswetterA(at)spardat(dot)at>,Koichi Suzuki <suzuki(dot)koichi(at)oss(dot)ntt(dot)co(dot)jp>, josh(at)agliodbs(dot)com,pgsql-hackers(at)postgresql(dot)org, pgsql-patches(at)postgresql(dot)org
Subject: Re: [PATCHES] Full page writes improvement, code update
Date: 2007-03-30 10:27:10
Message-ID: 460CE5FE.5050906@archonet.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
Simon Riggs wrote:
> On Fri, 2007-03-30 at 10:22 +0200, Zeugswetter Andreas ADI SD wrote:
>>> Without a switch, because both full page writes and 
>>> corresponding logical log is included in WAL, this will 
>>> increase WAL size slightly 
>>> (maybe about five percent or so).   If everybody is happy 
>>> with this, we 
>>> don't need a switch.
>> Sorry, I still don't understand that. What is the "corresponding logical
>> log" ?
>> It seems to me, that a full page WAL record has enough info to produce a
>>
>> dummy LSN WAL entry. So insead of just cutting the full page wal record
>> you 
>> could replace it with a LSN WAL entry when archiving the log.
>>
>> Then all that is needed is the one flag, no extra space ?
> 
> The full page write is required for crash recovery, but that isn't
> required during archive recovery because the base backup provides the
> safe base. 

Is that always true? Could the backup not pick up a partially-written 
page? Assuming it's being written to as the backup is in progress. (We 
are talking about when disk blocks are smaller than PG blocks here, so 
can't guarantee an atomic write for a PG block?)

-- 
   Richard Huxton
   Archonet Ltd

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