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

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
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>, "Hannu Krosing" <hannu(at)skype(dot)net>, "Joshua D(dot) Drake" <jd(at)commandprompt(dot)com>, josh(at)agliodbs(dot)com, pgsql-hackers(at)postgresql(dot)org, pgsql-patches(at)postgresql(dot)org
Subject: Re: [HACKERS] Full page writes improvement, code update
Date: 2007-04-13 15:47:41
Message-ID: 19755.1176479261@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
"Simon Riggs" <simon(at)2ndquadrant(dot)com> writes:
> On Fri, 2007-04-13 at 10:36 -0400, Tom Lane wrote:
>> That's what bothers me about this patch, too.  It will be increasing
>> the cost of writing WAL (more data -> more CRC computation and more
>> I/O, not to mention more contention for the WAL locks) which translates
>> directly to a server slowdown.

> I don't really understand this concern.

The real objection is that a patch that's alleged to make WAL smaller
actually does the exact opposite.  Now maybe you can buy that back
downstream of the archiver --- after yet more added-on processing ---
but it still seems that there's a fundamental misdesign here.

> Koichi-san has included a parameter setting that would prevent any
> change at all in the way WAL is written.

It bothers me that we'd need to have such a switch.  That's just another
way to shoot yourself in the foot, either by not enabling it (in which
case applying pg_compresslog as it stands would actively break your
WAL), or by enabling it when you weren't actually going to use
pg_compresslog (because you misunderstood the documentation to imply
that it'd make your WAL smaller by itself).  What I want to see is a
patch that doesn't bloat WAL at all and therefore doesn't need a switch.
I think Andreas is correct to complain that it should be done that way.

			regards, tom lane

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