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From: Decibel! <decibel@decibel.org>
To: Adriaan van Os <postgres@microbizz.nl>
Cc: pgsql-performance@postgresql.org
Subject: Re: TRUNCATE TABLE
Date: Mon, 6 Aug 2007 00:48:54 -0500
Message-ID: <20070806054853.GK25704@nasby.net> (raw)
In-Reply-To: <46B4F213.3000203@microbizz.nl>
References: <46972DFA.9000900@microbizz.nl>
	<20070713124435.GL31358@mathom.us>
	<21495.1184337630@sss.pgh.pa.us>
	<4697A58E.8060904@microbizz.nl>
	<23092.1184344246@sss.pgh.pa.us>
	<20070713173501.GD12109@alvh.no-ip.org>
	<162867790707131212g17cbc5a9q5d098dbf01c8e502@mail.gmail.com>
	<25418.1184631498@sss.pgh.pa.us>
	<46B050B8.EE98.0025.0@wicourts.gov>
	<46B4F213.3000203@microbizz.nl>

On Sat, Aug 04, 2007 at 11:39:31PM +0200, Adriaan van Os wrote:
> Kevin Grittner wrote:
> >>>>On Mon, Jul 16, 2007 at  7:18 PM, in message 
> >>>><25418.1184631498@sss.pgh.pa.us>,
> >Tom Lane <tgl@sss.pgh.pa.us> wrote: 
> >>Somehow, autovac is doing something that makes the filesystem go nuts
> >>every so often, and take an astonishingly long time to create an empty
> >>file.  But autovac itself doesn't create or delete any files, so what's
> >>up here?
> > 
> >Have you ruled out checkpoints as the culprit?
> 
> That's a good question. I will do some more tests, but I also suspect fsync 
> "cascading"
> <http://www.uwsg.iu.edu/hypermail/linux/kernel/0708.0/1435.html;.

Interesting. I'm guessing that ext3 has to sync out the entire journal
up to the point in time that fsync() is called, regardless of what
files/information the journal contains. Fortunately I think it's common
knowledge to mount PostgreSQL filesystems with data=writeback, which
hopefully eliminates much of that bottleneck... but if you don't do
noatime you're probably still spewing a lot out to the drive.
-- 
Decibel!, aka Jim Nasby                        decibel@decibel.org
EnterpriseDB      http://enterprisedb.com      512.569.9461 (cell)

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