From: Andres Freund <andres@anarazel.de>
To: pgsql-performance@postgresql.org
Subject: Re: performance regression with Linux 2.6.33 and glibc 2.12
Date: Fri, 4 Jun 2010 17:25:05 +0200
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Cc: Tom Lane <tgl@sss.pgh.pa.us>,
 Marc Cousin <cousinmarc@gmail.com>
References: <201006041539.03868.cousinmarc@gmail.com>
	<2135.1275659945@sss.pgh.pa.us>
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On Friday 04 June 2010 15:59:05 Tom Lane wrote:
> Marc Cousin <cousinmarc@gmail.com> writes:
> > I hope I'm not going to expose an already known problem, but I couldn't
> > find it mailing list archives (I only found
> > http://archives.postgresql.org/pgsql- hackers/2009-12/msg01543.php).
> 
> You sure this isn't the well-known "ext4 actually implements fsync
> where ext3 didn't" issue?
I doubt it. It reads to me like he is testing the two methods on the same 
installation with the same kernel 

> > with wal_sync_method = open_datasync (new default)
> > marc=# INSERT INTO test SELECT generate_series(1,100000);
> > INSERT 0 100000
> > Time: 16083,912 ms
> > 
> > with wal_sync_method = fdatasync (old default)
> > 
> > marc=# INSERT INTO test SELECT generate_series(1,100000);
> > INSERT 0 100000
> > Time: 954,000 ms
Its not actually surprising that in such a open_datasync is hugely slower than 
fdatasync. With open_datasync every single write will be synchronous, very 
likely not reordered/batched/whatever. In contrast to that with fdatasync it 
will only synced in way much bigger batches.

Or am I missing something?

I always thought the synchronous write methods to be a fallback kludge and 
didnt realize its actually the preferred method...

Andres