Message-ID: <4CD4811F.60905@agliodbs.com>
Date: Fri, 05 Nov 2010 15:11:43 -0700
From: Josh Berkus <josh@agliodbs.com>
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To: pgsql-performance@postgresql.org
Subject: Re: Major Linux performance regression; shouldn't we be
	worried about RHEL6?
References: <4CD465D8.4070007@agliodbs.com> <4CD47678.5050002@2ndquadrant.com>
In-Reply-To: <4CD47678.5050002@2ndquadrant.com>
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> The main change here was discussed back in January:
> http://archives.postgresql.org/message-id/4B512D0D.4030909@2ndquadrant.com
> 
> What I've been doing about this is the writing leading up to
> http://wiki.postgresql.org/wiki/Reliable_Writes so that when RHEL6 does
> ship, we have a place to point people toward that makes it better
> documented that the main difference here is a reliability improvement
> rather than a performance regression.  I'm not sure what else we can do
> here, other than organizing more testing for kernel bugs in this area on
> RHEL6.  The only way to regain the majority of the "lost" performance
> here is to turn off synchronous_commit in the default config.

Yeah, I was looking at that.  However, there seems to be some
indications that there was a drop in performance specifically in 2.6.32
which went beyond fixing the reliability:

http://www.phoronix.com/scan.php?page=article&item=linux_2636_btrfs&num=1

However, Phoronix doesn't say what sync option they're using; quite
likely it's O_DSYNC.  Unfortunately, the fact that users now need to be
aware of the fsync_method again, after having it set automatically for
them for the last 4 years, is a usability regression for *us*.  Anything
that's reasonable for us to do about it?

-- 
                                  -- Josh Berkus
                                     PostgreSQL Experts Inc.
                                     http://www.pgexperts.com