linux-ext4 - Re: Severe slowdown caused by jbd2 process

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Message-ID: <20110121235641.GM3043@thunk.org>
Date:	Fri, 21 Jan 2011 18:56:41 -0500
From:	Ted Ts'o <tytso@....edu>
To:	Josef Bacik <josef@...hat.com>
Cc:	Jon Leighton <j@...athanleighton.com>, linux-ext4@...r.kernel.org
Subject: Re: Severe slowdown caused by jbd2 process

On Fri, Jan 21, 2011 at 09:31:45AM -0500, Josef Bacik wrote:
> 
> Yup, whatever you are doing in your webapp is making your database do lots of
> fsyncs, which is going to suck.  If you are on a battery backed system or just
> don't care if you lose your database and rather it be faster you can mount your
> ext4 fs with -o nobarrier.  Thanks,

Note that if you don't use -o barrier on ext3, or use -o nobarrier on
ext4, the chance of significant file system damage if you have a power
failure, since without the barrier, the file system doesn't wait for
disk to acknowledge that the data has hit the barrier.  The problem is
that if you are using a barrier operation, you're not going to be able
to get more than about 30-50 non-trivial[1] fsync's per second on a
standard HDD; barriers are inherently slow.

[1] Where there was some kind of data write between the two fsync's.
You may be able to get faster back-to-back fsync() with no intervening
data writes, but that's not terribly interesting.  :-)

A UPS should protect you against most of the dangers of not using
barriers.  The other choice is to be more intelligent with your coding
(and/or with your database choice) to avoid needing a huge number of
fsync's, as they are going to be costly.  If you can batch multiple
database operations under a single commit, for example, you should be
able to eliminate the need for so many fsync's.

		        	       	       - Ted

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