On 01/21/2011 06:56 PM, Ted Ts'o wrote:

On Fri, Jan 21, 2011 at 09:31:45AM -0500, Josef Bacik wrote:

quoted

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

Just a note that databases usually already think hard about batching updates 
into transactions which all go to disk on a commit.

Various databases have statistics to show the average size of a transaction, etc 
and that can help you tune your workload,

Ric