On Wed, 25 Apr 2007, Pawe�~B Gruszczy�~Dski wrote:
> I was just reading some informations on the web (for example:
> http://www.nabble.com/a-comparison-of-ext3,-jfs,-and-xfs-on-hardware-raid-t144738.html).
You were doing your tests with a database scale of 50. As Heikki already
pointed out, that's pretty small (around 800MB) and you're mostly
stressing parts of the system that may not change much based on filesystem
choice. This is even more true when some of your tests are only using a
small amount of transactions in a short period of time, which means just
about everything could still be sitting in memory at the end of the test
with the database disks barely used.
In the example you reference above, a scaling factor of 1000 was used.
This makes for a fairly large database of about 16GB. When running in
that configuration, as stated he's mostly testing seek performance--you
can't hold any significant portion of 16GB in memory, so you're always
moving around the disks to find the data needed. It's a completely
different type of test than what you did.
If you want to try and replicate the filesystem differences shown on that
page, start with the bonnie++ tests and see if you get similar results
there. It's hard to predict whether you'll see the same differences given
how different your RAID setup is from Jeff Baker's tests.
It's not a quick trip from there to check if an improvement there holds up
in database use that's like a real-world load. In addition to addressing
the scaling factor issue, you'll need to so some basic PostgreSQL
parameter tuning from the defaults, think about the impact of checkpoints
on your test, and worry about whether your WAL I/O is being done
efficiently before you get to the point where the database I/O is being
measured usefully at all via pgbench.
--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD