On Sun, 10 Aug 2008, Henrik wrote:
Normally, when a SATA implementation is running significantly faster than a
SAS one, it's because there's some write cache in the SATA disks turned on
(which they usually are unless you go out of your way to disable them).
Lucky for my I have BBU on all my controllers cards and I'm also not using
the SATA drives for database.
From how you responded I don't think I made myself clear. In addition to
the cache on the controller itself, each of the disks has its own cache,
probably 8-32MB in size. Your controllers may have an option to enable or
disable the caches on the individual disks, which would be a separate
configuration setting from turning the main controller cache on or off.
Your results look like what I'd expect if the individual disks caches on
the SATA drives were on, while those on the SAS controller were off (which
matches the defaults you'll find on some products in both categories).
Just something to double-check.
By the way: getting useful results out of iozone is fairly difficult if
you're unfamiliar with it, there are lots of ways you can set that up to
run tests that aren't completely fair or that you don't run them for long
enough to give useful results. I'd suggest doing a round of comparisons
with bonnie++, which isn't as flexible but will usually give fair results
without needing to specify any parameters. The "seeks" number that comes
out of bonnie++ is a combined read/write one and would be good for
double-checking whether the unexpected results you're seeing are
independant of the benchmark used.
--
* Greg Smith gsmith@xxxxxxxxxxxxx http://www.gregsmith.com Baltimore, MD
