Hi again all,
Just wanted to give you an update.
Talked to Dell tech support and they recommended using write-
through(!) caching in RAID10 configuration. Well, it didn't work and
got even worse performance.
Anyone have an estimated what a RAID10 on 4 15k SAS disks should
generate in random writes?
I'm really keen on trying Scotts suggestion on using the PERC/6 with
mirror sets only and then make the stripe with Linux SW raid.
Thanks for all the input! Much appreciated.
Cheers,
Henke
11 aug 2008 kl. 17.56 skrev Greg Smith:
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
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