[Linux-PowerEdge] poor disk IO - suggestions please?

Faris Raouf asterisk at raouf.net
Tue Jul 23 06:48:19 CDT 2013

Thanks David.

Switching to software RAID isn't possible in this case.

But beeping beepety beep -- thanks to your "nudge" I enabled the disk cache
and BOOM, iowaits down by a factor of 10 and load average down very
significantly too. Fantastic!

I also happened to notice that in the Help file in OMSA that it says: "For
virtual disks based on SATA drives, the default Disk Cache Policy is
Enabled; and for virtual disks based on SAS drives, it is Disabled", and if
that's not an additional reason to enable then I don't know what is.

Unfortunately OMSA doesn't seem to offer any option other than "enable disk
cache" - there doesn't seem to be an option to change the type or change a
write cache policy or anything. I'm assuming this is a limitation of the
H200 as opposed to OMSA.

I've learned a big lesson here -- don't assume the default is
correct/best/appropriate. I'm not normally this useless. On being given new
hardware, I normally fiddle with every setting until I'm happy that I know
the best way for it to be configured. Unfortunately in this case there was
no time and everything was done in a rush, and I made quite a few incorrect
assumptions and decisions - not just relating to the cache - that I'm
regretting now.

Thanks again David!

I just have to decide on the mp2sas driver -- stock kernel 12.x or Dell 7.x,
so if anyone has any suggestions on that, it would be appreciated.

> The H200 is definitely not very good; can you run software mirror instead
> using it?  I have a set of those same drives in a R410 with the on-board
> doing software raid on CentOS 6 and I'm seeing really great performance;
> 100+ MB/sec writes.
> I'd definitely enable the write back cache, and set it to read ahead, not
> ahead adaptive.  I found on the H200 the read ahead adaptive works against
> you rather than for you.
> David

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