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

David B. haazeloud at gmail.com
Tue Jul 23 07:08:17 CDT 2013

Hi Faris,

Be very carefull with disk cache policy enabled.

If one of your drive failed, his own cache will be cleaned, you'll loose 
some write commands...
In this case, the FS could be corrupted.

Le 23/07/2013 13:48, Faris Raouf a écrit :
> 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.

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