Disk IO issues

Mike McGrath mmcgrath at redhat.com
Wed Dec 31 23:35:42 UTC 2008


On Wed, 31 Dec 2008, Corey Chandler wrote:

> Mike McGrath wrote:
> > Lets pool some knowledge together because at this point, I'm missing
> > something.
> >
> > I've been doing all measurements with sar as bonnie, etc, causes builds to
> > timeout.
> >
> > Problem: We're seeing slower then normal disk IO.  At least I think we
> > are.  This is a PERC5/E and MD1000 array.
> >
>
> 1. Are we sure the array hasn't lost a drive?

I can't physically look at the drive (they're a couple hundred miles away)
but we've seen no reports of it (via the drac anyway).  I'll have to get
the raid software on there to be for sure.  I'd think a degraded raid
array would affect both direct block access and file level access.

> 2. What's your scheduler set to?  CFQ tends to not work in many applications
> where the deadline scheduler works better...
>

I'd tried other schedulers earlier but they didn't seem to make much of a
difference.  Even still, I'll get dealine setup and take a look.

At least we've got the dd and cat problem figured out.  Now to figure out
why there's such a discrepancy between file level reads and block level
reads.  Anyone else have an array of this type and size to run those tests
on?  I'd be curious to see what others are getting.

	-Mike




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