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[Xen-users] Slow disk IO

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Subject: [Xen-users] Slow disk IO
From: Mike McGrath <mmcgrath@xxxxxxxxxx>
Date: Sun, 28 Dec 2008 13:39:10 -0600 (CST)
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I'm seeing slower then expected disk IO.  Also, using sar, it seems the
disk is measuring different utilization on the domU then the dom0.  I've
attached to the disk with:


That logical volume has a physical volume of an attached 15 disk raid6
array (an MD1000 from dell).

Here is the sar data I'm referencing:




The disk does have one other guest attached to it but that guest has very
little disk IO.

Other items of note: the data periods above happened during a bacula
backup.  I would have expected much higher data throughput.  We're talking
about an average read speed of 3.58 MBytes/s and it says its 95% utilized.
Meanwhile the dom0 says its only 51% utilized.

Additionally on both the domU and dom0 we're not seeing much iowait.  On
the domU we're only talking about an average of 14%.  If the disks truly
were fully utilized, I'd expect that to be much higher. These are both
RHEL5 boxes.  dom0=2.6.18-92.1.18.el5xen.  domU=2.6.18-105.el5xen.  Also
both are x86_64.



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