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[Xen-users] Poor disk io performance in domUs

To: xen-users@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-users] Poor disk io performance in domUs
From: Andrej Radonic <ar@xxxxxxxxxxxxx>
Date: Thu, 21 Jun 2007 07:49:52 +0200
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Hi,

in a previously well behaving xen setup we are suddenly experiencing massive loss of disk performance.

Setup: two identical machines (dell, 2 quadcore cpus each, 8 GB RAM each), attached to *one* SAS storagebox (Raid5 SAS disks), CentOS 5, Xen 3.04 running three domUs each. Storage partitions are exposed as block-devices to the guests. Filesystem is ext3.

Since a domU running a rather heavily loaded mailserver (many small files, many reads/writes) has been added, io performance has deteriorated significantly.

In dom0 about 100 MB/s can be achieved, in domU only about 50 MB/s. This looks even worse when there are massive concurrent / parallel io jobs, then throughput goes down to 20 MB/s.

During all this the cpus are mostly idle...

Any ideas? I know some performance degradation is to be expected but shurely not on this scale?
Thanks,
Andrej


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