[Xen-users] Poor disk io performance in domUs

[Andrej Radonic <ar@xxxxxxxxxxxxx>]

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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