[CentOS] Slow HVM IO performance with newer kernels

henry ritzlmayr centos at rc0.at
Thu Jul 10 14:04:16 UTC 2008

Hi list.

I am testing CentOS 5.2 now. I experience much slower IO under HVM
Guests with the newer kernels, so I started some measurements.

DOM-0 is a fully upgraded CentOS 5.2. DOM-U is CentOS 5.0 not upgraded
at all for testing purposes. DOM-U sits on an LVM Volume. No other
services are running. Hardware was freshly rebooted every time. 

within DOM-0
dd if=/dev/vgpentanol/lvol2 of=/dev/null bs=1M count=1000
gives 134 MB/s

so thats the bare metal value.

within DOM-U
dd if=/dev/hda of=dev/null bs=1M count 1000

50 MB/s with DOM-0 kernel 2.6.18-53.1.19.el5xen
10 MB/s with DOM-0 kernel 2.6.18-92.1.1.el5xen
10 MB/s with DOM-0 kernel 2.6.18-92.1.6.el5xen

so there has been a drop to 20% of the performance within 2.6.18-53.
Any Ideas? 
The machine is a test system - so I am open to any changes in
configuration you might suggest. 

As a side note: A PV Guest gets 85 MB/s no matter which kernel. 


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