libvirt users - Feb 2012 - Regarding file vs block devices for the guest

Hi

There is a general consensus in regards to use of a block device (exported
partition) over a file device (.img or .qcow2) image. It is expected to see
a big performance jump in case of a block device. However in my test I have
seen it is not always the case.

My testbed has Fedora-14.0 as the host and Ubuntu-10.0 as guest running on
Intel x86 4 core/8 thread VTX enabled system. I ran file system benchmarks
from the guest using iozone and measure throughput as well as CPU
utilization. I observed that upto device sizes of 8 GB, file device (.img
with ext3 partition) consistently outperformed block device (ext3 partition
exported directly to the guest). Is similar anomaly seen by the community
too? If not, i.e. block device should always outperform file, which layer
should I be looking at to understand what could be happening. Just to make
sure, I repeated same test a bunch of times to eliminate noise.

As I increased the HDD size, then block device performance drastically
improved and consistently performed better than fileIO. In both tests, my
guest had 512 MB RAM with 1 vcpu. I ran all 11 iozone test to do
performance comparisons.

Any pointer/insight would be extremely helpful.

Thank you in advance.

-- 
Udayan
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