[CentOS-virt] disk i/o benchmarking inside vm

Dennis Jacobfeuerborn dennisml at conversis.de
Sat Oct 8 13:45:31 EDT 2011

I'm having trouble benchmarking disk i/o in my vm's. The data I get seems 
I have two centos 6 guests which use a raw image as volume. Each volume is 
stored on its own physical disk and both disks are the same model. The host 
system is fedora 15 with the virt-preview repo enabled. The disks for the 
guests use virtio and caching is set to none.

My problem is that I get very different results when I benchmark I/O in 
these guests even though I shouldn't. Doing a seekmark I get:
guest 1: 120 seeks/s
guest 2: 220 seeks/s

"hdparm -t" shows:
guest 1: 100 MB/s
guest 1: 160 MB/s

What's worse is that when I restart the guests the results change and 
suddenly guest 1 is a lot faster and guest 2 is a lot slower however as 
long as the guests are running repeating the benchmarks give consistent 

When I test the disks on the host directly I get:
/dev/sdb: 75 seeks/s
/dev/sdc: 75 seeks/s

hdparm -t:
/dev/sdb: 140 MB/s
/dev/sdc: 140 MB/s

What bugs me is not so much the absolute numbers (for now) but the fact 
that these guests give so wildly inconsistent results. Even if the jump 
from 75 seeks/s to 120 seeks/s from host to guest is explainable by the way 
block i/o is handled in the virtualization layer I would still expect both 
guests to return similar results and I would also expect to see similar 
results across restarts of a single guest.

I've attached the definition for both guests even though they are virtually 

Does anyone have an idea what's happening here?

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