Hi,
I'm having trouble benchmarking disk i/o in my vm's. The data I get seems
bogus.
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
results.
When I test the disks on the host directly I get:
seekmark:
/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
identical.
Does anyone have an idea what's happening here?
Regards,
Dennis
