Virtualization and disk performance

[David Peall]

Hi

I've been using KVM for a bunch VM's on hardy and now Lucid and with
CPU and memory performing quite well its been no problem.  I'm now
looking at our ageing DB server and wanting to put it in a VM but the
disk performance is dismal or am I doing it wrong?
I'm quite comfortable if I loose 25% or even 33% through
virtualization as the benefits are worth it.

Here are the numbers I have so far (using dbench, ubuntu lucid and
ext4):

Bare metal using a slice for the host OS:
       Throughput 2586.65 MB/sec  10 clients  10 procs
max_latency=18.029 ms
       Throughput 3631.62 MB/sec  50 clients  50 procs
max_latency=239.773 ms
       Throughput 3635.12 MB/sec  100 clients  100 procs
max_latency=458.094 ms

Guest KVM machine using a block device
       Throughput 1130.52 MB/sec  10 clients  10 procs
max_latency=262.047 ms
       Throughput 513.972 MB/sec  50 clients  50 procs
max_latency=6561.761 ms
       Throughput 465.593 MB/sec  100 clients  100 procs
max_latency=2520.585 ms

I tried VMware just as a comparison using a vmdk file (not even a block
device):
       Throughput 1482.44 MB/sec  10 clients  10 procs
max_latency=53.682 ms
       Throughput 2049.45 MB/sec  50 clients  50 procs
max_latency=492.187 ms
       Throughput 2098.71 MB/sec  100 clients  100 procs
max_latency=681.216 ms

Using LVM was worse and Qcow2 was even worse as expected.

Thats a big pill to swallow for KVM.

Any ideas what is the best way to get disk performance using KVM.

Thanks
-- 
David Peall
Domain Name Services
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