OT (perhaps): i/o performance on usb drives with vmware guests

[Eric Shubert]

I have a pair of USB drives that I've run as a raid-1 pair for quite a 
while (a year or so) attached to a backup server. A couple months ago, I 
got the idea of making the backup server virtual, and indeed it's 
working. I have a raid-1 pair of external USB drives attached to a 
virtual guest, and mounted and accessed by the guest machine.

Now the problem. When the backup server ran on bare iron, I got a pretty 
consistent 20M/s throughput (usb2 std) on each hd (easy to see when the 
array is resyncing), with fairly low cpu use (15-20% iirc). On the 
virtual/guest backup server, I get a pretty consistent 15M/s, with 
45%cpu. It used to be worse, like 100% cpu and 12M/s, before I "tuned" 
vmware memory and i/o a bit.

While the VM disk drive is using elevator=noop for the virtual drive, 
I've tried using the deadline scheduler (what the VM host uses for its 
software raid-1 devices) on the usb drives, with no difference.

Does anyone have an idea what might be the bottleneck?

Thanks for any input. I'm about to post to the VMware community, but I 
wanted to run it by this group first.

-- 
-Eric 'shubes'