disappointing speed with virtio_blk

[Gerd von Egidy <lists@egidy.de>]

Hi,

I just tried the virtio block device with the intent
to boost disk throughput for my vm.

I ran bonnie++ -r 512 -s 2048 -u nobody -d /tmp:

Version  1.03       ------Sequential Output------ --Sequential Input- --Random-
                    -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine        Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
virtio_blk       2G 14274  42 20206  14 22363 37 31116  92 66731  81 140.8   13
kvm-ide          2G 26065  83 26435  28 24146 33 26587  84 57991  18  91.5    2

The host is a Xeon 3040, 1G RAM (I know that that is a bit 
few, it's just a test machine...), the vm gets 512MB of that.
The data is stored on two SATA disks, mirrored (RAID1) with
md, lvm running on top of that. 

Host and Client are running 2.6.25-0.200.rc8.git3.i686. This
is a kernel from Fedora-Rawhide with kvm manually enabled by me.
KVM version is 64.

Especially writing seems to be slower using virtio, but
reading isn't that much faster.

I thought virtio would improve io speed significantly because
of fewer steps needed to communicate betweeen host and client.
What might be the reason that I can't see a speed boost?

- Wrong setup (The virtio-client boots from /dev/vda1, 
  so I think virtio is working)
- virtio_blk is not matured/tuned enough to give a real
  speed boost
- I'm missing a patch that is not included into 2.6.26-rc8
  but can be found in kvm-git
- The output of bonnie++ is bogus because timing is not that
  accurate within a kvm-client

Any ideas welcome.

Kind regards,

Gerd

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