Re: [kvm-devel] disappointing speed with virtio_blk

[Anthony L. <an...@co...>]

Gerd von Egidy wrote:
> Hi,
>
> I just tried the virtio block device with the intent
> to boost disk throughput for my vm.
>   

We really haven't optimized virtio block yet under KVM.  Most of the 
effort so far has been focused on virtio_net.  We'll get there though in 
the near future.

Regards,

Anthony Liguori

> 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
>
>