[fedora-virt] Virt Disk Performance

[Richard W.M. Jones]

On Sun, Oct 30, 2011 at 08:28:30AM +0000, Ken Smith wrote:
> Hi, I have two virtual machine setups, one is FC13 and the other is 
> FC14, using kvm and guests created using the Virtual Machine Manager.
> 
> I have noticed that the disk performance is slower than I would have 
> expected. Here are some figures
> 
> FC14 Host (i7, 8Gb memory, Two 1Tb sata disks in soft Raid0)
> 
>      Host, 9.76 MBytes/Sec
> 
>      Centos 5.6 Guest, 6.45 MBytes/Sec
> 
>      RedHat 8 Guest, 0.426 Mbytes/Sec
> 
> 
> FC13 Host (i7, 4Gb Memory, One 1Tb sata disk)
> 
>      Host, 47.6 MBytes/Sec
> 
>      Centos 5.6 Guest, 12.52 MBytes/Sec
> 
>      RedHat 7.3 Guest, 1.42 MBytes/Sec
> 
>      FC6 Guest, 16 MBytes/Sec
> 
>      Centos 6 Guest, 15.72 MBytes/Sec
> 
> The tests were run by copying large files (650 Mbytes) and timing the 
> result.
> 
> I didn't expect the legacy 2.4 kernel guests to perform so slowly.
> 
> The performance of the soft raid0 host machine is disappointing.
> 
> On the FC13 host, without Raid, the guest performance at 1/3 of the host 
> is also a surprise.
> 
> What experience and guidance is out there on this area?

There's not enough information in this post to say
what is going on.

What device are you exporting to the guest?  virtio?  IDE?

What are you using on the host to store the disks?  qcow2?  raw file?
sparse or not?  an LV?  a partition?

What precise settings for cache etc are being used?  Use 'virsh
dumpxml' and look at the <disk> section.

Also you should try a later host.  Some performance improvements have
been made in more recent versions of qemu, and in any case F14 is
almost out of support.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
New in Fedora 11: Fedora Windows cross-compiler. Compile Windows
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