[fedora-virt] Virt Disk Performance

[Ken Smith]

Richard W.M. Jones wrote:
> 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
>>      
Host file system Ext4 formatted on md0, Intel MB
>>       Centos 5.6 Guest, 6.45 MBytes/Sec
>>      
Virtio, QEMU/RAW, No Specific cache setting
>>       RedHat 8 Guest, 0.426 Mbytes/Sec
>>      
Correction this is FC8, IDE Device, QEMU/RAW, No Specific cache setting
>>
>> FC13 Host (i7, 4Gb Memory, One 1Tb sata disk)
>>
>>       Host, 47.6 MBytes/Sec
>>      
Host file system Ext4 formatted LV on partition(s), ASUS MB
>>       Centos 5.6 Guest, 12.52 MBytes/Sec
>>      
IDE, QEMU/RAW, No Specific cache setting
>>       RedHat 7.3 Guest, 1.42 MBytes/Sec
>>      
IDE, QEMU/RAW, No Specific cache setting
>>       FC6 Guest, 16 MBytes/Sec
>>      
IDE, QEMU/RAW, No Specific cache setting
>>       Centos 6 Guest, 15.72 MBytes/Sec
>>      
Virtio, QEMU/RAW, No Specific cache setting
>> 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.
>
>    
I've pulled most of the information you requested. See above. I don't 
see any specific cache settings on either machine. Is there somewhere I 
would see the default setting on the machine? The raw files are the full 
size of the filesystem they are intended to hold.

Thanks

Ken

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