For those of you running multiple virtual disks off of a single storage device or the cache pool, we may have found a key tweak to the XML for virtual machines to improve performance.  We will be implementing this change in an upcoming release, but it will only take affect on newly created VMs, or when existing VMs are updated using the webGui.  To add this change to your VMs right now, the process is fairly simple:

 

• Stop the virtual machine you wish to update.
  
• Click the icon for the VM and click Edit XML
  
• Scroll to the <disk> section of your XML, which should look something like this:
  

    <disk type='file' device='disk'>
      <driver name='qemu' type='raw' cache='none' io='native'/>
      <source file='/mnt/cache/domains/yourvm/vdisk1.img'/>
      <target dev='hda' bus='virtio'/>
      <boot order='1'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </disk>

• Modify the <driver> line by removing the io='native' part and changing the cache to cache='writeback'
  
• Your new <disk> section should look like this:

    <disk type='file' device='disk'>
      <driver name='qemu' type='raw' cache='writeback'/>
      <source file='/mnt/cache/domains/yourvm/vdisk1.img'/>
      <target dev='hda' bus='virtio'/>
      <boot order='1'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </disk>

• Click Update to update the XML and then attempt to start your VM again.

 

Please report back how this impacts virtual machine disk performance in your setup.  Thanks!

 

A side note:  by doing this, you can also refer to your vdisk by going through user shares (eliminating the need to use /mnt/cache or /mnt/disk# in referencing your virtual disk).

Does the contents of this post still stand as the recommended setup ?

You don't need to do anything in this post anymore. It's automatic.

OK thanks, so just need to apply to any existing systems only that don't have these settings in place.

Yes, but its been this way for quite a while now.