Libvirt Disk Configuration

Libvirt Disk Configuration

Storage issues

Disk configuration for maximum performance on Libvirt/Qemu guests is quite fiddly.

Uffizi (storage VM) suffered from the symptoms described in the following thread : http://www.spinics.net/lists/kvm/msg51361.html

Configuration Sources

The current configuration (which should be optimized for performance) originates from those pages:

• http://blog.vmsplice.net/2013/03/new-in-qemu-14-high-performance-virtio.html
• http://wiki.mikejung.biz/KVM_/_Xen
• https://www.suse.com/documentation/sles11/book_kvm/data/sect1_1_chapter_book_kvm.html

Summary of the configuration

• virtio-blk-pci
• cache=directsync
• io=native
• scsi=off
• config-wce=off
• x-data-plane=on

XML configuration

<domain type='kvm' xmlns:qemu='http://libvirt.org/schemas/domain/qemu/1.0'>
  [...]
  <devices>
    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='directsync' io='native'/>
      <source dev='/dev/vg-louvre/uffizi-root'/>
      <target dev='vda' bus='virtio'/>
    </disk>
    [...]
  </devices>
  [...]
  <qemu:commandline>
    <qemu:arg value='-set'/>
    <qemu:arg value='device.virtio-disk0.scsi=off'/>
    <qemu:arg value='-set'/>
    <qemu:arg value='device.virtio-disk0.config-wce=off'/>
    <qemu:arg value='-set'/>
    <qemu:arg value='device.virtio-disk0.x-data-plane=on'/>
    [...]
  </qemu:commandline>
</domain>