[libvirt-users] Proper way to reference a disk

Michael Schwager mschwager at mochotrading.com
Wed May 6 20:05:19 UTC 2015

We are running on CentOS 7.0. We want to move our VMs (kvm/qemu) from
qcow/image files to logical volumes under LVM.

I have moved one of them but I had a little trouble editing the disk
properly and I wonder what is the correct way to do it. Specifically, in
the XML under the devices/disk section why does the source say "source file=",
and not "source dev="?  The reason I ask is that I have a number of other
machines to move and I don't want to mess them up.

The source file entry for the disk was like this in our xml file:
<source file='/var/lib/libvirt/images/pxetest.qcow2'/>

I got the vm to work by converting the image to raw, then dumping via dd
into the LV, and editing the XML using virsh as follows:
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw' cache='none'/>
      <*source file*='/dev/kvm_temp/pxetest'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04'

By comparison I have a test machine on another real host upon which I
loaded a VM and put its disk into a logical volume. Its disk section looks
like this:

    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='none' io='native'/>
      <*source dev*='/dev/kvm_hosts/pxetest2'/>
      <target dev='hda' bus='ide'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>

The source file / source dev thing really has me wondering.

*- Mike Schwager*

*  Linux Network Engineer, Mocho Trading LLC*

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