libvirt users - Jun 2015 - qemu-img snapshots configuration

Hello.

What it the difference in external snapshot configuration with and
without <domainsnapshot> ?

What is the difference between vda and vdb in the following example?
(From https://libvirt.org/formatsnapshot.html)


<domainsnapshot>
...
  <memory snapshot='no'/>
  <disks>
    <disk name='vda' snapshot='external'>
      <driver type='qcow2'/>
      <source file='/path/vda-delta.qcow2'/>
    </disk>
    <disk name='vdb' snapshot='no'/>
  </disks>
  <domain>
    ...
    <devices>
      <disk type='file' device='disk'>
        <driver name='qemu' type='raw'/>
        <source file='/path/vda-img.qcow2'/>
        <target dev='vda' bus='virtio'/>
      </disk>
      <disk type='file' device='disk'
snapshot='external'>
        <driver name='qemu' type='raw'/>
        <source file='/path/vdb-img.qcow2'/>
        <backingStore type='file'>
          <format type='qcow2'/>
          <source file='/path/vdb-delta.qcow2'/>
        </backingStore>
        <target dev='vdb' bus='virtio'/>
      </disk>
      ...
    </devices>
  </domain>
</domainsnapshot>


-- 
sergio.