docs/en-US/working-with-volumes.xml

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<section id="working-with-volumes">
    <title>Using Swift for Secondary Storage</title>
    <para>A volume provides storage to a guest VM.  The volume can provide for
      a root disk or an additional data disk.  &PRODUCT; supports additional
      volumes for guest VMs.
    </para>
    <para>Volumes are created for a specific hypervisor type. A volume that has
      been attached to guest using one hypervisor type (e.g, XenServer) may not
      be attached to a guest that is using another hypervisor type (e.g. 
      vSphere, KVM).  This is because the different hypervisors use
      different disk image formats.
    </para>
    <para>&PRODUCT; defines a volume as a unit of storage available to a guest
      VM. Volumes are either root disks or data disks. The root disk has "/"
      in the file system and is usually the boot device. Data disks provide
      for additional storage (e.g. As "/opt" or "D:"). Every guest VM has a root
      disk, and VMs can also optionally have a data disk. End users can mount
      multiple data disks to guest VMs. Users choose data disks from the disk
      offerings created by administrators. The user can create a template from
      a volume as well; this is the standard procedure for private template
      creation. Volumes are hypervisor-specific: a volume from one hypervisor
      type may not be used on a guest of another hypervisor type.
    </para>    
</section>