'Return size of an existing Vdisk for manage_existing.
existing_ref is a dictionary of the form:
{\'source-id\': <uid of disk>} or
{\'source-name\': <name of the disk>}
Optional elements are:
\'manage_if_in_use\':  True/False (default is False)
If set to True, a volume will be managed even if it is currently
attached to a host system.'

def manage_existing_get_size(self, volume, ref):

    vdisk = self._manage_input_check(ref)
    manage_if_in_use = ref.get('manage_if_in_use', False)
    if ((not manage_if_in_use) and self._helpers.is_vdisk_in_use(vdisk['name'])):
        reason = _('The    specified    vdisk    is    mapped    to    a    host.')
        raise exception.ManageExistingInvalidReference(existing_ref=ref, reason=reason)
    return int(math.ceil((float(vdisk['capacity']) / units.Gi)))
