'Return size of volume to be managed by manage_existing.
existing_ref is a dictionary of the form:
{\'source-name\': <name of the volume>}'

def manage_existing_get_size(self, volume, existing_ref):

    target_vol_name = existing_ref['source-name']
    self.client_login()
    try:
        size = self.client.get_volume_size(target_vol_name)
        return size
    except exception.DotHillRequestError as ex:
        LOG.exception('Error    manage    existing    get    volume    size.')
        raise exception.Invalid(ex)
    finally:
        self.client_logout()
