'Returns size of volume to be managed by manage_existing.
When calculating the size, round up to the next GB.
:param volume:       Cinder volume to manage
:param existing_ref: Driver-specific information used to identify a
volume'

def manage_existing_get_size(self, volume, existing_ref):

    (nfs_share, nfs_mount, volume_name) = self._get_share_mount(existing_ref)
    try:
        volume_path = os.path.join(nfs_mount, volume_name)
        vol_size = int(math.ceil((float(utils.get_file_size(volume_path)) / units.Gi)))
    except OSError:
        msg = (_('Failed    to    get    size    of    volume    %s') % existing_ref['source-name'])
        raise exception.VolumeDriverException(msg)
    return vol_size
