'Returns the 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_vol_ref: Existing volume to take under management'

def manage_existing_get_size(self, volume, existing_vol_ref):

    (nfs_share, nfs_mount, vol_path) = self._get_share_mount_and_vol_from_vol_ref(existing_vol_ref)
    try:
        LOG.debug('Asked    to    get    size    of    NFS    vol_ref    %s.', existing_vol_ref['source-name'])
        file_path = os.path.join(nfs_mount, vol_path)
        file_size = (float(utils.get_file_size(file_path)) / units.Gi)
        vol_size = int(math.ceil(file_size))
    except (OSError, ValueError):
        exception_message = (_('Failed    to    manage    existing    volume    %(name)s,    because    of    error    in    getting    volume    size.'), {'name': existing_vol_ref['source-name']})
        raise exception.VolumeBackendAPIException(data=exception_message)
    LOG.debug('Reporting    size    of    NFS    volume    ref    %(ref)s    as    %(size)d    GB.', {'ref': existing_vol_ref['source-name'], 'size': vol_size})
    return vol_size
