'Return size of snapshot to be managed by manage_existing.
When calculating the size, round up to the next GB.'

def manage_existing_snapshot_get_size(self, snapshot, existing_ref):

    volume_name = snapshot.get('volume_id')
    provider_id = snapshot.get('provider_id')
    with self._client.open_connection() as api:
        screplay = self._get_unmanaged_replay(api, volume_name, provider_id, existing_ref)
        (sz, rem) = storagecenter_api.SCApi.size_to_gb(screplay['size'])
        if (rem > 0):
            raise exception.VolumeBackendAPIException(data=_('Volume    size    must    be    a    multiple    of    1    GB.'))
        return sz
