'Generates a GCS backup object name prefix.
prefix = volume_volid/timestamp/az_saz_backup_bakid
volid is volume id.
timestamp is time in UTC with format of YearMonthDateHourMinuteSecond.
saz is storage_availability_zone.
bakid is backup id for volid.'

def _generate_object_name_prefix(self, backup):

    az = ('az_%s' % self.az)
    backup_name = ('%s_backup_%s' % (az, backup.id))
    volume = ('volume_%s' % backup.volume_id)
    timestamp = timeutils.utcnow().strftime('%Y%m%d%H%M%S')
    prefix = ((((volume + '/') + timestamp) + '/') + backup_name)
    LOG.debug('generate_object_name_prefix:    %s', prefix)
    return prefix
