@Override
  public Map<ServerName, List<RegionInfo>> roundRobinAssignment(List<RegionInfo> regions,
    List<ServerName> servers) throws IOException {
    return assign(regions, servers);
  }
a balancer which is only used in maintenance mode.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/balancer/MaintenanceLoadBalancer.java