@Override
  public List<RegionPlan> balanceCluster(
    Map<TableName, Map<ServerName, List<RegionInfo>>> loadOfAllTable) throws IOException {
    // do not need to balance in maintenance mode
    return Collections.emptyList();
  }
a balancer which is only used in maintenance mode.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/balancer/MaintenanceLoadBalancer.java