Disk balancer follows/reuses one rule designed by HDFS balancer, that is, only one instance is allowed to run at the same time. This is correct in production system to avoid any inconsistencies, but it's not ideal to write and run unit tests. For example, it should be allowed run plan, execute, scan commands under one setup of disk balancer. One instance rule will throw exception by complaining 'Another instance is running'. In such a case, there's no way to do a full life cycle tests which involves a sequence of commands.