Turning safe mode on HA NameNodes

You can use the applicable dfsadmin command options to turn safe mode on for a single or both the active and standby NameNodes.

To turn safe mode on for both NameNodes, run hdfs dfsadmin -safemode enter.
To turn safe mode on for a single NameNode, run hdfs dfsadmin -fs hdfs://<host>:<port> -safemode enter

note
For a full list of dfsadmin command options, run: hdfs dfsadmin -help.