3. Decommission DataNodes

On the NameNode host machine, edit the <HADOOP_CONF_DIR>/dfs.exclude file and add the list of DataNodes hostnames (separated by a newline character).

where <HADOOP_CONF_DIR> is the directory for storing the Hadoop configuration files. For example, /etc/hadoop/conf.

Update the NameNode with the new set of excluded DataNodes. On the NameNode host machine, execute the following command:

su <HDFS_USER> 
hdfs dfsadmin -refreshNodes

where <HDFS_USER> is the user owning the HDFS services. For example, hdfs.

Open the NameNode web UI (http://<NameNode_FQDN>:50070) and navigate to the DataNodes page. Check to see whether the state has changed to Decommission In Progress for the DataNodes being decommissioned.

When all the DataNodes report their state as Decommissioned (on the DataNodes page, or on the Decommissioned Nodes page at http://<NameNode_FQDN>:8088/cluster/ nodes/decommissioned), all of the blocks have been replicated. You can then shut down the decommissioned nodes.

If your cluster utilizes a dfs.include file, remove the decommissioned nodes from the <HADOOP_CONF_DIR>/dfs.include file on the NameNode host machine, then execute the following command:

su <HDFS_USER> 
hdfs dfsadmin -refreshNodes

	Note
	If no dfs.include file is specified, all DataNodes are considered to be included in the cluster (unless excluded in the dfs.exclude file). The dfs.hosts and dfs.hosts.exclude properties in hdfs-site.xml are used to specify the dfs.include and dfs.exclude files.