This is the documentation for CDH 4.6.0.
Documentation for other versions is available at Cloudera Documentation.

Step 12: Start up a DataNode

Begin by starting one DataNode only to make sure it can properly connect to the NameNode. Use the service command to run the /etc/init.d script.

 
$ sudo service hadoop-hdfs-datanode start

You'll see some extra information in the logs such as:

10/10/25 17:21:41 INFO security.UserGroupInformation: 
Login successful for user hdfs/fully.qualified.domain.name@YOUR-REALM.COM using keytab file /etc/hadoop/conf/hdfs.keytab

If you can get a single DataNode running and you can see it registering with the NameNode in the logs, then start up all the DataNodes. You should now be able to do all HDFS operations.