What do you need to do when adding new slave nodes to a cluster?

A.
Halt and resubmit any running MapReduce jobs to guarantee correctness.

B.
Restart the NameNode daemon.

C.
Add a new entry to /etc/slavenodes on the NameNode host.

D.
Increase the value of dfs.number.of.nodes in hdfs-site.xml.

E.
add the new node’s DNS name to the conf/slaves file on the master node.

Explanation:
http://www.quora.com/How-to-add-a-node-in-Hadoop-cluster