title: Active datanode not displayed by namenode
body: <p>I have a hadoop cluster with 1 name node and 4 data nodes, and the replication value in HDFS-site.xml was set to 3. I recently replaced a data node machine and restarted the cluster. SSH from name node to any data node was set up to use passwordless login, and the firewall on the new data node was also turned off. The cluster works fine. However, the name node only displays 3 data nodes with the newly added data node missing. What's interesting is that JPS displays that DataNode and TaskTracker processes are running on the data node that is not being displayed by the name node. What could be wrong? Thanks!</p>
  
  <p>Update: Just found out that the core-site.xml and mapred-site.xml on the problematic node were pointing to wrong namenode host. After correcting these values and restarting the cluster, all the data nodes are being displayed by the cluster summary.</p>