One or more of the following is happening:
1) There are 0 DataNodes in your Hadoop cluster according to an error message
2) There is 0 B configured as capacity (as shown from a "hdfs dfsadmin -report" command).
3) There is one fewer DataNode in your Hadoop cluster than you expect.
4) You run "hdfs dfsadmin -report | grep Hostname" and do not see a node that has its DataNode service (as seen with the jps command) started and stopped from the NameNode with corresponding start-dfs.sh and stop-dfs.sh script runs.
What should you do?
Warning: This will delete all data in your Hadoop cluster.
1. Go to the node that should be a DataNode (it could be the NameNode itself), do the following three steps:
1.1 Delete everything in /app/hadoop/tmp (including subdirectories).
1.2 Are you ready to delete all the content/data in your cluster? If so, run this: hdfs namenode -format #all your data will be deleted permanently!!!
1.3 bash /usr/local/hadoop/sbin/start-dfs.sh
If you are still having problems, you may want to view this posting.