Map Reduce jobs are failing on a cluster that was just restarted. They worked before restart. What could be wrong? | Hadoop admin questions

The cluster is in a safe mode. The administrator needs to wait for namenode to exit the safe mode before restarting the jobs again
This is a very common mistake by Hadoop administrators when there is no secondary namenode on the cluster and the cluster has not been restarted in a long time. The namenode will go into safemode and combine the edit log and current file system timestamp