I agree with @sagarshimpi , please provide the full log trace from the datanode and see if you are able to find any disk level exception in the logs are not. Also you can open NN UI-->"Datanode volume failures" tab and to confirm this. Leaving safe mode manually will not solve the problem as NN did not receive any block reports from these data nodes, if you will leave the safe mode manually then you will get missing blocks alerts for all the blocks you have (approx 34 million blocks). Did you change any configuration at hdfs level before restarting the whole HDFS service? If yes, then please share the property name. Perform a disk i/o check on all the 4 data nodes using iostat command and check if disks are working fine without any heavy %util.    ... View more