The SafeMode of Hadoop

When the cluster starts, it enters security mode first. The system checks the integrity of the data block in Safe mode. Assuming that we set the number of replicas (that is, parameter dfs.replication) is 5, then there should be 5 copies on the Datanode, assuming there are only 3 copies, then the ratio is 3/5=0.6. A minimum replica rate of 0.999 is defined in profile hdfs-default.xml, so the system automatically replicates the replica to the other Datanode, striving for the minimum replica rate of >=0.999. If there are 8 replicas in the system that exceed the 5 copies we set, the system will also delete more than 3 copies.

Command Hadoop fs–safemode Get view security mode status

Command Hadoop fs–safemode enter into Safe mode state

Command Hadoop fs–safemode leave leave Safe Mode state

The SafeMode of Hadoop