Safemode is
the maintenance state of Namenode. During this state, Namenode is a read-only
mode for the HDFS cluster, where it does not allow any modifications to file
system or blocks.
During start
up the NameNode loads the file system state from the fsimage and the edits log
file. It then waits for DataNodes to report their blocks so that it does not
prematurely start replicating the blocks though enough replicas already exist
in the cluster. During this time NameNode stays in Safemode. NameNode leaves
Safemode automatically after the DataNodes have reported that most file system
blocks are available
Administrators
can manually enter/leave from safemode.
How to enter safemode
Use “hdfs
dfsadmin -safemode enter” to enter into safemode.
$ hdfs dfsadmin
-safemode enter
Safe mode is
ON
During
safemode, Namenode won’t allow any write/delete operations on HDFS.
$ hadoop fs
-rmdir /user/harikrishna_gurram/dir2
rmdir:
Cannot delete /user/harikrishna_gurram/dir2. Name node is in safe mode
How to leave safemode
Use “hdfs
dfsadmin -safemode leave” to leave from safemode.
$ hdfs
dfsadmin -safemode leave
Safe mode is
OFF
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