Namenode goes into safe mode with error "Space available on volume 'null' is 0".
WARN org.apache.hadoop.hdfs.server.namenode.NameNodeResourceChecker: Space available on volume 'null' is 0, which is below the configured reserved amount 104857600 WARN org.apache.hadoop.hdfs.server.namenode.FSNamesystem: NameNode low on available disk space. Entering safe mode.
However "dfs.namenode.name.dir" are all showing enough disk space.
NFS server is unreachable for some time due to some reason, eg, network issue.
After fixing the NFS server issue, manually leave safe mode.
hdfs dfsadmin -safemode leave