Namenode goes into safe mode with error "Space available on volume 'null' is 0"

Symptom:

Namenode goes into safe mode with error "Space available on volume 'null' is 0".
eg:

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.

Root cause:

NFS server is unreachable for some time due to some reason, eg, network issue.

Fix:

After fixing the NFS server issue, manually leave safe mode.

hdfs dfsadmin -safemode leave