We encountered a situation where the namenode dropped into safe mode after a temporary outage of an NFS mount.
At 12:10 the NFS server goes offline
Oct 8 12:10:05 <namenode> kernel: nfs: server <nfs host> not responding, timed out
This caused the namenode to conclude resource issues:
2011-10-08 12:10:34,848 WARN org.apache.hadoop.hdfs.server.namenode.NameNodeResourceChecker: Space available on volume '<nfs host>' is 0, which is below the configured reserved amount 104857600
Temporary loss of NFS mount shouldn't cause safemode.