HDFS-4666. Define ".snapshot" as a reserved inode name so that users cannot create a file/directory with ".snapshot" as the name. If ".snapshot" is used in a previous version of HDFS, it must be renamed before upgrade; otherwise, upgrade will fail.
hadoop/common/branches/HDFS-2802/hadoop-hdfs-project/hadoop-hdfs/CHANGES.HDFS-2802.txt