From the README, section 4
Additionally, it is strongly recommended that you enable 'dfs.datanode.synconclose' in your hdfs-site.xml configuration file to ensure that, in the face of unexpected power loss to a datanode, files are wholly synced to disk. Additionally, it is strongly recommended that you enable 'dfs.datanode.synconclose' (only available in Apache Hadoop >=1.1.1 or >=0.23) in your hdfs-site.xml configuration file to ensure that, in the face of unexpected power loss to a datanode, files are wholly synced to disk.