Whereas HDFS is to be used programatically or through a shell, it appears (though the documentation is sparse) that Ceph was designed to be mountable like most traditional unix FSs.  There's the MountableHDFS [1] project for Hadoop, and so these could end up being equivalent interface-wise.  At that point it's all up to how they implement create/append/delete/seek/replication semantics, which Hadoop has way more documentation on than Ceph.<p>The Ceph docs also imply that they have designed it so that it's easy to snapshot directories---I'm not sure whether HDFS has facilities for this.<p>[1] <a href="http://wiki.apache.org/hadoop/MountableHDFS" rel="nofollow">http://wiki.apache.org/hadoop/MountableHDFS</a>