In current DFSInputStream impl, there're a couple of coarser-grained locks in read/pread path, and it has became a HBase read latency pain point so far. In HDFS-6698, i made a minor patch against the first encourtered lock, around getFileLength, in deed, after reading code and testing, it shows still other locks we could improve.
In this jira, i'll make a patch against other locks, and a simple test case to show the issue and the improved result.
This is important for HBase application, since in current HFile read path, we issue all read()/pread() requests in the same DFSInputStream for one HFile. (Multi streams solution is another story i had a plan to do, but probably will take more time than i expected)