Major Description
Why does FSDataOutputStream.close() call flush()? This stream itself does not store any data that it needs to flush. It is a wrapper and it should just invoke its outputstream's close().
For. e.g one bad side effect is that, in the case of DFSOutputStream which extends FSOutputSummer, flush() inside close sends the current data even though FSOutputSummer might have some data.. this left over data will be sent in side close() (so it sends data in two different packets instead of one). Other filesystems might have similar side effects.
I will submit a patch.