FSDataOutputStream#hsync checks if a stream implements Syncable, and if so, calls hsync. Otherwise, it just calls flush. This is used, for instance, by YARN's FileSystemTimelineWriter.
DFSStripedOutputStream extends DFSOutputStream, which implements Syncable. However, DFSStripedOS throws a runtime exception when the Syncable methods are called.
We should refactor the inheritance structure so DFSStripedOS does not implement Syncable.