As discussed in
HDFS-5042, there is a chance to lose blocks on power failure if rbw file creation entry is not yet sync to device. Then the block created is nowhere exists on disk. Neither in rbw nor in finalized.
As suggested by kihwal, will discuss and track it in this JIRA.
As suggested by vinayrpet, May be first hsync() request on block file can call fsync on its parent directory (rbw) directory.