Spill InternalCachedBag into only 1 file

I encountered a reducer's OOM because of java.io.DeleteOnExitHook. And I found out that the InternalCachedBag creates a seperate tmp file, and the tmp files is deleted on exit. So the file delete hook caused the OOM.
Why not just hold the tmp file handle and spill only one tmp file?
Too many tmp files may block the tasktracker start process, if the tmp files are not cleaned on time and the tasktracker restarts at this specific time.