You'll need to use lsof with a pid specifier (lsof -p PID). The PID must be your target RegionServer's java process (find via 'ps aux | grep REGIONSERVER' or similar).

In the output, you should be able to classify the items as network (sockets) / filesystem (files) / etc., and the interest would be in whatever holds the highest share. For ex. if you see a lot more sockets hanging around, check their state (CLOSE_WAIT, etc.). Or if it is local filesystem files, investigate if those files appear relevant.

If you can pastebin your lsof result somewhere, I can take a look.