-journal COW credits
     journal COW credits have to be recomputed for extent file.

-partial-write
     Partial-write need to be handle more elegently for snapshot.
     A private block_write_begin() is needed.

-extent merge
     Try to merge extent after MOW. A more sophiscated extent splitting
     function is needed. 

-flex bg
     Shrink snapshots correctly in non-first flex group
