along the lines of how reiser chooses to pack things on disk (heuristic that makes numbers of where and how to pack things).

onodes get a unique id.
– to be used in indexing (onode_index primarily, but also higher level indicies)

onodes get a packing id
– onodes with similar packing ids get put on disk together (theoretically). Numbers can overlap. i.e. non-unique. if two onodes share the same packing id, we REALLY want them to be packed together.