Isn't fnCreatedRow, fnCreatedCell supposed to be called when fnUpdate is called?

Right now, at least unless I ve messed things up, when I call fnUpdate, only fnRowCallback is called. The problem is that fnRowCallback is called no matter whether a row has been updated or it is just been redrawn and this causes the following problem. If I put complex rendering stuff in fnRowCallback, there is a great performance penalty because this stuff gets called even when not needed (row data not changed). It is needed only in initial table construction and when a row gets updated. And if I put the complex rendering stuff in fnCreatedRow/Cell, everything works fast even with a lot of data and rendering, but I cant rerender just the row that gets updated, because fnCreatedCell/Row doesnt get called in fnUpdate.
Is there a workaround for this? Can I somehow manually call fnCreatedCell/Row ? Any other ideas?
Thanks a lot, Bill