It'd be sweet if we could kick a regionserver and have it start recording the read/write load. Then after we'd taken a sample, we could turn off the recording.
Chatting at the meetup today, replaying the WALs would give you the write side (though missing would be the rate at which the client should play the edits – perhaps we could add this to the WALEdit if its not already there?).
Read side we'd need something new recording the read load (Perhaps we'd have a single trace for read and write but somehow you could get the write from the WAL logs). It would be nice too if we could verify that we read the right thing somehow (hash of the return when the trace switch is thrown? Would need to cater to differences in timestamp possibly?)