Yeah, I was going to say what Ryan says, that down in SequenceFile, its synchronized anyway so we'd be narrowing the synchronization for sure but ain't sure the narrowing would make for that much in performance improvement.
Chatting too w/ J-D, we probably want the current synchronization to ensure that the get of the next sequence number happens in the same synchronization block that does the actual write. Otherwise, we could have edits going in out of order which could be a prob. if a high number edit goes in before a lower number edit.... we might lose a few edits if we crash after the high number goes in but before the lower edits have a chance to go in.
We might want to revisit if we want to run a pool of WAL-writers.