Another thing: Many applications, can get away with updating to disk much less often, say every 30 seconds or even longer if appropriate. When a machine fails then, you lose 30 seconds worth of updates stored in that machine's RAM, which may be fine. For some applications, you could probably stand losing even a few minutes of updates on a failure, while gaining huge decreases in loads on disk.
Request-response, XHR, long-polling, "COMET" (blarg, I hate that moniker) when talking to the backend?
