When serializing operations (eg. for two-phase commit systems), it's useful to be able to wait until data has been sent to all replicas. Currently, this requires knowing the correct, maximum value of "w" to pass to getLastError (or setting up replica tags), and even that won't be correct if some replicas are offline.

I'd recommend allowing

, akin to 'majority', to allow waiting for all online replicas. ('$online' may be better than "online", so it can't clash with replica tags in existing deployments.)