I think it's also interesting to note that once you do a cluster, you have a distributed system and instantaneous writes are no longer a cheap operation... Plus

I think it's also interesting to note that once you do a cluster, you have a distributed system and instantaneous writes are no longer a cheap operation... Plus how many operations do you need to access full data in your database? For simple one off access, you could propagate the changes into ram on each of your cluster machines (memcached) and only relatively "unimportant" search operations would see stale data.