[mmalone]

My point was that for an interactive application it doesn't take a very long time for user experience to degrade substantially.

And we've glossed over my points about newer databases exposing these knobs to clients. It's possible to do exactly what you're describing using Cassandra, for example (er, you might have to do some consistency level tweaking to make writes fail if you can't get a quorum of authoritative nodes, but it wouldn't be hard - and I'm not even sure if that's necessary). It's not possible to do it with MySQL or PostgreSQL without building some intelligent partitioning layer on top. And that layer will probably make it impossible to do joins and add relationship constraints, so you lose any benefits these systems bring to the table.