[YZF]

It can be hard to tell if the machine is down as in permanently dead including the disk drives or if it's temporarily down, unreachable or just busy. Obviously if you want to maintain the same replication factor you need to create new copies of data that is temporarily down to a lower replication number. In theory though you could still serve reads as long as a single replica survives and you could serve writes regardless.

You can kind of get this with Cassandra if you write with CL_ANY and read with CL_ONE but hinted handoffs don't work so great (better in 3.0?) and reading with ONE may get you stale data for a long while. It would be nice, and I don't think there's any theory that says it can't be done, if you could keep writing new data into the database at the right replication factor in the presence of a partition and you could also read that data. Obviously accessing data that is solely present on the other side of the partition isn't possible so not much can be done about that...