[rbranson]

Cassandra has it's own hurdles, but I think if we're talking about getting your mind in the right place, it might be a better answer. Cassandra definitely has a much more mature scalability implementation that isn't caveat-ridden like MongoDB is. It's operating at scale at both Twitter and Facebook.

Cassandra has online compaction, but still requires up to 2x space for compaction. However, Cassandra does not have to do a full scan of the entire database to do compaction, and almost never actually uses the 2x space. It's also much easier to maintain a Cassandra cluster, because each instance shares equal responsibility, and replication topology is handled for you.

Despite what their fans will say, these are both beta-quality products.