> What's the difference between a replex and multiple projections of data with different partitions and sort-orders used in C-store and Vertica?

The 3-replicas for failure tolerance are reused, so that the first 3 ordering projections don't add storage costs to the system.

Also the paper doesn't mention it, but the rebuild traffic is also better distributed on failure if the failure of a replica causes a rebuild that draws from a wider set of machines rather than a single one.