Partitions are based on a hash of the primary key. The number of buckets in the system has to be a power of 2. But we can split buckets to increase the number,

Partitions are based on a hash of the primary key. The number of buckets in the system has to be a power of 2. But we can split buckets to increase the number, or even have some buckets that have split and some that haven't yet. Each bucket is stored on 3 separate servers (and the assignment makes sure the three servers are on separate racks).