Jonathan Ellis about using Cassandra with mixed spinning disks and SSDs:
Finally, I should point out that taking advantage of SSDs in a Cassandra cluster doesn’t have to be all or nothing. You can mix SSD and spinning disks either at the individual node level, or at the cluster level. For the former, Cassandra allows putting “hot” tables on SSD while leaving “cold” ones on spinning disks. But if you want to use a group of nodes for analytical workloads the way DataStax Enterprise does, Cassandra will also be comfortable with having just those nodes be entirely based on cheaper spinning disks, with the remaining, “realtime” nodes based on SSDs. This latter configuration is a good fit for EC2 deployments.
Original title and link: EC2 Solid State Disks and Cassandra ( ©myNoSQL)