By default SmartOS aggregates all your disks together into a single ZFS pool (SmartOS names this pool "zones"). From this pool you create ZFS datasets which can either look like block devices (used for KVM virtual machines) or as filesystems (used for SmartOS zones).
You can setup more than one pool in SmartOS, but you will have to do it manually. The Solaris documentation is still quite good and applicable to modern Illumos distributions (including SmartOS). Chapter 4 has all the relevant information for creating a new ZFS pool, but it can be as simple as:
zpool create some_new_pool_name c1t0d0 c1t1d0
This assumes that you have access to the global zone.
If I were running a Cassandra cluster on bare metal and I wanted to benefit from things like ZFS and DTrace I would probably use OmniOS instead of SmartOS. I don't want any contention for resources with my database machines, so I wouldn't run any other zones or VMs on that hardware (which is what SmartOS is really good at).