Hans de Leenheer blogged about Nexenta again 
good post about Nexenta failover techniques.
1) Disk management:
The first part of failover management is still plane old ZFS. It’s the way disks work together, how I/O is handled and how data corruption is avoided.
- Storage Pool: a set of (equally configured) vDEVs
- vDEV: a set of disks with a certain protection level
- RAIDZ: first form of physical disks protection level. Examples:
- RAIDZ-5: 4 disks data, 1 disk parity
- RAIDZ2-6: 4 disks data, 2 disks parity
- RAIDZ3-9: 6 disks data, 3 disks parity
- Mirrored Disks: second form of physical disks protection level. Here too you have multiple choices of failover (dual-mirror, triple-mirror, …)