Determine if you want Expansion-unit level protection on the following:
- Expansion unit failures are rare compared with other disk-related hardware failures.
- If an expansion unit fails, disk I/O operations may continue, but other hardware is lost (such as work stations, printers and communication lines) making the system unusable.
To achieve expansion-unit level protection, all disk units that are present in the expansion unit must have a mirrored unit present in another expansion unit. The figure shows the elements of expansion-unit level protection: two expansion units that each contain two buses that are attached to separate IOPs, IOAs, and disk units, respectively.