Mirrored protection always provides disk-unit level protection because the storage units are duplicated. If your main concern is protection of data and not high availability, then disk-unit level protection might be adequate. The disk unit is the most likely hardware component to fail, and disk-unit level protection keeps your system available after a disk unit failure.
Concurrent maintenance is often possible for certain types of disk unit failures that have disk-unit level protection.
This figure shows the elements of disk unit level protection: one bus, connected to one I/O processor, connected to one I/O adapter, which is attached to two separate disk units. The two storage units make a mirrored pair. With disk-unit level protection, the system continues to operate after a disk unit failure. If the I/O adapter or I/O processor fails, the system cannot access data on either of the storage units of the mirrored pair, and the system is unusable.