In order to communicate with the rest of the system, disk units are attached to I/O adapters, which are attached to I/O processors, which are attached to buses. The number of each of these types of disk-related hardware that are available on the system directly affects the level of protection that is possible.
To provide the best protection and performance, each level of hardware should be balanced under the next level of hardware. That is, the disk units of each device type and model should be evenly distributed under their I/O adapters. The same number of I/O adapters should be under each I/O processor for that disk type. The I/O processors should be balanced among the available buses.
To plan what disk-related hardware is needed for your mirrored system, you must plan the total number and type of disk units (old and new), that are needed on the system, as well as the level of protection for the system. It is not always possible to plan for and configure a system so that all mirrored pairs meet the planned level of protection. However, it is possible to plan a configuration in which a large percentage of the disk units on the system achieve the required level of protection.