When mirrored protection is started, most systems show little difference in performance; in some cases, mirrored protection can improve performance. Generally, functions that do mostly read operations see equal or better performance with mirrored protection. This is because read operations have a choice of two storage units to read from, and the one with the faster expected response time is selected. Operations that do mostly write operations (such as updating database records) might see slightly reduced performance on a system that has mirrored protection because all changes must be written to both storage units of the mirrored pair. Thus, restore operations are slower.
In some cases, if the system ends abnormally, the system cannot determine whether the last updates were written to both storage units of each mirrored pair. If the system cannot determine whether the last changes were written to both storage units of the mirrored pair, the system synchronizes the mirrored pair by copying the data in question from one storage unit of each mirrored pair to the other storage unit. The synchronization occurs during the IPL that follows the abnormal system end. If the system can save a copy of main storage before it ends, the synchronization process takes just a few minutes. If not, the synchronization process can take much longer. The extreme case might be close to a complete synchronization.
If you have frequent power outages, you might want to consider adding an uninterruptible power supply to your system. Should main power be lost, the uninterruptible power supply allows the system to continue. A basic uninterruptible power supply allows the system time to save a copy of main storage before ending, which avoids long recovery. Both storage units of the load source mirrored pair must be powered by the basic uninterruptible power supply.