01 08 09 rack 20 21 22 2301 cpu 07 03 control unit 02 03 rack 24 25 02 cpu 04 05 control unit 04 05 rack 28 29 03 cpu 06 02 control unit 06 07 rack 2c 2d ______________________________________ the ability to bury the addressing within variable drawer configurations is of great importance during the upgrading of the storage devices during operation it has been noted that upgrading previously installed storage devices or other system requirements that require data to be moved to an alternate device medium are very disruptive to a customer and the business operations of that customer it is desirable for that customer to continue with operations during any upgrade procedure examples of upgrades to previously installed disk drives include upgrading the subsystem capacity upgrading the device capacity the logic hardware disk drive or microcode technology additional upgrades may include upgrading the performance characteristics of the disk drives environmental impact of the devices or new installations the storage controller manages the subsystem storage device upgrade the storage controller is capable of maintaining concurrent data access by using unused devices during the upgrade procedure the storage controller is capable of reconfiguring the string size of any dasd string which initially has a fixed number of device addresses assigned to the string the microcode used in the storage controller.