Whenever a device fails, you can use the SWAP command to invoke dynamic device reconfiguration (DDR), which allows you to move or swap a demountable volume from the device.
Using the SWAP command, you can also turn on or off system-initiated swapping requests. When DDR is on, the system dynamically performs the swapping function whenever the originally-allocated device encounters device errors. DDR tells you to mount the volume on another available device. When the swapping function is turned off, you can invoke operator-initiated DDR by issuing the SWAP command and specifying the “from” and “to” device numbers. (See the SWAP command in MVS system commands reference.)
When swapping tape devices, the “from” and “to” devices should have the same density whenever possible. Swapping devices of unlike but compatible densities (for example, 1600 and 1600/6250) can cause the failure of jobs that are in device allocation at the time of the swap.
On JES3 systems, DDR interfaces with JES3 to ensure that the “to” device has not been assigned to another job or function. When the swap is complete, DDR notifies JES3.