When a device fails, operators can enter the SWAP command to perform dynamic device reconfiguration (DDR). DDR allows the operator to move or swap a demountable volume from a device. When DDR is active, the system dynamically requests the swapping whenever a device encounters device errors. DDR tells the operator to mount the volume on another available device.
Operators can invoke DDR by issuing the SWAP command and specifying the from and to device numbers. See SWAP command for more details.
See z/OS MVS System Commands for the SWAP command.
When swapping tape devices, the from and to devices should have the same density, whenever possible. Swapping devices of unlike but compatible densities can fail the jobs in device allocation at the time of the swap.
On JES3 systems, DDR checks 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.
When a data check occurs in an IBM 3495 Tape Library Dataserver, the system cleans the tape device and retries the failing operation. If the error persists, the system initiates a swap to another eligible system-managed tape library device without involving the operator. The system will try to swap to up to five other devices. If these efforts fail, it issues an error message to the operator and fails the job.