Allocating Devices during Deferred Checkpoint/Restart

When you do a deferred restart from a checkpoint, the type of device allocated for the data set depends on the specification in the UNIT parameter of the DD statement. In addition to assuring the same device type for a checkpoint and restart, the system also attempts to allocate a device with the same optional features present at the time the checkpoint was taken.

If a device number was specified (for example, UNIT=190), then a device of that type is allocated. It may or may not be the device requested.
If a device name was specified, then a device of that type is allocated.
If a user-defined name for a single type of device was specified (for example, UNIT=DISK1), then a device of the defined type is allocated.
If a name for a mixed group of devices was specified (for example, UNIT=SYSDA), then a device of the same type as that used when the checkpoint was taken is allocated.
However, if the mixed group includes devices with varying optional characteristics (for example with and without RPS or DASD shared and not shared between processors), a device with the same optional characteristics is not guaranteed. To insure that the device allocated by checkpoint/restart has the same optional characteristics as the device used when the checkpoint was taken, define some generics at system definition time that include only a single group with the same optional characteristics.

In jobs that can restart, you should avoid using generic unitnames or esoteric unitnames (for example, UNIT=TAPE) that contain more than one device type. Allocation failure may result during restart if too few units of a specific device type are available. For example, if UNIT=TAPE includes more than one type of an IBM® 3400 Tape Drive (that is 3400-3, 3400-4, and 3400-6), allocation failure may occur if the device type available at restart time is not the same device type as allocated in the original run of the job. To avoid this allocation failure, define some esoteric unitnames at system initialization that include only a single device type.