For SMS-managed volumes, you specify a backup device category for the entire system with the BACKUP parameter of the SETSYS command. Suppose that you specified SETSYS BACKUP(DASD). In this environment, DFSMShsm uses DASD unless a command backup for a volume specifies another device category. DFSMShsm uses DASD daily backup volumes even if the storage administrator has specified the ADDVOL command to add tape daily backup volumes.
Selection of DASD daily backup volumes depends on the daily limit you specify for the backup cycle. The daily limit determines at what point volume backup stops assigning more DASD daily backup volumes to this day in the backup cycle and instead tries to reuse the full DASD daily backup volumes already assigned to this day in the backup cycle. The daily limit has little or no value when backup output is to tape.
Until the daily limit is reached, DFSMShsm attempts to associate another backup volume with the day in the cycle. DFSMShsm then reuses as many of these daily backup volumes as necessary during this day in the backup cycle as long as enough space is available on the DASD daily backup volumes. You use the BACKUP parameter of the DEFINE command to define the daily limit.
If you choose a daily limit that is too small, your system will have poorer performance because it spends too much time doing cleanup for the amount of space made usable. The daily limit is too small if cleanup does not make the daily backup volumes reusable.
If you choose a daily limit that is too large, too many volumes can be assigned to a particular day in the backup cycle; therefore, available space cannot be used because it is restricted to certain days in the backup cycle. The daily limit is too large if DFSMShsm uses only a small percentage of the daily backup volumes assigned to the day in the backup cycle on a day when a normal amount of data is backed up.
To reuse DASD daily backup volumes that were previously full, DFSMShsm uses spill processing or cleanup processing. You specify spill processing with the SPILL parameter of the SETSYS command; only cleanup processing is done if you specify the NOSPILL parameter.
If the reuse process has tried today to use all volumes not currently in use, an additional volume is assigned from the set of unassigned DASD backup volumes, even though the number of assigned volumes exceeds the daily limit. If no unassigned DASD backup volume exists but at least one acceptable volume is being used, DFSMShsm waits for the task to finish using the acceptable volume.