Explanation: TAPESPANSIZE(nnnn) is an optional parameter used to reduce the occurrences of data sets spanning migration or backup tape volumes. For nnnn, specify a value between 0 and 9999 in units of megabytes (MB). This value represents the maximum number of megabytes of tape (ML2 or backup) that DFMSMShsm may leave unused while it tries to eliminate spanning of data sets. To state this differently, this value is the minimum size of a data set that is allowed to span tape volumes. Data sets whose size is less than the value do not normally span volumes. Only those data sets whose size is greater than or equal to the specified value are allowed to span volumes.
This parameter offers a trade-off: to reduce the occurrences of a user data set spanning tapes in exchange for some unused media at the end of each cartridge. The amount of unused media can vary from 0 to nnnn physical megabytes, but roughly averages 50% of the customer's median data set size. For example, if you specify 4000MB and your customer's median-sized data set is 2MB, then on average only 1MB of media is unused per cartridge.
If the SETSYS TAPEUTILIZATION parameter is set to NOLIMIT, DFSMShsm makes no attempt to reduce data set tape volume spanning. If the SETSYS TAPEUTILIZATION parameter is set to a percent, then DFSMShsm performs reduced data set spanning.
DFSMShsm calculates a target data set size to use when it is determining whether a data set should fit entirely on the currently mounted tape volume. The size is calculated based on the used tracks of the data set and an estimated IDRC compaction accomplishment. If data is written to tape in IDRC format and software compaction is not being used (by either DFSMShsm or CPU) for the data set, DFSMShsm assumes a 2.5 to 1 compaction savings for the data set. If you want to use a different compaction assumption, see the topic about tuning DFSMShsm in z/OS DFSMShsm Implementation and Customization Guide. If IDRC format is not used or software compaction is used for the data set, then DFSMShsm does not consider any compaction accomplishments when it estimates the amount of tape media the data set requires.
DFSMShsm recommends a value of 4000MB for all IBM® 3490 and 3590 tape cartridges.
SMS relationship: Parameter has the same meaning when applied to SMS-managed or non-SMS-managed DASD volumes or data sets.
SETSYS default: None.
DFSMShsm default: If you do not specify TAPESPANSIZE, the default is 500MB.