Letting the system assign the specific tracks is easiest and most frequently used. Specify only how the space is to be measured—in tracks, cylinders, blocks, or records—and how many of those tracks, cylinders, blocks, or records are required.
Requests for blocks (blklgth)
Without SMS, it is easiest to specify an average block length: the system allocates the least number of tracks required to contain the number of blocks specified. Specifying block length also maintains device independence; you can change the device type in the UNIT parameter without altering the space request or you can code in the UNIT parameter a group name that includes different direct access devices.
When you request space in terms of average block length or average record length, the system allocates tracks to contain the request. However, if you code ROUND as the last subparameter in the SPACE parameter, the system allocates the smallest number of cylinders needed to contain the request.
Requests for tracks or cylinders (TRK or CYL)
You can specify TRK or CYL. You will need to compute the number of tracks or cylinders required. Consider such variables as the device type, track capacity, tracks per cylinder, cylinders per volume, data length (blocksize), key length, and device overhead. These variables and examples of estimating space requirements for partitioned data sets are described in z/OS DFSMS Using Data Sets.
Cylinder allocation (and therefore ROUND used with average block or average record) allows faster input/output of sequential data sets than does track allocation.
Requests for records (reclgth)
With SMS, specify an average record length in bytes, as well as the primary, secondary, and directory quantity on the SPACE parameter, to request space or to override the space allocation in the data class of the data set.
You must also specify the AVGREC parameter with the SPACE parameter in order to specify a record request and indicate whether the primary and secondary quantity represents units, thousands, or millions of records.