z/OS MVS JCL Reference
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SPACE parameter

z/OS MVS JCL Reference

Parameter type: Keyword, optional
Note: With SMS, code the SPACE parameter when you want to
  • Request space for a new data set, or
  • Override the space allocation defined in the DATACLAS parameter for the data set.
See the DATACLAS parameter (described in ) and the AVGREC parameter (described in ).
Purpose: Use the SPACE parameter to request space for a new data set on a direct access volume. You can request space in two ways:
  • Tell the system how much space you want and let the system assign specific tracks.
  • Tell the system the specific tracks to be allocated to the data set.

Letting the system assign the specific tracks is most frequently used. You specify only how space is to be measured — in tracks, cylinders, blocks, or records — and how many of those tracks, cylinders, blocks, or records are required.

The SPACE parameter has no meaning for tape volumes; however, if you assign a data set to a device class that contains both direct access devices and tape devices, for example, UNIT=SYSSQ, you should code the SPACE parameter.

If you code the SPACE parameter on a DD statement that defines an existing data set, the SPACE value you specify temporarily overrides the SPACE value used to create the data set. For example, a data set created with SPACE=(CYL,(5,1)) causes 5 cylinders to be allocated to the data set, and, if it needs more space, it can obtain 1 additional cylinder.

Suppose, though, that there is one particular job that specifies DISP=MOD and will write many records to this data set. JCL for this job can define, for example, SPACE=(CYL,(5,10)) to obtain an additional 10 cylinders instead of just 1 cylinder. The override, however, is in effect only for the single step of the job which specifies the overriding SPACE parameter. Any other job or step within the same job that requires a secondary extent and does not have a SPACE parameter override gets just the 1 additional cylinder specified in the JCL that created the dataset.

  • When creating VSAM data sets, be aware that there is no direct one-to-one correspondence between ‘define cluster’ parameters and JCL keyword parameters.
  • The average value in the SPACE keyword is meant to be an average block length value for space calculations and is not meant to represent an LRECL value.
  • The AVGREC keyword is only to be used as a multiplier in determining how much space is to be allocated.
  • When defining VIO data sets, be aware that a SPACE parameter in the JCL or the SPACE value defined for a data class will override the system default space value.
  • The size of a data set is limited to 65,535 tracks per volume except for the following types of data sets:
    • Hierarchical File System (HFS)
    • Extended format sequential
    • Partitioned data set extended (PDSE)
    • VSAM
    • Large format

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