You can allocate space for single volume and multivolume data sets by specifying a storage class with the Guaranteed Space attribute. SMS fails the request when space is insufficient. The Guaranteed Space attribute does not apply to objects, but it does apply to both VSAM and non-VSAM-managed data sets.

For a multivolume system-managed data set, the system preallocates primary space on all the volumes. The first volume becomes the primary volume. The remaining volumes become candidate volumes with preallocated space. The system operates differently depending on what the user codes for DISP.

When the existing allocated space on the current volume becomes full with DISP=NEW or MOD, the system attempts to create secondary extents on the current volume. If not enough space is left on the current volume, the system uses the preallocated primary extent on the next volume. The system converts this next volume from a candidate volume to a primary volume.

With DISP=MOD, you can specify different primary space amounts to be allocated to each volume. To accomplish this you can specify multiple DD statements using the same data set name, each with different values for primary space. Refer to Preallocating space for multivolume data sets for examples.

When the allocated space (possibly including secondary amounts) on the current volume becomes full with DISP=OLD or SHR, the system switches to the next volume. The system allocates new space only on the last volume. The system converts each candidate volume to a primary volume.

For VSAM key-sequenced data sets with key ranges specified, KEYRANGES will be ignored if it is specified for the DEFINE CLUSTER command of IDCAMS.

For information about key ranges, see the description of the DEFINE CLUSTER command in z/OS DFSMS Access Method Services Commands.