Relationship of the UNIT and VOLUME parameters (SMS-managed data sets)

The system can obtain device eligibility information from:

the catalog for cataloged tape data sets
a passed data set
an earlier DD statement
a previous request for the volume
the tape configuration database

Cataloged data sets

When a tape data set is cataloged, the system obtains device eligibility and volume information from the catalog. If the DD statement for a cataloged tape data set contains a volume serial number that is not in the SMS configuration, the system does not use the catalog; instead, it obtains device eligibility from the tape configuration data base.

For data sets with no volume serial specified, the system always searches the catalog when a data set is OLD (or MOD treated as OLD). For data sets with a non-SMS volume serial specified, the system assumes the data set is non-SMS-managed and resides on that volume, and it does not search the catalog. For data sets with an SMS volume serial number specified (whether it is a real volume or a non-existent volume that is in a DUMMY storage group), the system always searches the catalog; SMS controls volume selection, and the data set might not be on the volume that is specified.

Volume References to Cataloged Data Sets

For tape, if a data set is to use the same volume(s) as a cataloged data set, code VOLUME=REF to refer to the cataloged data set. The system obtains unit and volume information from the catalog and places the data set on the same volume(s). For DASD, storage class and volume information are retrieved from the catalog. The data sets share the same storage class but can be in different storage groups as long as the storage groups are of compatible types (such as POOL or VIO).

Overridden Procedure DD Statements

When a step calls a cataloged or in-stream procedure, an overriding DD statement in the calling step statement can specify a cataloged data set in its DSNAME parameter. If so, the overriding DD statement should nullify the VOLUME parameter; otherwise, the system uses the VOLUME parameter on the overridden DD statement and does not search the catalog.

Passed data sets

When receiving a data set passed from a previous step, omit the VOLUME parameter. The system obtains volume information from the passing step.

Earlier DD statement

For NEW data sets, if VOL=REF references a non-SMS-managed data set, the ACS routines are given control. The ACS routines can either allow the non-SMS allocation or fail it, but they cannot make the new data set SMS-managed. For NEW data sets, if VOL=REF references an SMS-managed data set, then:

For tape, the storage group and the volume must be the same as the referenced data set.
For DASD, the storage class must be the same as the referenced data set, but the storage groups can be different as long as they are compatible, such as POOL and VIO.

For OLD data sets, if VOL=REF references a non-SMS-managed data set, the system assumes the data set is non-SMS-managed and on the same volume as the referenced data set. If VOL=REF references an SMS-managed data set, the system searches the catalog because the referencing data set might not be on the same volume as the referenced data set.

Previous request for the volume

A volume/unit association can be established during device allocation so that any subsequent request for the volume within the same step will receive the same unit.