Processing data class attributes in JCL

The order of precedence for data class attributes in JCL is as follows:

Explicit specifications
LIKE and REFDD keywords
Data class definitions (explicit or derived) in REFDD statement
Data class definitions in the referencing DD

A data class does not need to be a self-contained, complete data organization. You can partially define the data set attributes in the data class definition (as a base) and the user can explicitly specify the remaining attributes. However, the merging of all attributes according to the order specified above must result in a valid data organization. In the example below,

//DD1  DD    ...,DATACLAS=DC2,RECORG=ES,...
//DD2  DD    ...,DATACLAS=DC1,LRECL=180,REFDD=DD1,...

For the attributes for DD2, LRECL of 180 is used first, and then the RECORG of ES in DD1 is used, regardless of the values specified in either the DC1 or the DC2 data class. For the remaining attributes that are not explicitly specified on the DD statements, SMS uses the values defined in the data class definition of DC2 and then DC1.

In this next example, the attributes of the data set referenced by the LIKE keyword are used after all other explicit specification but before data class DC3 attributes:

//DD3  DD    ...,DATACLAS=DC3,LRECL=180,LIKE=SAMPLE.DATA,....

Here, an LRECL of 180 is used. Then, SMS uses the Data Set Control Block (DSCB) information from the SAMPLE.DATA data set. Finally, the remaining attribute values are drawn from the DC3 data class.

A final example illustrates the use of REFDD:

//DD4  DD   ...,DATACLAS=DC4,...
//DD5  DD   ...,DSN=DS1,REFDD=DD4,DATACLAS=DC5,LRECL=180,...

In this example, the REFDD keyword specifies that the explicit attributes on the DD4 JCL statement are to be used second, because explicit attributes on the DD4 JCL statement are used after the explicitly specified attributes. Next, the attributes from data class DC4 referenced in DD4 should be used. The remaining attributes are taken from data class DC5.

You can use the DATACLAS keyword to specify data class attributes for SMS-managed or non-SMS-managed data sets. Or, you can use the DCB=*.ddname or DCB=dsname keywords to copy the attributes of existing data sets for new non-SMS-managed data set allocations. You can specify the attributes of new SMS-managed data sets with the LIKE or REFDD keyword. The LIKE and REFDD keywords are mutually exclusive.

Restriction: You cannot use the REFDD keyword to copy DCB attributes from the DSCB. Use the LIKE keyword to copy from the DSCB.

For more information on using the LIKE and REFDD keywords, refer to z/OS MVS JCL Reference. For information on determining data classes through ACS routines, see Defining ACS routines. For information on access method services DEFINE defaults and data class defaults, refer to z/OS DFSMS Access Method Services Commands.