Allocating data sets

Access method services identifies the verb name ALLOCATE and attaches the terminal monitor program (TMP), which runs Time Sharing Option Extended (TSO/E) commands in the background. You should use the ALLOCATE command only to allocate new data sets to the job step. If you use ALLOCATE through access method services for anything else (such as the handling of SYSOUT data sets), you can get unpredictable results. For more information on using this command, see z/OS TSO/E Programming Guide. Table 1 separates the parameters to be used under access method services from the parameters that cause unpredictable results.

When you use ALLOCATE, the data set is allocated to the job step. If your job contains multiple allocations, you might need to use the DYNAMNBR parameter in the job control language (JCL) EXEC statement. DYNAMNBR establishes a control limit that TMP uses when allocating a data set. The control limit is the sum of the number of data definition (DD) statements that are coded plus the value coded in DYNAMNBR. If you do not use DYNAMNBR, the system sets it to 0 (the default). If you code DYNAMNBR incorrectly, the system uses the default and issues a JCL warning message. For a description of how to code the DYNAMNBR parameter, see z/OS MVS JCL User's Guide. For an example that illustrates the use of DYNAMNBR, see Allocate a data set using SMS class specifications: Example 1.

When you use the ALLOCATE command within access method services, you must follow the data set naming conventions of TSO/E when you run TMP in batch mode:
  • If the data set name is not in quotation marks and a USER parameter is given in the JCL, the value in the USER parameter is prefixed to all data set names given by ALLOCATE.
  • If the USER parameter is not in the JCL, no prefix is added to any data set name given by ALLOCATE.
For information about the naming conventions of TSO/E and other considerations when you use access method services commands from a TSO/E background job, see z/OS TSO/E User's Guide. For information about the USER parameter and its Resource Access Control Facility (RACF®) requirements, see z/OS MVS JCL Reference.
You can use the ALLOCATE command to define data set attributes in several ways:
  • You can use the storage management subsystem (SMS) parameters STORCLAS, MGMTCLAS, and DATACLAS. You can either define these parameters explicitly or let them use the parameters assigned by the ACS routines that your storage administrator defines. For information about storage administration policies and about how the ACS routines might apply, contact your storage administrator.

    You cannot override attributes that the STORCLAS and MGMTCLAS parameters assign. You can override attributes that the DATACLAS parameter assigns. For example, if you use both the DATACLAS parameter and the SPACE parameter, SMS assigns all the attributes defined in the DATACLAS parameter but uses the values you defined in the SPACE parameter when allocating a data set.

  • You can use the LIKE parameter to allocate a data set with the same attributes as an existing (model) data set. The model data set must be a cataloged data set. You can override any of the model data set attributes by stating them in the ALLOCATE command.
  • You can identify a data set and explicitly describe its attributes.