Dynamic allocation with IMS

If data sets are specified with JCL in a control region procedure, they are initially allocated when the control region starts up. Some data sets can also be dynamically allocated.

You can specify that the following data sets be dynamically allocated when needed and deallocated when no longer in use.

Using the IMS macro DFSMDA, you declare those data sets that are subject to dynamic allocation and deallocation.

• Database data sets can be dynamically allocated explicitly with the /START command or implicitly when an application program is scheduled. Database data sets can be deallocated with the ⁄DBRECOVERY command.
• For DEDB area data sets, an implicit allocation occurs at first access by an application program; the ⁄STOP command also deallocates the data set.
• An IMS Monitor data set can be dynamically allocated at the time the IMS Monitor is started with the ⁄TRACE SET ON command and deallocated by the ⁄TRACE SET OFF command.
• RECON data sets, online log data sets (OLDSs), write-ahead data sets (WADSs), and system log data sets (SLDSs) that are required as input to restart can be dynamically allocated.
• ACB libraries can be dynamically allocated.
• For High Availability Large Databases (HALDBs), dynamic allocation allocates only the DBRC registered data sets. Allocation does not look for or process any DFSMDA members. The DD names allocated for HALDBs contain the letters A through J, X, or L, suffixed to the 7-byte HALDB partition name.

  If an allocation already exists with a DD name that matches the HALDB partition DD name generated, the data set name is compared to the DBRC registered data set name in online IMS environments. If the data set names do not match, an allocation failure occurs.

All data sets using dynamic allocation must be cataloged, except an IMS Monitor data set, which must not be cataloged. A data set that is initially allocated with JCL can be dynamically deallocated and reallocated during the execution of the control region.

Related reading: For more information on the IMS macro DFSMDA, see IMS Version 15.5 System Definition.