Subsystem initialization is the process of readying a subsystem for use in the system. IEFSSNxx members of SYS1.PARMLIB contain the definitions for the primary subsystems, such as JES2 or JES3, and the secondary subsystems, such as VPSS and DB2®. For detailed information about the data contained in IEFSSNxx members for secondary systems, please refer to the installation information for the specific system.

During system initialization, the defined subsystems are initialized. You should define the primary subsystem (JES) first, because other subsystems, such as DB2, require the services of the primary subsystem in their initialization routines. Problems can occur if subsystems that use the subsystem affinity service in their initialization routines are initialized before the primary subsystem. After the primary JES is initialized, then the subsystems are initialized in the order in which the IEFSSNxx parmlib members are specified by the SSN parameter. For example, for SSN=(aa,bb) parmlib member IEFSSNaa would be processed before IEFSSNbb.

Using IEFSSNxx to initialize the subsystems, you can specify the name of a subsystem initialization routine to be given control during master scheduler initialization, and you can specify the input parameter to be passed to the subsystem initialization routine. IEFSSNxx is described in more detail in z/OS MVS Initialization and Tuning Reference.