Before creating the first generation data set, build a generation data group base entry in a VSAM, or integrated catalog facility catalog. This base entry must provide for as many generation data sets, up to 255, as you would like to have in the GDG. The system uses the base to keep track of the chronological order of the generation data sets.
Use the access method services DEFINE command to build generation data group bases in a catalog. This command is described in z/OS DFSMS Access Method Services Commands.