Coexistence ¹ occurs when two or more systems at different software levels share resources. The resources could be shared at the same time by different systems in a multisystem configuration, or they could be shared over a period of time by the same system in a single-system configuration. Examples of coexistence are two different JES releases sharing a spool, two different service levels of DFSMSdfp sharing catalogs, multiple levels of SMP/E processing SYSMODs packaged to exploit the latest enhancements, or an older level of the system using the updated system control files of a newer level (even if new function has been exploited in the newer level).

z/OS V2R1 systems can coexist with specific prior releases of z/OS systems. (The releases are listed in Which releases are supported for coexistence, fallback, and migration?.) This is important because it gives you flexibility to migrate systems in a multisystem configuration to z/OS V2R1 using rolling IPLs rather than requiring a systems-wide IPL. (See Rolling z/OS across a multisystem configuration.) The way in which you make it possible for earlier-level systems to coexist with z/OS V2R1 is to install coexistence service (PTFs) on the earlier-level systems.

You should complete the migration of all earlier-level coexisting systems to z/OS V2R1 as soon as you can. Keep in mind that the objective of coexistence PTFs is to allow existing functions to continue to be used on the earlier-level systems when run in a mixed environment that contains later-level systems. Coexistence PTFs are not aimed at allowing new functions provided in later releases to work on earlier-level systems.