Understanding coexistence

Coexistence1 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 use the latest enhancements, or an older level of the system by using the updated system control files of a newer level (even if new function has been exploited in the newer level).

The sharing of resources is inherent in multisystem configurations that involve Parallel Sysplex® implementations. But other types of configurations can have resource sharing too. Examples of configurations where resource sharing can occur are:
  • A single processor that is time-sliced to run different levels of the system, such as during different times of the day
  • A single processor running multiple images with logical partitions (LPARs)
  • Multiple images running on several different processors
  • Parallel Sysplex or non-Parallel Sysplex configurations
Note: The term coexistence does not refer to z/OS® residing on a single system along with z/VSE®, VSE/ESA, or z/VM® in an LPAR or as a VM guest.

z/OS 3.1 systems can coexist with specific prior releases of z/OS systems. (The releases are listed in Which releases are supported for coexistence, fallback, and upgrade?.) This is important because it gives you flexibility to migrate systems in a multisystem configuration to z/OS 3.1 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 3.1 is to install coexistence service (PTFs) on the earlier-level systems.

Complete the migration of all earlier-level coexisting systems to z/OS 3.1 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 intended to allow new functions that are provided in later releases to work on earlier-level systems.

1 In some documentation, you might find the terms compatibility or toleration used instead of coexistence.