Optimize LU-to-LU Sessions

To keep LU-to-LU sessions active during interruptions in either APPC/MVS or VTAM® service, and to preserve new conversation requests until APPC/MVS service resumes, use VTAM persistent sessions. Single-node persistent sessions (SNPS) helps provide continuity of service to APPC/MVS TPs in situations where the APPC address space is cancelled, forced, terminated, or automatically restarted. The sessions also persist during interruptions in scheduler service. Multi-node persistent sessions (MNPS) extends persistence to sessions across VTAM failures as well and gives you the option to start the same LU on another system in the sysplex with all of its LU-to-LU sessions still intact.

If APPC/MVS or scheduler service is interrupted, new conversation requests targeted to LUs that were deactivated by the interruption will be queued until service returns or the PSTIMER time limit expires, whichever comes first. For MNPS, even if VTAM service is interrupted, VTAM will queue the new inbound conversation requests until the LU is reactivated on any system in the sysplex.

For an APPC/MVS LU that handles protected conversations, however, persistent sessions are not preserved if the LU fails while a syncpoint operation is in progress. Sessions on which syncpoint operations were in progress do not persist. All other sessions still have the persistent attribute. In this case, the session is unbound so that outstanding resynchronization work can proceed when the LU is reactivated.

With MNPS, persistent sessions go beyond the scope of just keeping sessions active when various services go down. An installation may choose to move a particular LU to another system in the sysplex. This can be done:

to redistribute workload in the sysplex
to move all work off of one system so that maintenance can be performed.
to satisfy other availability issues.

Since the sessions associated with the LU move with the LU, this provides a non-disruptive mechinism to shift the current workload.

The PSTIMER parameter in the APPCPMxx LUADD statement controls whether persistent sessions are in effect for a particular local LU and how long the sessions persist. For more information about coding the PSTIMER parameter, see the APPCPMxx parmlib member description in z/OS MVS Initialization and Tuning Reference.

SNPS support requires ACF/VTAM Version 3 Release 4. MNPS requires eNetwork Communications Server Release 5. For general information about persistent sessions, see z/OS Communications Server: SNA Network Implementation Guide.