Distributing Session Workload

Your installation's goals for workload distribution also might affect how you define APPC/MVS LUs as VTAM® generic resources. Together with the MVS workload manager (WLM), VTAM can balance session (not conversation) workload. For session allocation to a generic resource, VTAM binds the session to one LU in the generic resource group, balancing sessions among the members of the group based on workload information from WLM, if possible, or on session counts. Because VTAM balances sessions only when they are bound, if a particular LU or system becomes constrained, the work running on that LU cannot be redistributed until the sessions are unbound and re-established.

To more closely achieve conversation-level workload balancing, your installation can define LUs as limited resources, as well as defining them as members of a generic resource group. If the LUs are defined as limited resources, VTAM can terminate sessions between LUs if those sessions are not active for an installation-defined time period. For more information about terminating idle sessions, see z/OS Communications Server: SNA Network Implementation Guide.

As you try to decide which APPC/MVS LUs to assign to a generic resource group, or to define as limited resources, also keep the following points in mind:

When an APPC/MVS TP allocates a conversation from a local LU that is a member of a generic resource group, and other group members reside on the same system, VTAM balances sessions only within the local system, as long as eligible group members are available. If more than one eligible member is available, VTAM selects the one with less work. Depending on your installation's configuration, this selection might constrain a particular system, if other eligible members of a generic resource group reside on different systems in the sysplex.
Session workload balancing also varies for generic resource LUs that are associated with a specific transaction scheduler. For these LUs, workload balancing varies depending on the transaction scheduler associated with the LU:
- Inbound sessions to ASCH or NOSCHED LUs are balanced by analysis of two factors: the number of sessions bound and the capacity of each LU's system
- Inbound sessions to IMS™ V5R1 LUs are balanced by the same factors as those for ASCH and NOSCHED LUs, but also by analysis of the actual performance of each LU compared to the performance goals of the transactions it runs.
- How VTAM balances inbound sessions to alternate transaction schedulers depends on how the alternate transaction scheduler notifies WLM of the completion of the scheduler's transactions.
See z/OS MVS Planning: Workload Management for more information about workload balancing.