One XRF complex with one CPC
An XRF complex can be set up on one CPC with two IMS systems defined. However, the full benefit of XRF IMS is only achieved when IMS as well as z/OS®, VTAM®, and the CPC are replicated. When only one CPC is used, only IMS has an alternate. Failures in z/OS, VTAM, and the CPC affect both IMS systems.
This configuration consists of:
- One CPC, running z/OS and VTAM
- Two IMS systems, the active and the alternate IMS systems
- IMS system logs shared by the active and the alternate IMS systems
- Databases shared by the active and the alternate IMS systems
- Remote terminals that use SNA protocol and that can communicate with either of the IMS systems
- If your XRF complex uses USERVAR, NCP running in each 37x5 Communication Controller to which the SNA terminals are attached
This arrangement is recommended only as a test environment, and then only if the CPC is fast enough and the system has enough real and virtual storage to support XRF.
Processing in a one-CPC XRF complex
In
an XRF complex that runs on one CPC, XRF processing differs little
from XRF running on two CPCs. One important difference is the role
of AVM during takeover. In a 2-CPC XRF complex, the alternate IMS issues a RESERVE command
to the IMS log to prevent the
active IMS from accessing the
databases. In a 1-CPC complex, this action does not prevent the active IMS from changing the databases.
I/O prevention quiesces the outstanding I/O requests to the system
log, as well as to the database data sets. When I/O prevention is
complete, the operator can respond GO
to AVM006A to complete
the takeover. If AVM cannot perform I/O prevention, the alternate IMS waits for termination of the
failing active IMS before completing
the takeover.
Planning considerations in a one-CPC complex
In a 1-CPC XRF complex, all messages from AVM appear at the one z/OS console as if two z/OS operating systems were in the complex.
If AVM fails to perform I/O prevention at takeover, the IMS operator cannot reset the CPC to prevent the active IMS from writing to the databases. This action terminates the XRF complex. In a 1-CPC complex, the operator should terminate IMS and ensure that all the address spaces in the active IMS terminate before assuming that I/O prevention is complete.