IBM 990 - Manual (Page 171)

Chapter 7. Sysplex functions 159
Data access, allowing dynamic workload balancing and improved availability
Dynamic transaction routing, providing dynamic workload balancing and improved
availability
End-user interface, allowing access to an application as opposed to a specific image
Operational interfaces that allow Systems Management across the sysplex from a single
point
7.1.2 Parallel Sysplex summary
Through this state-of-the-art cluster technology, the power of multiple z/OS and/or OS/390
images can be harnessed to work in concert on common workloads. The zSeries Parallel
Sysplex cluster takes the commercial strengths of the z/OS or OS/390 platform to improved
levels of system management, competitive price/performance, scalable growth, and
continuous availability.
7.2 Sysplex and Coupling Facility considerations
Described here are the supported Parallel Sysplex configurations, required set-up information
when connected to an Sysplex Timer, different forms of Coupling Facilities (CFs) supported
on z990 servers, CFRM policy considerations, and ICF processor assignments. The z990
models support both Central Processors (CPs) and Internal Coupling Facility (ICF)
processors.
The z990 family of servers does not provide a special model for a CF-only processor. You
can, however, have a z990 server with up to 16 PUs defined as ICFs.
7.2.1 Sysplex configurations and Sysplex Timer considerations
Parallel Sysplex configurations today can have system images and Coupling Facilities
located across multiple servers. This can be anything from S/390 to zSeries servers.
However, z990 brings some additional considerations to these types of configurations.
Message Time Ordering
As server and Coupling Facility link technologies have improved over the years, the
synchronization tolerance between operating systems in a Parallel Sysplex has become more
rigorous. In order to ensure that any exchange of timestamped information between operating
systems in a sysplex involving the Coupling Facility observe the correct time ordering,
timestamps are now included in the message-transfer protocol between the server operating
systems and the Coupling Facility. This is known as Message Time Ordering.
Message Time Ordering requires a connection between the z990 CPC and the Sysplex Timer
whenever a Coupling Facility is located in a logical partition on a z990.
Therefore, when a Coupling Facility is configured as an ICF on any z990 model, the Coupling
Facility will require connectivity to the same 9037 Sysplex Timer that the systems in its
Parallel Sysplex are using for the time synchronization. If the ICF is on the same server as
one or more a member of its Parallel Sysplex, no additional Sysplex Timer connectivity is
required, since the server already has connectivity to the Sysplex Timer. However, when an
ICF is configured on any z990 model that does not host any systems in the same Parallel
Sysplex, it is necessary to attach the z990 server to the 9037 Sysplex Timer.