The number of CTC links available to the complex determines, to a great extent, the availability, ease of operation, and performance of the complex.

A fully-connected complex exists when every system in the complex has a communication link to every other system in the complex. Each system in a fully-connected n-system complex has n-1 communication links, where n is the number of systems in the complex. For example, each system in a fully-connected four-system complex requires three link connections.

In contrast, a partially-connected complex is one where not every system has a communication link to every other system. That is, some systems in an n-system complex have fewer than n-1 communication links. Systems operating outside of a multisystem sysplex or where the sysplex does not match the complex might be in a partially-connected complex. For example, some systems in a partially-connected four-system complex have less than three communication links. The level of connectivity of the complex affects the operation of the complex in two basic ways. One is the order in which one system starts the complex and other systems join the complex. A partially-connected complex of four or more systems requires coordination of IPLs so that global resource serialization can build a valid ring. The second effect is on recovery from a system or CTC link failure that cannot be repaired immediately. A partially-connected complex limits the reconfiguration options available to recover from the failure.

Figure 1 illustrates the problem of a system failure on a partially-connected complex. It shows an active four-system ring with five communication link connections.

Figure 1. Recovery Problems with a Partially-Connected Complex

If system D fails, global resource serialization can automatically rebuild systems A, B, and C into a three-system ring using CTC2, CTC3, and CTC4, as shown in the figure. The three-system ring can resume processing requests for global resources. This four-system ring can recover just as quickly from a failure on system B.

If system A fails, however, the problem is entirely different because the complex is not fully connected. Global resource serialization cannot rebuild a three-system ring. It can automatically rebuild a two-system ring consisting of systems D and C (using CTC5) or a two-system ring consisting of systems B and C (using CTC4), and there is no way to predict which ring it would build. The same problem occurs if system C fails.

If the complex shown in the figure were fully connected, it would include a CTC link between system B and system D. With this fully-connected complex, global resource serialization can respond to a failure on any system by rebuilding a three-system ring that can quickly resume processing requests for global resources.

The distinction between a fully-connected complex and a partially-connected complex does not exist for a two-system complex or a three-system complex. For these complexes, the number of communication link connections required for full connectivity and the minimum number of communication link connections required are the same.

IBM® recommends that you design a fully-connected global resource serialization complex, especially if you are operating a mixed complex that includes a multisystem sysplex. Even a fully-connected complex, however, might not meet the level of reliability or performance your installation requires. Designing a complex that includes alternate links offers significant advantages for both reliability and performance.