Link placement

Global resource serialization requires fast communication between systems. Without channel interference, this communication normally requires only a few milliseconds. To achieve the optimum communication speed, place the 3088 on a 3-megabyte or faster data streaming block multiplexor channel.

Other data links used by other MVS™ subsystems can share the channel, but do not connect data links on the same channel as any devices that keep the channel busy for a long period of time. For example, some DASD control units, such as the IBM® 3880, might be compatible while tape and terminal control units are not. The 3088, however, might monopolize the channel, causing delays in I/O operations to DASD.

Note: If one or more of the systems in the complex are running as VM guest systems, provide real CTC links (not virtual CTCs) and dedicate the CTC links to global resource serialization.

Other configuration recommendations are:

Design a fully-connected complex — one where every system has a communication link to every other system. See Level of connectivity for more information.
Provide at least one alternate (a second connection) for each communication link. Alternate links are required if you want to use ring acceleration to speed up the processing of global resource requests. See Alternate links for more information.
Provide a backup IBM 3088. See Backup considerations for more information.

Following these recommendations ensures a complex that provides the best possible performance and availability. Configuration decisions have a significant effect on recovery planning.