Global resource serialization uses one link, the primary link, to send the RSA-message from one system in the ring to another. Alternate links provide additional connections. At IPL time, global resource serialization uses one link as the primary connection and marks any other links as alternates. Alternate links provide two very important benefits.

If alternate links exist, you can use ring acceleration during normal ring processing. Ring acceleration speeds up the process of granting requesters access to global resources. See Ring acceleration (ACCELSYS) for more information.

Alternate links also provide improved recovery. If the primary link fails during ring processing, a ring disruption occurs. If an alternate link is available, however, global resource serialization automatically selects an alternate link to replace the primary link, and ring processing can resume almost immediately. If global resource serialization was using the alternate link to send the ring acceleration signal, and no other link is available, the ring acceleration signal can no longer be sent between the two systems. This loss might affect performance, but the ring continues processing.

Figure 1 shows the problem of a link failure in a three-system complex. If CTC1 fails, system A and system B cannot communicate. Global resource serialization can rebuild a two-system ring, but the two-system ring omits either system A or system B. One of the two systems cannot continue to serialize access to global resources.

Figure 1. CTC Link Failure — No Alternate Available

Figure 2 shows how alternate links can solve this problem. If there is an alternate link between system A and system B, global resource serialization can use the alternate to rebuild the original three-system ring. The alternate link means that the two systems can continue to serialize access to global resources even when the primary link fails; global resource serialization uses the alternate in place of the primary, and ring processing continues.

Figure 2. CTC Link Failure — Alternate Available

Because of the recovery and performance benefits, IBM® recommends that you provide alternate links for the global resource serialization complex.

For additional information, see Alternate link failure.