Requirements for Dual-String Mediators

A dual-string mediator, or mediator host, is a cluster node that stores mediator data. Mediator data provides information on the location of other mediators and contains a commit count that is identical to the commit count stored in the database replicas. This commit count is used to confirm that the mediator data is in sync with the data in the database replicas.

Dual-string mediators are required for all Solstice DiskSuite or Solaris Volume Manager disk sets that are configured with exactly two disk strings and two cluster nodes. A disk string consists of a disk enclosure, its physical drives, cables from the enclosure to the node(s), and the interface adapter cards. The use of mediators enables the Sun Cluster software to ensure that the most current data is presented in the instance of a single-string failure in a dual-string configuration. The following rules apply to dual-string configurations that use mediators.

• Disk sets must be configured with exactly two mediator hosts. Those two mediator hosts must be the same two cluster nodes that are used for the disk set.

• A disk set cannot have more than two mediator hosts.

• Mediators cannot be configured for disk sets that do not meet the two-string and two-host criteria.

These rules do not require that the entire cluster must have exactly two nodes. Rather, only those disk sets that have two disk strings must be connected to exactly two nodes. An N+1 cluster and many other topologies are permitted under these rules.