Mediators Overview

A 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.

Mediators are required for all Solstice DiskSuite disksets configured with exactly two disk strings and two cluster nodes. A disk string consists of a disk enclosure, its physical disks, 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 using mediators.

• Disksets must be configured with exactly two mediator hosts, and those two mediator hosts must be the same two cluster nodes used for the diskset.

• A diskset cannot have more than two mediator hosts.

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

These rules do not require that the entire cluster have exactly two nodes. Rather, they only require that those disksets 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.

How to Add Mediator Hosts

Perform this procedure if your configuration requires mediators.

1. Become superuser on the node currently mastering the diskset to which you want to add mediator hosts.

2. Run the metaset(1M) command to add each node with connectivity to the diskset as a mediator host for that diskset.

   # metaset -s setname -a -m mediator_host_list

   -s setname

   Specifies the diskset name

   -a

   Adds to the diskset

   -m mediator_host_list

   Specifies the name of the node to add as a mediator host for the diskset

See the mediator(7) man page for details about mediator-specific options to the metaset command.

Example--Adding Mediator Hosts

The following example adds the nodes phys-schost-1 and phys-schost-2 as mediator hosts for the diskset dg-schost-1. Both commands are run from the node phys-schost-1.

# metaset -s dg-schost-1 -a -m phys-schost-1
# metaset -s dg-schost-1 -a -m phys-schost-2

Where to Go From Here

To check the status of mediator data, go to "How to Check the Status of Mediator Data".