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## Configuring Dual-String Mediators

This section provides information and procedures to configure dual-string mediator hosts.

A single disk string consists of a disk enclosure, its physical drives, cables from the enclosure to the node or nodes, and the interface adapter cards. A dual-string disk set includes disks in two disk strings, and is attached to exactly two nodes. If a single disk string in a dual-string disk set fails such that exactly half the Solaris Volume Manager replicas remain available, the disk set will stop functioning. Dual-string mediators are therefore required for all Solaris Volume Manager dual-string disk sets. The use of mediators enables the Oracle Solaris Cluster software to ensure that the most current data is presented in the instance of a single-string failure in a dual-string configuration.

A dual-string mediator, or mediator host, is a cluster node that stores mediator data. Mediator data provides information about the location of other mediators and contains a commit count that is identical to the commit count that is 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.

The following table lists the tasks that you perform to configure dual-string mediator hosts. Complete the procedures in the order that is indicated.

Table 4-3 Task Map: Configuring Solaris Volume Manager Dual-String Mediators

Task | Instructions
--- | ---
Configure dual-string mediator hosts. | Requirements for Dual-String Mediators How to Add Mediator Hosts
Check the status of mediator data and, if necessary, fix bad mediator data. | How to Check For and Fix Bad Mediator Data

### Requirements for Dual-String Mediators

The following rules apply to dual-string configurations that use mediators.

- Disk sets must be configured with either two or three mediator hosts. Two of these mediator hosts must be the same two cluster nodes that are used for the disk set. The third can be another node in the cluster or a non-clustered host on the cluster's public network, such as a quorum server.

- 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 consist of only 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 dual-string mediators.

1. If you will use a third mediator host for a dual-string disk set and that host does not already have disk sets configured, modify the `/etc/group` file and create a dummy disk set.
   1. Add the entry `root` to the `sysadmin`group in the `/etc/group` file.
   2. Create a dummy disk set by using the `metaset` command.

          phys-schost-3# metaset -s dummy-diskset-name -a -h hostname

      `-s` dummy-diskset-net

        Specifies the dummy disk set name.

      `-a`

        Adds to the disk set.

      `-h` hostname

        Specifies the name of the node.

2. Assume the `root` role on the node that currently masters the disk set to which you intend to add mediator hosts.
3. Add each node with connectivity to the disk set as a mediator host for that disk set.

       phys-schost# metaset -s setname -a -m mediator-host-list

   `-s` setname

     Specifies the disk set name.

   `-m` mediator-host-list

     Specifies the name of the node to add as a mediator host for the disk set.

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

Example 4-6 Adding Mediator Hosts

The following example adds the nodes `phys-schost-1` and `phys-schost-2` as mediator hosts for the disk set `dg-schost-1`. If needed, repeat the command a third time for a third mediator host. All commands are run from the node that masters the disk set for which you are adding mediator hosts, in this case `phys-schost-1`.

    phys-schost# metaset -s dg-schost-1 -a -m phys-schost-1
    phys-schost# metaset -s dg-schost-1 -a -m phys-schost-2
    phys-schost# metaset -s dg-schost-1 -a -m phys-schost-3

Next Steps

Check the status of mediator data. Go to How to Check For and Fix Bad Mediator Data.

## How to Check For and Fix Bad Mediator Data

Perform this procedure to repair bad mediator data.

Before You Begin

Ensure that you have added mediator hosts as described in How to Add Mediator Hosts.

1. Display the status of the mediator data.

       phys-schost# medstat -s setname

   `-s` setname

     Specifies the disk set name.

   See the `medstat`(1M) man page for more information.

2. Check the Status field of the `medstat` output for each mediator host.
   - If the status is `Ok`, no error condition exists.
   - If the status is `Bad`, repair the affected mediator host.
3. Assume the `root` role on the node that owns the affected disk set.
4. Remove all mediator hosts with bad mediator data from all affected disk sets.

       phys-schost# metaset -s setname -d -m mediator-host-list

   `-s` setname

     Specifies the disk set name.

   `-d`

     Deletes from the disk set.

   `-m` mediator-host-list

     Specifies the name of the node to remove as a mediator host for the disk set.

5. Restore each mediator host that you removed in Step 4.

       phys-schost# metaset -s setname -a -m mediator-host-list

   `-a`

     Adds to the disk set.

   `-m` mediator-host-list

     Specifies the name of the node to add as a mediator host for the disk set.

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

Next Steps

Determine from the following list the next task to perform that applies to your cluster configuration.

- If you want to create cluster file systems, go to How to Create Cluster File Systems.

- To find out how to install third-party applications, register resource types, set up resource groups, and configure data services, see the documentation that is supplied with the application software and the Oracle Solaris Cluster Data Services Planning and Administration Guide.

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