Relationship Between Resource Groups and Disk Device Groups

Sun Cluster has the concept of a node list for disk device groups and resource groups. These node lists are ordered lists of nodes that are potential masters of the disk device group or resource group. Associated with the node list is a failback policy. This policy describes the action to be taken when the node that masters the disk device group or resource group (the primary) leaves the configuration and later rejoins-that is, whether the disk device group or resource group is once again mastered by the primary when it rejoins the cluster.

To ensure high availability of a failover resource group, make the group's node list match the node list of any associated disk device group. For a scalable resource group, the resource group's node list cannot always match the device group's node list because, currently, a device group's node list can contain exactly two nodes only. For a greater than two-node cluster, the node list for the scalable resource group can have more than two nodes.

For example, assume you have a disk device group dg-schost-1 that has nodes phys-schost-1 and phys-schost-2 in its node list and the failback policy is set to Enabled. Assume you also have a failover resource group, rg-schost-1, which uses dg-schost-1 to hold its application data. When you set up rg-schost-1, also specify phys-schost-1 and phys-schost-2 for its node list and set its failback policy to True.

To ensure high availability of a scalable resource group, make the group's node list a superset of the node list for the disk device group. Doing so ensures that the nodes that are directly connected to the disks are also nodes that can run the scalable resource group. The advantage is that, when at least one node connected to the data is up and in the cluster, the scalable resource group is running on those same nodes, making the scalable services available also.

For information on setting up disk device groups, refer to the Sun Cluster 3.0 Installation Guide. For more details on the relationship between disk device groups and resource groups, see the Sun Cluster 3.0 Concepts document.

SUNW.HAStorage Resource Type

The resource type SUNW.HAStorage serves the following purposes:

• Coordinates the boot order by monitoring the global devices and cluster file systems and causing the START methods of the other resources in the same resource group that contains the SUNW.HAStorage resource to wait until the disk device resources become available.

• With AffinityOn set to True, enforces colocation of resource groups and disk device groups on the same node, thus enhancing the performance of disk-intensive data services.

If the device group is switched to another node while the SUNW.HAstorage resource is online, AffinityOn has no effect and the resource group does not migrate along with the device group.

Recommendations

To determine whether to create SUNW.HAStorage resources within a data service resource group, consider the following criteria:

• In cases where a data service resource group has a node list in which some of the nodes are not directly connected to the storage, you must configure SUNW.HAStorage resources in the resource group and set the dependency of the other data service resources to SUNW.HAStorage. This requirement is to coordinate the boot order between the storage and the data services.

• If your data service is disk-intensive, such as Sun Cluster HA for Oracle and Sun Cluster HA for NFS, then we recommend that you add a SUNW.HAStorage resource to your data service resource group, set the dependency of your data service resources to the SUNW.HAStorage resource, and set AffinityOn to True. That way, the resource groups and disk device groups are colocated on the same node. On the other hand, if your data service is not disk-intensive-such as one that reads all its files at startup (for example, Sun Cluster HA for DNS), configuring SUNW.HAStorage is optional.

• If your cluster contains only two nodes, configuring SUNW.HAStorage is optional. However, if you plan to add nodes and run scalable services later on, you must configure SUNW.HAStorage at that time. To get prepared, you can go ahead and set up SUNW.HAStorage now and add nodes to the node list later.

See the individual chapters on data services in this document for specific recommendations.

For the procedure on how to set up SUNW.HAStorage, see "How to Set Up SUNW.HAStorage Resource Type for New Resources". Additional details are in the SUNW.HAStorage(5) man page.