Additional Considerations

VCS provides the option of creating zones for systems in a cluster to further fine-tune application failover decisions. It also provides options to identify a suitable system to host a service group when the cluster starts.

System Zones

The SystemZones attribute enables you to create a subset of systems to use in an initial failover decision. This feature allows fine-tuning of application failover decisions, and yet retains the flexibility to fail over anywhere in the cluster.

If the attribute is configured, a service group tries to stay within its zone before choosing a host in another zone. For example, in a three-tier application infrastructure with Web, application, and database servers, you could create two system zones: one each for the application and the database. In the event of a failover, a service group in the application zone will try to fail over to another node within the zone. If no nodes are available in the application zone, the group will fail over to the database zone, based on the configured load and limits.

In this configuration, excess capacity and limits on the database backend are kept in reserve to handle the larger load of a database failover. The application servers handle the load of service groups in the application zone. During a cascading failure, the excess capacity in the cluster is available to all service groups.

Load-Based AutoStart

VCS provides a method to determine where a service group comes online when the cluster starts. Setting the AutoStartPolicy to Load instructs the VCS engine, HAD, to determine the best system on which to start the groups. VCS places service groups in an AutoStart queue for load-based startup as soon as the groups probe all running systems. VCS creates a subset of systems that meet all prerequisites and then chooses the system with the highest AvailableCapacity.

You can use AutoStartPolicy = Load and SystemZones to establish a list of preferred systems on which to initially run a group.