Resource configuration planning
Resources in a PowerHA cluster include the application and all the shared file systems that belong to it. PowerHA provides a highly available environment by identifying a set of cluster-wide resources essential to uninterrupted processing, and then defining relationships among nodes that ensure these resources are available to client processes. When a cluster node fails or detaches from the cluster for a scheduled outage, the Cluster Manager redistributes its resources among any number of the surviving nodes.
PowerHA considers the following as resource types:
- Volume Groups
- File Systems
- File Systems to be NFS mounted
- File Systems to be NFS exported
- Service IP addresses
Each resource in a cluster is defined as part of a resource group. This allows you to combine related resources that need to be together to provide a particular service. A resource group also includes the list of nodes that can acquire those resources and serve them to clients. Resources can either be cascading or rotating.
In cascading resource groups, all nodes are assigned priorities for that resource group. The set of resources cascades up or down to the highest priority node active in the cluster. When a node that is serving the resources fails, the surviving node with the highest priority takes over the resources.
A rotating resource group is associated with a group of nodes, rather than a particular node. A node can possess one rotating resource group per network. As participating nodes join the cluster for the first time, they acquire the first available rotating resource group per network until all the groups are acquired. The remaining nodes maintain a standby role. When a node holding a rotating resource group leaves the cluster, either because of a failure or gracefully while specifying the takeover option, the node with the highest priority and available connectivity takes over. Upon reintegration, a node remains as a standby and does not take back any of the resources that it had initially served.