Choosing the cluster type

When creating a cluster for a multiple site configuration with PowerHA® SystemMirror 7.1.2 Standard Edition you can configure a cluster in one of two modes.

Stretched cluster
A stretched cluster contains nodes from sites at the same geographical locations. Stretched clusters share at least one repository disk between sites. Stretched clusters support a configuration across sites with Logical Volume Manager (LVM) Mirroring and HyperSwap®.

When you use a stretched cluster in your environment, a single Cluster Aware AIX® (CAA) cluster is deployed across the sites in the cluster. If internet protocol (IP) communication fails and your environment is using stretched clusters, all nodes in the cluster can use the disk heartbeat function to communicate and keep your environment functioning.

A stretched cluster must meet the following requirements:
  • Site communication for the cluster must use multicast IP addresses. Therefore, all network elements for your environment, such as switches and routers, must allow multicast packets to travel between sites.
  • The cluster must share at least one repository disk between the sites.
Linked clusters
start of changeA linked cluster contains nodes from sites at different geographical locations. Linked clusters do not require sites to share a repository disk or to support multicast communication. Although the sites do not share a repository disk, all the nodes within a particular site must share the same repository amongst them. So all the nodes in siteA must share one repository, and all the nodes in siteB must share another one.end of change
Linked clusters do not support HyperSwap or high availability disaster recovery (HADR) with Storage Replication Management.

CAA uses linked clusters to support multiple sites that are geographically far apart, usually in different cities. Linked clusters link the individual CAA clusters to these sites. PowerHA SystemMirror uses the links between the sites for heartbeat and cluster communications. These links use regular IP communication between the sites, instead of the multicast IP communication that each CAA cluster uses internally.

Using linked clusters is a useful option if you cannot use multicasting between the sites or if you cannot share a common repository disk between the sites.

For more information about cluster types for multiple site configurations, see the Stretched clusters topic and the Linked clusters in the AIX Information Center.