Linked cluster

IBM® AIX® 7.1 with Technology Level 2 Cluster Aware AIX (CAA) introduces the concept of linked cluster.

Linked cluster provides the reliable exchange of data and control messages between two or more nodes that are part of the same cluster but that are separated by geographical boundaries. Each location is called a site. The AIX 7 with 7100-02 CAA supports up to two sites.

The only mode of communication between nodes that are in two sites is through TCP/IP. There is no Storage Area Network (SAN) or disk communication.

The nodes within a site share a common repository. The repositories between sites are synchronized by CAA. When sites are divided or merge, CAA provides a mechanism to reconcile the two repositories. The reconciliation can be done either through a reboot (of all the nodes on site whose repository needs to be updated) or through an application programming interface (API) implemented exclusively for Reliable Scalable Cluster Technology (RSCT).

Disks are not shared across sites. Therefore, the addition or deletion of disks is limited to a single site.

Autonomic Heath Advisor File System (AhaFS) events are propagated across the entire linked cluster to provide a consistent view across the sites and links. Similarly, the lscluster command displays the clusterwide information, that is, the command includes information from both the sites.

The suggested way of creating a two-site cluster is to first create a single-site cluster by issuing the mkcluster command and then to add the remote site and node by issuing the chcluster command. The nodes and the site can be removed by issuing the rmcluster command.

You can upgrade an existing AIX 7 with 7100-01 or AIX 7 with 7100-01 SP4 of CAA, which does not support clusters with sites to a cluster with site support associated through the process of rolling upgrade.

Several tunable parameters are provided to tune the rate of exchange of heartbeat messages between nodes at different sites.