This chapter provides a summary of the various cluster functions and features. From establishing cluster quorum to node fencing for isolation, these disparate features comprise the core functionality of the High Availability Add-On.
In order to maintain cluster integrity and availability, cluster systems use a concept known as quorum to prevent data corruption and loss. A cluster has quorum when more than half of the cluster nodes are online. To mitigate the chance of data corruption due to failure, Pacemaker by default stops all resources if the cluster does not have quorum.
Quorum is established using a voting system. When a cluster node does not function as it should or loses communication with the rest of the cluster, the majority working nodes can vote to isolate and, if needed, fence the node for servicing.
For example, in a 6-node cluster, quorum is established when at least 4 cluster nodes are functioning. If the majority of nodes go offline or become unavailable, the cluster no longer has quorum Pacemaker stops clustered services.
The quorum features in Pacemaker prevent what is also known as split-brain, a phenomenon where the cluster is separated from communication but each part continues working as separate clusters, potentially writing to the same data and possibly causing corruption or loss.
Quorum support in the High Availability Add-On are provided by a Corosync plug-in called
votequorum, which allows administrators to configure a cluster with a number of votes assigned to each system in the cluster and ensuring that only when a majority of the votes are present, cluster operations are allowed to proceed.
In a situation where there is no majority (such as a two-node cluster where the internal communication network link becomes unavailable, resulting in a 50% cluster split),
votequorum can be configured to have a tiebreaker policy, which administrators can configure to continue quorum using the remaining cluster nodes that are still in contact with the available cluster node that has the lowest node ID.