HP Serviceguard manual Arbitrator Nodes, Quorum Server

Arbitration for Data Integrity in Serviceguard Clusters

Arbitration in Disaster-Tolerant Clusters

with shared data, there is no one disk that is actually connected to both data centers that could act as a lock disk. Arbitration in this case can be obtained by using arbitrator nodes or a quorum server.

Arbitrator Nodes

For example, a metropolitan cluster with three nodes in Data Center A and three nodes in Data Center B could be partitioned such that two equal-sized groups remain up and running, trying to re-form. To address this problem, the supported configurations included one or two arbitrator nodes located in a third data center. These nodes are configured into the cluster for the purpose of providing a majority of nodes when combined with one half the nodes in an equal partition. In other words, if the metropolitan cluster should lose one data center, the surviving data center would still remain connected to the arbitrator nodes, so the surviving group would be larger than 50% of the previously running nodes in the cluster. It could therefore obtain the quorum and re-form the cluster.

Note that in a metropolitan cluster, it is the simple existence of the node(s) in the third data center that provides arbitration combined with the requirement that the configuration have an equal number of nodes in Data Center A and Data Center B. The arbitrator nodes located in Data Center C may do useful work, but they are not attached to the storage devices used by the main nodes in the cluster. They are fully configured as cluster nodes, but their main job is to provide arbitration.

Quorum Server

With the advent of the quorum server, another MetroCluster configuration is now possible. A quorum server process, located in a third data center, can be used for arbitration. The third data center is needed, as it was in the case of arbitrator nodes, to provide the appropriate degree of disaster tolerance. That is, the QS could arbitrate cluster re-formation if either of the other two entire sites should be destroyed.

One advantage of the quorum server is that additional cluster nodes do not have to be configured for arbitration.

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