Arbitration For Data Integrity in Serviceguard Clusters, July 2007

Arbitration for Data Integrity in Serviceguard Clusters

Use of a Quorum Server as the Cluster Lock

A quorum server can be used in clusters of any size. The quorum server

is an alternate form of cluster lock that uses a server program running

on a separate system for tie-breaking rather than a lock disk. Should two

equal sized groups of nodes (exactly 50% of the cluster in each group)

become separated from each other, the quorum server allows one group

to achieve quorum and form the cluster, while the other group is denied

quorum and cannot start a cluster.

The quorum server process runs on a machine outside of the cluster for

which it is providing quorum services. The quorum server listens to

connection requests from the Serviceguard nodes on a known port. The

server maintains a special area in memory for each cluster, and when a

node obtains the cluster lock, this area is marked so that other nodes will

recognize the lock as “taken.” The cluster will maintain this mark during

an off-on power cycle of the quorum server.

The operation of the quorum server is shown in Figure 7. When there is a

loss of communication between node 1 and node 2, the quorum server

chooses one node (in this example, node 2) to continue running in the

cluster. The other node halts.

Figure 7 Quorum Server Operation