Quorum is the number of votes that must be present for the cluster to function.
Two system parameters, VOTES and EXPECTED_VOTES, are key to the computations performed by the quorum algorithm.
VOTES specifies a fixed number of votes that a computer contributes towards the quorum. For satellite nodes, the value is 0; for all other computers, the value is 1.
EXPECTED_VOTES specifies the sum of all votes held by the cluster members. This parameter is set on each active Alpha or Integrity system in the cluster including satellites.
Calculating Custer Votes
When nodes in the OpenVMS Cluster boot, the connection manager uses the largest value for EXPECTED_VOTES of all systems present to derive an estimated quorum value according to the following formula:
Estimated quorum = (EXPECTED_VOTES + 2)/2 | Rounded down
During a state transition (whenever a node enters or leaves the cluster or when a quorum disk is recognized), the connection manager dynamically computes the cluster quorum value to be the maximum of the following:
- The current cluster quorum value (calculated during the last cluster transition).
- Estimated quorum, as described above
- The value calculated from the following formula, where the VOTES system parameter is the total votes held by all cluster members:
QUORUM = (VOTES + 2)/2 | Rounded down
The connection manager compares the cluster votes value to the cluster quorum value, and if the total number of cluster votes is equal to at least the quorum value, the cluster continues running, otherwise the remaining OpenVMS Cluster members suspend all process activity and all I/O operations to cluster-accessible disks and tapes until sufficient votes are added (that is, enough computers have joined the OpenVMS Cluster) to bring the total number of votes to a value greater than or equal to quorum.
For example, consider a cluster consisting of three computers, each computer having its VOTES parameter set to 1 and its EXPECTED_VOTES parameter set to 3. The connection manager dynamically computes the cluster quorum value to be 2 (that is, (3 + 2)/2). In this example, any two of the three computers constitute a quorum and can run in the absence of the third computer. No single computer can constitute a quorum by itself. Therefore, there is no way the three OpenVMS Cluster computers can be partitioned and run as two independent clusters.
A quorum disk is a virtual cluster member whose purpose is to add one vote to the total cluster votes to increase the availability of a two-node cluster. A quorum disk must contain a valid format file named QUORUM.DAT in the Master File Directory created automatically after a system specifying a quorum disk has booted into the cluster for the first time. This file is used on subsequent reboots. The following restrictions apply to quorum disks:
- a quorum disk must be mounted by all quorum disk watchers
- the OpenVMS Cluster can include only one quorum disk
- the quorum disk cannot be a member of a shadow set
At least one computer in the cluster must have a direct (not served) connection to the quorum disk - that computer is designated as the Quorum Disk Watcher.
Quorum Disk Watcher
A quorum disk watcher is a member of a cluster that has a direct connection to the quorum disk and supplies information about the status of votes contributed by the quorum disk. It is also used to configure the quorum disk by running CLUSTER_CONFIG.COM, at OpenVMS installation, or by changing DISK_QUORUM and QDISKVOTES using AUTOGEN.