Q. What are the Windows Server 2008 quorum models?

A. Windows Server 2008 has a single-quorum model based on a number of cluster voting elements. By allocating these votes differently, Server 2008 clusters have four quorum modes.

  • Node Majority—There's no file-share witness or disk witness and votes are assigned only to nodes. More than 50 percent of the nodes must be available to make quorum. Node majority works best if you have an odd number of nodes. I recommend the Node Majority mode if you have an odd number of nodes, and you have a multi-site cluster, a cluster with no shared storage, or a single-node cluster. Figure 1 shows an example of a Node-Majority quorum.

  • Node and Disk Majority—Nodes and a shared disk get votes. This configuration allows a loss of half the nodes, providing the disk witness is available, or over half the nodes are available without the disk witness being available. I recommend Node and Disk Majority mode if your cluster has an even number of nodes. Figure 2 shows an example of a Node-and-Disk-Majority quorum.

  • Node and File Share Majority—The same as Node and Disk Majority, except the shared disk is replaced with a file-share witness. I recommend a Node and File Share majority mode if you have multi-site cluster, an even-node cluster, a cluster with no shared storage, or a two-node cluster. Figure 3 shows an example of a Node-and-File-Share-Majority quorum.

  • No Majority: Disk Only—The disk witness must be available to have quorum, but the number of available nodes doesn't matter. If you have a four-node cluster and only one node is available, but the disk witness is available, you have quorum. If the disk witness isn't available, then even if all four nodes are available you can't have quorum. I never recommend the No-Majority: Disk-Only mode. Figure 4 shows how the disk witness must be available to have quorum.

    The table in Figure 5 summarizes my recommended options.

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