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So What are the Windows Server 2008 quorum models? |
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Written by David Noel-Davies
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Friday, 13 June 2008 |
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.
- 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.
- 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.
- 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.
A quorum isn't a physical object, such as a file, and it isn't the cluster database. Quorum is a concept. A cluster has quorum when a sufficient number of cluster voting elements with the same consistent cluster view are available and running. The number of cluster voting elements required to make quorum is determined by the quorum mode used. There are four modes: Node Majority, Node and Disk Majority, Node and File Share Majority, and No Majority: Disk Only. If a cluster is split for any reason, such as a network failure, and the cluster becomes partitioned, the quorum model is designed to ensure that only one partition can have quorum. The partition with quorum is determined by the number of voting elements present in the partition, or which partition has access to a specific resource, such as a disk witness. The partition without quorum will shut down all its services. Voting-element status can be assigned to: o nodes in the cluster o a shared disk witness (formerly known as the quorum drive), but each cluster may have only one o a file share witness, which each cluster may also have only one You can't have both a disk-witness and file-share-witness in the same cluster. You choose which resources can vote. Once votes have been assigned, the number of nodes required for quorum is based on the following formula: Total Voters/2 + 1 = majority (rounding down) For example, if I have a five-node cluster, the formula would read 5/2 + 1 = 3.5 The formula rounds down when there is a fractional result, so in this example, I would need three nodes present to make quorum. If I had a four-node cluster with a disk witness, I would still have five votes (four nodes plus the disk witness' vote), and would still need three votes to make quorum (which could be three nodes or two nodes and the disk witness). This quorum model's advantage is that there isn't a single point of failure, which existed in previous models that relied on the quorum drive. As my first example showed, quorum can be made even if the disk witness is unavailable. |
