Quorum resource options when installing server clusters

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Quorum resource options when installing server clusters

With server clusters on the Windows Server 2003 operating systems, you can now choose between three ways to set up the quorum resource (the resource that maintains the definitive copy of the cluster configuration data and that must always be available for the cluster to run).

The first is a single node server cluster, which has been available in the past and continues to be supported. A single node cluster is often used for development and testing and can be configured with, or without, external cluster storage devices. For single node clusters without an external cluster storage device, the local disk is configured as the cluster quorum device.

The second option is a single quorum device server cluster, which has also been available in earlier Windows versions. This model places the cluster configuration data on a shared cluster storage device that all nodes can access. The general topology is:

This is the most common model and is recommended for most situations. You might choose the single quorum device model if all of your cluster nodes are in the same location and you want to take advantage of the fact such a cluster continues supporting users even if only one node is running.

The third option, which is new for Windows Server 2003, is a "majority node set." A majority node set is a single quorum resource from a Server Cluster perspective; however, the cluster configuration data is actually stored on multiple disks across the cluster. The majority node set resource ensures that the cluster configuration data is kept consistent across the different disks. This allows cluster topologies as follows:

In the majority node set model, every node in the cluster uses a directory on its own local system disk to store the cluster configuration data. If the configuration of the cluster changes, that change is reflected across the different disks. Be aware that it is also possible to have shared storage devices in a majority node set cluster. The exact configuration depends on your installation's requirements.

Only use a majority node set cluster in targeted scenarios, such as:

• Geographically dispersed cluster: A cluster that spans multiple sites.

• Eliminating single points of failure: Although when using a single cluster storage device the quorum disk itself can be made highly available via RAID, the controller port or the Host Bus Adapters (HBA) itself may be a single point of failure.

• Clusters with no shared disks: There are some specialized configurations that need tightly consistent cluster features without having shared disks.

• Clusters that host applications that can fail over, but where there is some other, application-specific way, to replicate or mirror data between nodes.

Do not configure your cluster as a majority node set cluster unless it is part of a cluster solution offered by your Original Equipment Manufacturer (OEM), Independent Software Vendor (ISV), or Independent Hardware Vendor (IHV).

Cluster model considerations

Before implementing your cluster, consider what type of quorum resource solution you plan to use. Take into consideration the following differences between single quorum device clusters and majority node set:

Note

• The following information is presented to help you make basic decisions about the placement and management of your cluster nodes and quorum resource. It does not provide all the details about the requirements for each cluster model, or how each model handles failover situations. If you are not sure which model to use or where you want to place your cluster nodes, refer to the information in Choosing a Cluster Model.

Node failover behavior

The failover behavior of the majority node set is significantly different from the behavior of the single quorum device model:

• Using the single quorum device model, you can maintain cluster availability with only a single operational node.

• If you use a majority node set, more than half, or (Number of nodes configured in the cluster/2) + 1 nodes must be operational in order to maintain cluster availability. The following table shows the number of node failures that a given majority node set cluster can tolerate yet continue to operate:

  Number of nodes configured in the cluster	Number of node failures allowed before cluster failure	Number of nodes needed to continue cluster operations
  1	0	1
  2	0	2
  3	1	2
  4	1	3
  5	2	3
  6	2	4
  7	3	4
  8	3	5

Geographic Considerations

You would commonly use a single quorum resource model if all nodes in your cluster will be in the same geographical location. As part of this requirement, your nodes must be connected to the same physical storage device.

A majority node set on the other hand would typically be appropriate if you have geographically dispersed nodes. The cluster configuration data is stored locally on each node on a file share that is shared out to the other nodes on the network. However, those shares must always be accessible or nodes can fail.

There are other specific requirements for geographically dispersed clusters, including the requirement that round-trip latency of the network between cluster nodes be a maximum of 500 milliseconds. For information on cluster solutions that meet all requirements for a geographically dispersed cluster, refer to the hardware compatibility information, which you can find by clicking the appropriate link in Support resources.

Hardware

Microsoft supports only complete server cluster systems that are compatible with the Windows Server 2003 family of products. Confirm that your entire cluster solution is compatible with products in the Windows Server 2003 family by clicking the appropriate link in Support resources.

For more information about majority node set clusters, see Choosing a Cluster Model and Create a cluster.

Important

• Before implementing a majority node set, it is highly recommended that you read the information in Choosing a Cluster Model. This information will help you understand all the considerations, requirements, and restrictions for each type of quorum resource solution.