Majority Node Set Quorum in Geographically Dispersed Clusters
Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2
Although geographically dispersed clusters can use a standard quorum, presenting the quorum as a single logical shared drive between sites can create design issues. Majority node set clusters solve these issues by allowing the quorum to be stored on the local hard disk of each node.
Applications in a geographically dispersed cluster are typically set up to fail over in the same manner as those in a single-site cluster; however, the total failover solution is inherently more complex. The Cluster service provides health monitoring and failure detection of the applications, the nodes, and the communications links, but there are cases where it cannot differentiate between various failure modes.
Consider two identical sites, each having the same number of nodes and running the same software. If a complete failure of all communication (both network and storage fabric) occurs between the sites, neither site can continue without human intervention because neither site has sufficient information about the other site’s continuity.
As discussed in "Choosing the Cluster Model" earlier in this chapter, the server cluster architecture requires that a single-quorum resource in the cluster be used as the tiebreaker to avoid split-brain scenarios. Although split-brain scenarios can happen in single-site clusters, they are much more likely to occur in geographically dispersed clusters.
If communication between two sites in a geographically dispersed server cluster were to fail, none of the cluster nodes in either site could determine which of the following is true:
Communication between sites failed and the other site is still alive.
The other site is dead and no longer available to run applications.
This problem is solved when one of the partitions takes control of the quorum resource. However, with majority node set clusters, the process is different than in a traditional (single-site, single-quorum) server cluster. Traditional clusters can continue as long as one of the nodes owns the quorum disk, and traditional server clusters can therefore continue even if only one node out of the configured set of nodes is running (if that node owns the quorum disk). By contrast, a server cluster running with a majority node set quorum resource will start up (or continue running) only if a majority of the nodes configured for the cluster are running, and if all nodes in that majority can communicate with each other.
When designing your server cluster with a majority node set quorum, consider this difference, because it can affect how majority node set clusters behave when the server cluster is partitioned and during split-brain scenarios. This "more than half" quorum requirement makes the number of nodes in your server cluster very important. For more information about majority node set quorum cluster models, see "Model 3: Majority node set server cluster configuration" in Help and Support Center for Windows Server 2003.