Evaluating Clustering Across Multiple Sites

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

Many organizations build disaster recovery and increased availability into their infrastructure by using multiple physical sites. Most designs involve a primary site and one or more remote sites, where a remote site mirrors the primary site. The level at which components are mirrored between sites depends on the Service Level Agreement and the business requirements.

Geographically Dispersed Clusters

Geographically dispersed server clusters are possible in Windows Server 2003 by using virtual local area networks (VLANs)to connect SANs over long distances. Majority node set, a new type of quorum resource in Windows Server 2003, makes it easier for cluster servers to be geographically separated while maintaining consistency in the event of a node failure.

A geographically dispersed cluster is a combination of pieces supplied by hardware OEMs and software vendors. The configurations required can be complex, and the clusters must use only components supported by Microsoft. Geographically dispersed clusters should be deployed only in conjunction with vendors who provide qualified configurations.

For more information about geographically dispersed clusters and SANs, see "Designing and Deploying Server Clusters" in this book. For complete information about geographically dispersed clusters, see the Geographically Dispersed Clusters link on the Web Resources page at https://www.microsoft.com/windows/reskits/webresources.

MultiSite Network Load Balancing Clusters

Like geographically dispersed server clusters, Network Load Balancing clusters that span more than one physical site (sometimes known as stretch clusters) are intended to protect data in the event of a disaster. Additionally, multisite configurations can be used to provide local access to clients that are spread nationally or globally. For example, a Network Load Balancing cluster split between a data center on the East Coast and a data center on the West Coast can provide services to users in either location.

Unlike geographically dispersed server clusters, in most cases a single Network Load Balancing cluster is not deployed across multiple sites. Instead, multiple, independent Network Load Balancing clusters are deployed, typically with one cluster at each location. The sites are then combined into a single service using features such as DNS round robinto load balance client requests across the sites. Figure 6.10 illustrates a simple version of this scenario.

Figure 6.10   Multisite Network Load Balancing Configuration

In Figure 6.10, the client uses DNS to resolve the service name to an IP address. Depending on the IP address that the client receives, the request is sent to Site 1 or Site 2. In this type of configuration, the sites are essentially clones of each other, and the sites provide the same set of services. A Web site, for example, would be identical at either site, and clients receive the same response to a given query regardless of the site they are directed to. Not only are client requests load-balanced between the sites, but in the event that one site fails, the other site can continue responding to client requests. It is worth noting that when one site goes down, there is potential for a longer response time to individual requests, depending on geographical factors and network demand.