Professional Microsoft® SQL Server® 2008 Administration

20.2. Clustering: The Big Picture

If you're going to get involved with clustering, you need to know how it works, including clustering configuration options.

20.2.1. How Clustering Works

Clustering is a complex technology with a lot of messy details. To avoid scaring you too much too soon, we'll take a look at the big picture of how clustering works. In this section, we take a look at active and passive nodes, the shared disk array, the quorum, public and private networks, and the virtual server. Finally, we explain how a failover works.

20.2.1.1. Active Nodes versus Passive Nodes

Although a SQL Server 2008 cluster can support up to eight nodes, clustering actually occurs between only two nodes at a time because a single SQL Server 2008 instance can run on only a single node at a time; and should a failover occur, the failed instance can failover to another individual node only. This adds up to two nodes. Clusters of three or more nodes are used only where you need to cluster multiple instances of SQL Server 2008. We discuss larger clusters later in this chapter.

In a two-node SQL Server 2008 cluster, one of the physical server nodes is considered the active node, and the second one is the passive node. It doesn't matter which of the physical servers in a cluster is designated as active or passive, but it is easier from an administrative point of view to simply assign one node as the active and the other as the passive. This way, there is no confusion about which physical server ...