A majority node set (MNS) cluster requires a majority of nodes to be online. If not, the cluster’s resources will not be available. Obviously, a two-node MNS cluster does not make sense because when one node goes down, the cluster is down. A third node is needed so that one node can go down without impacting the entire cluster.
In the past, this third node needed to be a regular cluster node, thus requiring Windows Server 2003 Enterprise or higher. Not anymore though! Just take a look at the following knowledge base article. It discusses a new feature, called the file share witness. You will need to install an update to have this feature available.
Also take a look here and follow the link to more information about Exchange 2007 cluster continuous replication (CCR). CCR requires an MNS cluster so the file share witness feature will make it easier and less costly to deploy such clusters. It appears that the CCR preliminary documentation already refers to the file share witness feature.
