Exchange Server 2007 introduced Continuous Cluster Replication (CCR), which utilizes the log shipping and replay features of Exchange 2007 combined with a Majority Node Set (MNS) cluster. CCR utilizes the Microsoft Cluster service to provide failover to the passive node. An Exchange 2007 CCR cluster can be configured with two nodes and a File Share Witness (FSW). FSW is an option that is recommended so that a third piece of hardware does not need be utilized. FSW takes the place of the voter node that was used prior to update 921181. This hotfix allows the use of FSW which is used as a quorum resource without using a shared disk. The FSW is used by the cluster node for arbitration. At least two of the three (when counting nodes or FSW) must be up and communicating to maintain a majority or any cluster resources and the cluster service will cleanly shutdown.
With a CCR cluster it is possible to place each node in a separate datacenter, however the nodes must be on the same subnet. This is a Windows Clustering limitation that will removed with Longhorn Server. With the nodes being in separate datacenters, where should you place the FSW? It is Microsoft's recommendation to place the FSW in the same datacenter with the preferred owning node of the CCR cluster. This recommendation is based on management and statistics. These statistics show the most common network failure is WAN, not LAN related. This means that the highest risk of communication failure between 'voters' in the CCR implementation is between the Active and Passive nodes when the cluster is geographically disbursed. If we place the FSW in the datacenter with the preferred owning node, a loss of communication between the datacenter's will not cause the cluster service to shutdown. We recommend the FSW to be placed on the Hub Transport Role Server.
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