Exchange Server 2003 Clusters

What's Changed in Exchange 2003
Although you can deploy Exchange 2003 on Windows 2000 servers, Microsoft is fond of saying that Windows 2003 and Exchange 2003 are better together and deliver the optimum functionality because they're designed to work as a team. This statement is true in many respects but is especially true for clusters. I wouldn't recommend deploying an Exchange cluster on anything but Windows 2003 servers. Here are the major improvements in Windows 2003 and Exchange 2003 clusters:

• With Windows 2003 and Exchange 2003, you can configure eight-node clusters. Compared with four nodes, eight nodes provide a lot more flexibility in how you can lay out the servers within a cluster and the roles that the servers take. However, at least one node has to be passive if the cluster supports numerous clients, many connectors, or a heavy processing load. Clusters that support a small number of clients and perhaps run only one SG with a few databases on each active node can typically operate in a fully active mode because virtual-memory fragmentation is less likely to occur.
• The dependency on Datacenter is gone, so you can now deploy clusters without the additional expense that Datacenter introduces.
• Windows 2003 and Exchange 2003 better control virtual-memory fragmentation, which increases the number of MAPI clients that a cluster can support. Windows 2003 and Exchange 2003 also make better use of large amounts of memory (i.e., more than 1GB) when that memory is available to a server. No formal testing has yet established how many concurrent MAPI clients Exchange 2003 supports before it runs into the virtual-memory fragmentation problem, but the fact that Microsoft has deployed clusters that support 4000 mailboxes per node reveals that the limit is high. If a passive node is always available in an active-passive configuration, clusters can support numerous users per active node—perhaps as many as 5000 mailboxes per node. The exact figure depends on the system configuration, the load that the users generate, the types of clients used, and careful monitoring of virtual memory on the active nodes as they come under load.
• You can use drive mount points (otherwise known as NTFS mounted drives) to eliminate the Win2K—Exchange 2000 restriction on the number of available drive letters, which limits the number of available disk groups in a cluster. This improvement is important when you deploy more than 10 SGs across multiple cluster nodes.
• Because of Exchange 2003's new resource-dependency model and some tweaks in the way that Exchange 2003 manages failover, Exchange 2003 appears to be faster than Exchange 2000 at transitioning SGs from failed servers to active nodes when problems occur.
• Microsoft made tweaks to Exchange 2003's management interfaces to make life easier for administrators. For example, as Figure 2 shows, the Exchange System Manager (ESM) console now displays details of server types, so you know immediately whether a server is running on a cluster.
• Assuming that you use appropriate hardware and backup software, you can use Windows 2003's Volume Shadow Copy Service (VSS) API to take hot snapshot backups. This improvement is crucial because clusters can't attain their full potential if administrators limit the size of the databases. Limiting the size of databases limits the number of mailboxes that a cluster can host. However, vendors have been slow to ship VSS-compliant products, so don't depend too much on this feature until you see solid products appear.
• The Recovery Storage Group feature lets administrators recover from individual database failures quickly and without having to deploy dedicated recovery servers. This feature is also available when you deploy Exchange 2003 on Win2K servers.

VSS and the Recovery Storage Group aren't cluster-specific features. However, both contribute to higher levels of service availability and reassure administrators who worry that consolidating many standard servers into a large cluster might be putting all their eggs into one basket.