In Exchange Server 2007, Microsoft offered various high-availability features, but the actual implementation was less than flexible. Solutions providers had to plan the mailbox server architecture in advance, and it wasn't easily changed once implemented.
Microsoft has taken steps to make highly available mailbox servers easier to deploy in Exchange 2010. If you have ever configured an Exchange 2007 mailbox server (running on Windows Server 2008) to act as a Cluster Continuous Replication (CCR) cluster, Standby Continuous Replication (SCR) cluster or Single Copy Clusters, then you know that the configuration process requires you to configure failover clustering at the operating system level.
Solutions providers no longer face this requirement when implementing Exchange 2010 high availability. In some cases, Exchange 2010 still uses failover clustering, but the underlying cluster is completely controlled by Exchange. Solutions providers never have to worry about creating, configuring or maintaining a failover cluster.
Exchange 2010 high availability completely does away with the concept of a clustered mailbox server. Local Continuous Replication (LCR), CCR and SCR are all replaced by a new feature called Database Availability Groups (DAGs). SCCs are also removed from Exchange 2010.
DAGs make Exchange database management techniques easier and more consistent. In Exchange 2007, each type of continuous replication was managed in a different way.
DAGs also provide high availability at a more granular level. As you may recall, Exchange Server 2007 allows you to host up to 50 databases on a single mailbox server. If a problem occurs with one database, failover occurs at the server level -- even though the other 49 databases are still healthy.
In Exchange 2010, high availability, failover and switchover occur at the database level. If a database failure occurs, you can activate the database on another server that contains a replica of the database -- without affecting any of the functional databases on the original server.
With improved high-availability in Exchange Server 2010, databases are no longer bound to a specific server. Exchange 2010 high availability makes it simple to move the active copy of a database from one server to another. Therefore, you won't have servers that only act as hosts for passive database copies. In fact, in larger organizations, Microsoft recommends scattering active database copies across multiple mailbox servers rather than having one server that contains all active databases and other servers that contain only passive databases.
By redesigning the way clustering works in Exchange 2010, Microsoft also improved Exchange Server's scalability. CCR limited Exchange 2007 mailbox servers to only two copies of a database -- one active and one passive. In contrast, Exchange 2010 clusters can have up to 16 copies of a database -- one active and up to 15 passive. Most organizations won't need to create a cluster that large, but it is nice to know that your cluster can grow as your business continuity requirements change.
Additionally, Exchange 2010 Servers can host up to 100 different databases per mailbox server, as opposed to the 50 database-per-server limit imposed by Exchange 2007. The goal there is to make Exchange 2010 databases more efficient than the databases used by previous versions of Exchange.
Microsoft has also improved efficiency by removing single-instance storage, storage groups and other features from Exchange 2010.
About the expert
Brien M. Posey, MCSE, is a five-time recipient of Microsoft's Most Valuable Professional award for his work with Exchange Server, Windows Server, Internet Information Services (IIS), file systems and storage. Posey has served as CIO for a nationwide chain of hospitals and was once responsible for the Department of Information Management at Fort Knox. As a freelance technical writer, Brien has written for Microsoft, TechTarget, CNET, ZDNet, MSD2D, Relevant Technologies and other technology companies. You can visit Brien's personal website at www.brienposey.com.