Product Reviews

Highly Available Exchange 2007

Service Pack 1 brings high-availability enhancements and added capabilities.

• By J. Peter Bruzzese
• 11/01/2008

Certainly one of the highlights of the new release is the Standby Continuous Replication (SCR) feature. SCR joins the other three HA solutions that Exchange 2007 SP1 provides, and is built on the "continuous replication" concept. This is where the database and the transaction logs are copied over to a secondary location -- whether that location is a disk, as in the case of Local Continuous Replication (LCR), or a passive node to a cluster, as in the case of Cluster Continuous Replication (CCR) -- and then replayed into the database.

SCR allows you to replicate your data to a different server or servers because it can accommodate multiple targets. It allows admins to establish a lag time to prevent a corrupt set of files from replicating before they can stop it. It also allows admins to replicate the passive side to a CCR or Single Copy Cluster (SCC), the fourth solution of HA that is offered by Exchange 2007.

The only downside is that SP1 doesn't include a way to accomplish SCR through the Exchange Management Console. It requires the use of the Exchange Management Shell (EMS) with PowerShell commands.

New Capabilities
SP1 includes new capabilities designed to exploit some of the advances made in Windows Server 2008. For example, there are cluster improvements within 2008, which are now called server clusters as opposed to failover clusters as in previous Server versions. In SP1, there is now support for geographically dispersed clusters, or geo-clusters, for SCC and CCR clusters-this way users can have cluster nodes dispersed across different routed networks.

New support for IPv6, Dynamic Host Configuration Protocol support for cluster nodes and new quorum models that add flexibility to how servers handle node failures have also been added. The quorum is similar to a referee that prevents what is called "split-brain" syndrome, a case where two nodes are attempting to be active. There are four quorum models to choose from when configuring clusters, including Node Majority, Node and Disk Majority, Node and File Share Majority and No Majority: Disk Only.

Cleaning up Messaging
Microsoft has added some improvements to the Transport Dumpster in SPI, which was being used in connection with a CCR solution. In the event the active node crashes and the passive node has to pick up that functionality, the Transport Dumpster is there to ensure little or no loss of mail. The Hub Transport servers maintain a copy of mail going through them.

The passive node, when it becomes active, checks in with the Transport Hub to see if there are any messages in the Transport Dumpster that aren't already in the transaction logs.

Duplicates are then discarded and new messages are added in. One of the improvements to this feature is the ability to manually check the Transport Dumpster in the event that the user is using an LCR cluster. There's also now the added ability through the EMS to check on Transport Dumpster statistics. Using the Get-StorageGroupCopyStatus cmdlet, there's a DumpsterStatistics value that shows the number of messages in the Dumpster, the age of the oldest message and more.

These enhancements and additions are certainly welcome, and no doubt the Exchange Team will continue to develop improvements to the HA feature set of Exchange going forward into SP2. I look forward to seeing what's next.