High Availability for Exchange Server 2007

High Availability for Exchange Server 2007

Circumstances such as component failure, power outages, operator errors, and natural disasters can affect a messaging system's availability. To help prevent against such circumstances, it is crucial that companies plan and implement reliable strategies for maintaining high availability. A highly available messaging system can save money by providing consistent messaging functionality to users.

Exchange Server 2007 has three main High Availability features: Single Copy Cluster (SCC), Local Continuous Replication (LCR) and Cluster Continuous Replication (CCR). Exchange 2007 SP1 has an additional feature named Standby Continuous Replication (SCR), which can be classified as a Disaster Recovery feature rather than a High Availability feature.

Local Continuous Replication (LCR)
Local continuous replication (LCR) is a single-server solution that uses built-in technology to create and maintain a copy of a storage group on a second set of disks that are connected to the same server as the production Mailbox Server. LCR provides asynchronous log shipping, log replay, and a quick manual switch to a copy of the data.

Cluster Continuous Replication (CCR)
Cluster continuous replication (CCR) combines the replication and replay features in Exchange 2007 with failover features in Microsoft Cluster services. CCR is a solution that can be deployed with no single point of failure in a single datacenter or between two datacenters. CCR provides several advantages over clustering in previous versions of Exchange Server and single copy clusters in Exchange 2007. This feature takes the new Exchange Server 2007 Log file shipping and replay features and combines them with the features that are available in a more traditional 2 node Windows 2003 active/passive cluster setup. A traditional 2 node active/passive cluster certainly has its benefits, but it also has one major drawback and that is you still have a single point of failure when it comes to the Data Storage. CCR overcomes this disadvantage by storing the Exchange Active Database and Passive Database on different storage devices.

Single Copy Clusters (SCC)
Single copy clusters (SCC), known as shared storage clusters in previous versions of Exchange Server, are present in Exchange 2007, with some significant changes and improvements. With SCC, all of the hardware, including the disks used for Exchange data, must be listed in the Cluster category of the Windows Server Catalog. With CCR, the disks used for Exchange databases are local to each system and are not controlled or failed over as part of the cluster.

SCC provides redundancy for the server, but not for storage. CCR provide redundancy with no single point of failure. CCR allows you to simplify backup administration and offload backup IO demands completely to the passive replica server.