In today's Blackberry-connected world, email is a mission critical application for nearly every organization. While users expect their Exchange server to be available 24 hours a day, 365 days a year, those of us who build and maintain Exchange servers know Microsoft hasn't given us the tools to meet that expectation – at least not yet.
Until Exchange 2007 hit the streets last year, Microsoft's only answer to data replication on Exchange was to run it on a Windows cluster. However, since Windows clusters use a shared storage model, building a cluster doesn't protect your customer's data against an array failure, a corrupt database or a disaster that takes out the entire data center. For that you had to add a storage replication program like DoubleTake, which costs too much for many SMBs.
For Exchange Server 2007 Microsoft added several new data protection features based on log shipping. Exchange has always been based on a transaction logging database: As messages and other items are posted to the in-memory version of the Exchange database, they're written to a series of transaction log files. Eventually Exchange flushes changes to the in-memory cache to the on-disk copy of the database.
When you set up Cluster Continuous Replication for a client, they can have an Exchange cluster where each mailbox server has its own disks. Once the active server in the cluster fills a transaction log and writes it to the database, it copies the file via the Common Internet File System to a file share on the passive server. When the passive server receives the file, it rolls it forward into its own copy of the Exchange database. If the primary server goes down, the standby server can take over from its database. This also means that if you have to perform routine maintenance on the primary server, you can issue a PowerShell command that will flush its database and make sure all the log files are transferred to the standby node before shutting down.
A word of warning: Since data isn't transferred until a transaction log file is filled, some data will be lost if the primary server fails. In order to reduce the amount of data at risk, Microsoft reduced the size of each transaction file to 1MB, from 5MB. You can even run backups from the passive node to lighten the load on the active node.
Service Pack 1 adds Standby Cluster Replication (SCR), which -- like Cluster Continuous Replication -- uses log shipping. However, SCR lets you replicate data from one source to multiple targets for even better data protection. More significantly, Standby Cluster Replication lets you add a delay before the logs are played forward on the standby server(s). That way, if a bad message that corrupts the Exchange database comes into the primary system, you can mount the database on the standby system that hasn't yet posted that log file.
Standby Cluster Replication also eliminates the requirement that the active and passive servers be on the same subnet and that there be less than 500ms round-trip latency between them. Notably, data in transit between passive and active servers still isn't encrypted by Exchange, so using private links or transmitting data using a VPN is a good idea.
Standby Cluster Replication is a perfect disaster recovery solution for SMB Exchange users. As long as your client can afford a standby server and a connection to a disaster recovery site to house it, they can have a full-fledged failover solution. Just remember they may lose messages from the last 10 to 15 minutes before their server went down because those logs may not have been transferred.
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