Enabling Standby Continuous Replication – Dude, Where’s My Database?

Standby Continuous Replication is one of many reasons that customers, partners, and the general Exchange community are eagerly awaiting the release of Service Pack 1 for Exchange Server 2007. SCR extends the continuous replication features in the RTM version of Exchange 2007 by adding a third type of continuous replication that enables multiple storage group copies on one or more additional servers.

Just like local continuous replication (LCR), when you enable a storage group for SCR using the Enable-StorageGroupCopy cmdlet with the -StandbyMachine parameter, a copy of the storage group is created on the replication endpoint, or what we call the SCR target computer.  The path used by the local storage group copy is created on the SCR target computer, and log shipping from the SCR source (the active copy) to the SCR target will commence. But unlike LCR where the storage group is seeded before log shipping occurs, when enabling SCR, there is a delay in the creation of the SCR target's copy of the database.

This can be confusing because when you enable SCR, the copy of the database will not be immediately created. Here's what we say in the documentation for SCR: "The built-in delay of 50 log files and the value of the ReplayLagTime parameter has implications for the creation of the initial SCR target database. An SCR target database will not be created until 50 transaction log files have been replicated to the SCR target computer, and until the time period specified by ReplayLagTime (or the default ReplayLagTime value of 24 hours) has elapsed."

SCR has some important differences from LCR (and for that matter, from CCR, but I am intentionally using LCR for comparison because (1) SCR is not a clustered solution and does not require a failover cluster - although it can work with CCR and SCC, and (2) because in a CCR environment all storage groups are enabled for replication, whereas with LCR and SCR, the admin specifies what storage groups are enabled).

One difference is a built-in delay for replay activity, and a management control to allow admins to configure an additional replay delay time. By design in SCR, Exchange prevents a fixed number of log files from being replayed on an SCR target, regardless of the value for ReplayLagTime, using the following formula: Maximum of ("value of ReplayLagTime" or "X log files") where X=50.

Because of these delays, the initial SCR target database is not created immediately, and this is normal, expected behavior. For more information on managing SCR, visit http://technet.microsoft.com/en-us/library/bb676465.aspx.

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