Exchange 2010 DAG Always Replicates from Active Database

Today's article is a tidbit of information, but important to call out for larger scale DAG deployments.

Exchange 2010 always uses the active database in the DAG as the source for log shipping during normal replication.  That means that if you have multiple passive copies in your DAG, Exchange ships transaction logs from the active copy to each passive copy, even if some of the copies are in the same site.  There is no peer-to-peer log shipping between passive copies in a DAG.

Simple four node DAG with three passive copies

In the example above we have a single DAG with the active database and one HA copy in DC1, and one DR copy and a lagged copy in DC2.  Log shipping occurs from the active database to the three passive copies, traversing the WAN twice for the copies in DC2.

This can have quite an affect on a complex enterprise deployment with multiple DAGs and many remote passive copies, so keep that in mind for your designs.

Note: Log shipping is different than seeding.  Seeding is a file copy of the database to another server.  Once seeding completes log shipping is used to keep that copy up to date. It is possible to seed a database from a specific server, perhaps one in the same site.  For more information see the "Selecting the Seeding Source" topic in http://technet.microsoft.com/en-us/library/dd335158.aspx.