Wednesday 4 February 2015

Database Availability Groups in Exchange 2013

Database Availability Groups (DAG's) provide a way for multiple mailbox servers to keep in sync (providing continuous replication) with each other's database, so if a member of the DAG goes down another mailbox server can take over.

When DAG is deployed the failover clustering feature is installed - since DAG's use quorum to determine when to failover. In the event that there is an even number of members in the DAG a file share witness is employed to act as an additional vote.

You also have the ability to store different mailbox databases on each member of the DAG, for example in the case you have three members and three mailbox databases you could host a "passive" copy of one of he databases on each of the members and a "lagged" copy on the other two members respectively.

There is a limit of 16 mailbox servers in one DAG, but they can operate over multiple AD sites.

You can read more about setting up a DAG here.


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