Summary
Depending on how the block level backup of an Exchange Server 2010 DAG environment is configured there can be different license usage implications. This can be confusing for customers and needs to be discussed upfront.
Symptoms
A database availability group (DAG) is a set of up to 16 Microsoft Exchange Server 2010 Mailbox servers that provide automatic database-level recovery from a database, server, or network failure. Mailbox servers in a DAG monitor each other for failures. When a Mailbox server is added to a DAG, it works with the other servers in the DAG to provide automatic, database-level recovery from database, server, and network failures.
Only 1 node will hold the active mailbox databases. The databases are replicated to the other nodes in the DAG and then referred to as the passive databases.
To avoid that DPX will backup data multiple times it is important to make the correct selection in the job definition. For Exchange Server 2010 DAG we advise to only select the EXCH object.
By selecting only the EXCH object DPX will backup up all volumes (allocated blocks) that contain active copies of Exchange databases across DAG nodes. It should automatically skip all volumes containing passive copies of Exchange databases. In this way, only volumes containing active databases will be counted toward source side capacity. If all physical nodes are selected too (as we usually advise to do for non Exchange DAG nodes) all volumes containing the passive databases will also be included in the backup which could be considered as backing up the same data multiple times.
To be able to do BMR it is also required to select BMR object and system volumes on the physical nodes.
Please be aware that if for some reason the node containing the active databases has a problem another node will take over and then contain the active databases. DPX is not not capable of detecting this and with a new backup the volume now holding the active databases will be counted toward source side capacity. This can lead to a considerable increase in license usage.