DAG Activation Preference Behavior Change in Exchange Server 2016 CU2

Every copy of a mailbox database in a DAG is assigned an activation preference number. This number is used by the system as part of the passive database activation process, and by administrators when performing database balancing operations for a DAG. This number is expressed as the ActivationPreference property of a mailbox database copy. The value for the ActivationPreference property is a number equal to or greater than 1, where 1 is at the top of the preference order. When a DAG is first implemented, by default all active database copies have an ActivationPreference of 1. However, due to the inherent nature of DAGs (e.g., databases experience switchovers and failovers), active mailbox database copies will change hosts several times throughout a DAG’s lifetime. As a result of this inherent behavior, a mailbox database may remain active on a database copy which is the not the most preferred copy.

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