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.
Prior to Exchange 2016 Cumulative Update 2 (CU2), Exchange Server administrators had to either manually activate their preferred database copy, or use the RedistributeActiveDatabases.ps1 script to balance the databases copies across a DAG. Starting with CU2 (which will be releasing soon), the Primary Active Manager in the DAG performs periodic discretionary moves to activate the copy that the administrator has defined as most preferred is now built into the product. A new DAG property called PreferenceMoveFrequency has been added that defines the frequency (measured in time) when the Microsoft Exchange Replication service will rebalance the database copies by performing a lossless switchover that activates the copy with an ActivationPreference of 1 (assuming the target server and database copy are healthy).
Note: In order to take advantage of this feature, ensure all Mailbox servers within the DAG are upgraded to Exchange 2016 CU2.
By default, the Replication service will inspect the database copies and perform a rebalance every one hour. You can modify this behavior using the following command:
Set-DatabaseAvailabilityGroup <Name> -PreferenceMoveFrequency <value in the format of 00:00:00>
To disable this behavior, configure the PreferenceMoveFrequency value to ([System.Threading.Timeout]::InfiniteTimeSpan).
If you are leaving the behavior enabled, and you have created a scheduled task to execute RedistributeActiveDatabases.ps1, you can remove the scheduled task after upgrading the DAG to CU2.
We recommend taking advantage of this behavior to ensure that your DAG remains optimally balanced. This feature continues our work to improve the Preferred Architecture by ensuring that users have the best possible experience on Exchange Server.
As always, we welcome your feedback.
Ross Smith IV
Principal Program Manager
Office 365 Customer Experience
6/21/16: Updated information on how to disable PreferenceMoveFrequency without requiring a Replication service restart. If you set it to [Timespan]::Zero, you will need to cycle the Replication service.