Datacenter Activation Coordination (DAC) mode is a property setting for a database availability group (DAG).
DAC mode is disabled by default and should be enabled for all DAGs with 2 or more members that use continuous replication.
That means the majority of Exchange DAG deployments need the DAC mode.
The DAC is most useful in a multi-datacenter configuration to prevent split brain syndrome, a condition that occurs when all networks fail, and DAG members can’t receive heartbeat signals from each other.
However, I suggest you always enable the DAC.
If you enable the DAC and you need to recover, the recovery takes less commands on the command line. Only the following commandlets will be necessary:
Also, if you did NOT enable the DAC, you cannot do so if you have a failure. The DAC must be enabled ahead of time.
Here is how to enable the DAC mode:
1. Go to Exchange Management Shell
2. Type the following, where DAG2 is your DAG name:
Set-DatabaseAvailabilityGroup -Identity DAG2 -DatacenterActivationMode DagOnly
For more information, read this article:
I was glad for a chance to present at the Workshop of the Association of Environmental Authorities of NJ (AEANJ). There were great questions from the audience.
Thank you to attendees, Leon McBride for the invitation, Peggy Gallos, Karen Burris, and to my colleague Lucy Valle for videotaping.
My presentation is called “Data Portability, Data Security, and Data Availability in Cloud Services”
Here are the collateral files for the session: