Datacenter Activation Coordination mode (DAC mode) is a feature of Exchange database availability groups that is designed to prevent split brain scenarios.
Within a database availability group (DAG) each database can have one active copy, and up to 15 passive copies at any given time. Changes that occur in the active database copy are replicated to the passive copies by a process of continuous replication. The active copy can be dismounted, and one of the passive copies mounted to become the active copy, when a switchover or failover occurs.
In a split brain scenario, two copies of a database would be active at the same time, mounted on DAG members that are unable to communicate with each other due to a network problem, causing the databases to diverge. This is a situation that must be avoided, because it creates a difficult recovery scenario and will likely result in data loss.
DAC mode enables a protocol called Datacenter Activation Coordination Protocol (DACP). You can read an example of how DAC mode and DACP work to avoid split brains in this article.
The recommended practice for Exchange Server DAGs is to enable DAC mode if:
- The DAG has more than one member (DAGs start with zero members, and can have one member)
- Third party replication mode is not enabled, or the third party replication vendor specifies that DAC mode can be used
In addition to preventing split brains, DAC mode enables the use of Exchange site-resilience cmdlets (Stop-DatabaseAvailabilityGroup, Restore-DatabaseAvailabilityGroup, and Start-DatabaseAvailabilityGroup). Those cmdlets are used perform datacenter switchovers.
To review the DAC mode configuration for a DAG, use the Get-DatabaseAvailabilityGroup cmdlet.
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[PS] C:\>Get-DatabaseAvailabilityGroup | Select Name,Servers,Datace* Name Servers DatacenterActivationMode ---- ------- ------------------------ EX2016DAG01 {EX2016SRV2, EX2016SRV1} Off EX2010DAG {EX2010SRV1} Off |
To enable DAC mode, use the Set-DatabaseAvailabilityGroup cmdlet.
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[PS] C:\>Set-DatabaseAvailabilityGroup EX2016DAG01 -DatacenterActivationMode DagOnly [PS] C:\>Get-DatabaseAvailabilityGroup | Select Name,Servers,Datace* Name Servers DatacenterActivationMode ---- ------- ------------------------ EX2016DAG01 {EX2016SRV2, EX2016SRV1} DagOnly EX2010DAG {EX2010SRV1} Off |
DAC mode can be enabled for DAGs that exist within a single datacenter location. A multi-site DAG is not required. Although less likely, a split brain can still occur for DAGs within a single datacenter location under the right conditions.
So recommendation is use dac when going dag ? not by the fact if using two datacenter on different geolocations?
We are going to build a 10 node dag over 2 high speed low latency datacenter with FSW in azure.
Following your article we will configure dac
Paul,
This is great info. Thanks very much.
I think it’d be very helpful if you included a link to this post in your 2013 DAG setup/info posts. I did not know about DAC until today and would’ve caught it earlier through your DAG posts.
Thanks again!
Paul,
I’m confused now in two datacenters on different geolocations. We should Enable DAC or not and why?
And what about Alternate File Share Witness?
I appreciate your reply.
Thanks..
Paul, I appreciate your reply.
What did he tell you? i have a very similar situation.
2 datacenters, 2 Ex2013’s in both, all in 1 dag.
What is the best way to take care if one site was to go down.
Paul,
When it comes to DAC mode and an even number of members, is the FSW required to be up and running in order to mount the databases assuming all DAG members go offline ?
Studying your pluralsight videos.
If all DAG members go offline there is nothing left to mount the databases. I suspect I am misunderstanding your question.
Sorry Paul let me be more clear. I am saying this:
If you have a 4 member DAG and all servers go offline then once you bring them backup – is the FSW required to be online as well for Quorum to be attained ?
Meaning in your 4 member dag, do you need all 4 dag members online and the FSW online at the same time for the DAG to get quorum and exchange to mount the databases?
Robert