In the first two articles I discussed on Exchange 2010 Active Manager and Datacenter Activation Coordination (DAC) mode in Exchange 2010 RTM. But Exchange 2010 Service Pack 1 (SP1) introduces changes related to DAC, which I’ll discuss in this post.
To start with, DAC mode support has been extended in Exchange 2010 SP1 to support all 2 DAG configurations with 2 or more members. This is great, since you can now enable DAC mode for 2-member DAGs. Like I explained in the 2nd part, split brain syndrome isn’t unlikely, all the more with 2 nodes given the 50/50 situation. Implementing SP1 enables you to leverage DAC mode for the simplest form of mailbox database resilience, using DAGs with 2 members over 2 sites configurations. When required, DAC in SP1 will use the Witness Server to provide the necessary arbitration.
Another thing is that SP1 doesn’t have the requirement of being enabled for DAGs in at least 2 Active Directory sites. This is good news for customers who have their Active Directory organized in a single site located over multiple locations, e.g. stretched VLANs.
When implementing SP1 on DAG members, you must implement SP1 on all DAG members as soon as possible. Reason is that DAG members running Exchange 2010 RTM can move their databases to a DAG member running Exchange 2010 SP1, but not vice versa. So, do not postpone implementation of SP1 on additional DAG members after implementing it on the first, as it impacts your failover and switchover options. Worst case when not doing so, is ending up in the situation where you cannot activate databases on a server because it doesn’t contain SP1.
Alternate Witness Server
In SP1 you can configure the Alternate Witness Server and Directory using the Exchange Management Console. This location can be used to preconfigure the Alternate Witness Server used during switchover or failover to the secondary datacenter. The configured value will be picked up automatically using the Restore-DatabaseAvailabilityGroup cmdlet during a datacenter switchover, when not explicitly specifying AlternateWitnessServer and AlternateWitnessDirectory there.
Note that this location could already be configured in Exchange 2010 RTM using the Set-DatabaseAvailabilityGroup using the AlternateWitnessDirectory and AlternateWitnessServer options.
DAC is a useful option that each administrator running DAGs on Exchange should consider enabling. But be aware of the caveats, like the requirement of all nodes to be able to communicate with each other during start up. All in all, DAC is a helpful option as it not only prevents issues like split brain syndrome, but it also makes the process of switching datacenters easier and less prone to error. Exchange 2010 SP1 extends the number of possible configuration in which to implement DAC, making DAC an option for the masses.
I hope you found this 3-part post useful, if you still got questions do not hesitate asking me.