Last week, the Exchange team released “Exchange 2010 datacenter switchover tool”.
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 two or more members that use continuous replication. DAC mode shouldn’t be enabled for DAGs in third-party replication mode unless specified by the third-party vendor.
If a catastrophic failure occurs that affects the DAG (for example, a complete failure of one of the datacenters), DAC mode is used to control the startup database mount behavior of a DAG. When DAC mode isn’t enabled, and a failure occurs that affects multiple servers in the DAG, when a majority of the DAG members are restored after the failure, the DAG will restart and attempt to mount databases. In a multi-datacenter configuration, this behavior could cause split brain syndrome, a condition that occurs when all networks fail, and DAG members can’t receive heartbeat signals from each other. Split brain syndrome can also occur when network connectivity is severed between the datacenters. Split brain syndrome is prevented by always requiring a majority of the DAG members (and in the case of DAGs with an even number of members, the DAG’s witness server) to be available and interacting for the DAG to be operational. When a majority of the members are communicating, the DAG is said to have quorum.
For example, consider a scenario where the first datacenter contains two DAG members and the witness server, and the second datacenter contains two other DAG members. If the first datacenter loses power and you activate the DAG in the second datacenter (for example, by activating the alternate witness server in the second datacenter), if the first datacenter is restored without network connectivity to the second datacenter, the active databases within the DAG may enter a split brain condition.
This tool is useful if you are using DAC and have not planned your DR scenario yet or you did but still not sure which parameters to use with Stop,Restore and Start-DatabaseAvailabilityGroup commands.
The only downside for me is, this tool is a Powerpoint Presentation so you have to click the buttons in order to proceed without knowing the further steps. That is why I wanted to put it on a diagram with the exact same content.
You can download the Diagram from here.