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Saturday, 2 June 2012

DAG and DAC (Datacenter Activation Coordination)

When you have a two-member DAG, the system can no distinguish between the different types of failure. It could be a single server failure, a multiple-server failure or a site failure. To manage a datacenter switchover for a two-member DAG that is extended across multiple datacenters you must use Windows failover cluster management tools.

More information on datacenter switchovers can be found here: ... 51049.aspx

If you can wait until Exchange Server 2010 Service Pack 1 changes has been made so that DAC supports two-member DAGs in separate datacenters. DAC, or Datacenter Activation Coordination mode is a property setting for a database availability group (DAG). It enables control of the activation behavior of the DAG members preventing split-brain syndrom by not allowing databases to mount automatically even though the DAG has a quorum.

More information on DAC can be found here: ... 79790.aspx
Datacentre Activation Mode is a mode specifically for multsite Data Availability Groups with 3 or more members. It is there to stop datacentre DAG split brain syndrome.

For example you have 2 Sites with DAG members in each. Catastrophic errors occur and your WAN linking the two sites goes down. You now have a potential scenario where both sites will make a passive database active. The site that already had the active database, and the second site seeing the loss of the active database and automatically activating a copy.

so DAC operates this using literally a bit that it flips from either 0 or 1. 0 meaning it cannot mount a database and upon talking to other DAG members using DACP and finding another server with 1, will mount the databases as it knows it is allowed to.

For a more detailed explanation see here:

DAC must be enabled manually if you are in a split datacentre scenario with 3 or more members. To see the cmdlet to activate again see:

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