Cross-site DAG enables DAC mode to prevent network partitions from occurring at the database level

Data Center activation Coordination Mode also known as DAC mode, this mode should be enabled for Dags with two or more nodes that use continuous replication . This mode controls the startup database mount behavior of the DAG, which prevents network partition failures at the database level during data center recovery. For example, the primary datacenter loses power, all replicas are disconnected, and the active copy switches to the disaster recovery site. a network partition error occurs when the primary datacenter resumes power (when the server recovers but the inter-site WAN Connection has not been restored) and the database in the primary datacenter is automatically Mount.

Activity Manager stores a digit (0 or 1) in memory thattells the DAG whether to allow a local database to be mounted in an active state on the server. when the DAG is running in DAC mode, the bit is set to 0 each time the Activity Manager starts , indicating that thedatabase is not allowed to mount. If the Dag is in DAC mode, the server must try to communicate with all other members of the DAG that it knows to get another Dag member, Tells it if it can mount a local database that is assigned to it in an active state. The answer will be provided as a digital setting for other activity managers in the DAG. If another server responds with its digital digits set to 1, This means that the server allows the database to be mounted so that the server starts with a number of bits set to 1and loads its database.

back to the failure described earlier, after the disaster recovery site mounts the active node, the Activity Manager's parameter is set to 1, and the primary datacenter has a parameter of 0, and the primary Datacenter's copy is not automatically mounted until communication with the disaster recovery site is completed, and once the communication is complete, The value returned by the disaster recovery site also prevents the primary datacenter from mounting the database, so you can avoid network partitioning errors.

enabled DAC mode:

Set-databaseavailabilitygroup-identity Dag2-datacenteractivationmode dagonly

Close DAC mode:

Set-databaseavailabilitygroup-identity Dag2-datacenteractivationmode Off

For more information, refer to:https://technet.microsoft.com/zh-CN/library/dd979790 (v=exchg.150). aspx

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Cross-site DAG enables DAC mode to prevent network partitions from occurring at the database level