The Dedicated Admin Connection was built to help you connect and run basic troubleshooting queries in cases of serious performance problems. The DAC uses a special reserved scheduler which has one thread for processing requests. This essentially means that SQL Server is keeping a backdoor open to processor resources available just for you. Two percent of you will be tempted to use this for your maintenance jobs on busy systems. The DAC was not designed for high performance. By default the DAC is only enabled for accounts logged onto the local machine. For production servers, that means it only works for remote desktop sessions to non-clustered SQL Server instances. Should you enable remote connections? I agree with Books Online , which points out:. For clusters, sign me up!
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How to connect to DAC
SQL Server provides a special diagnostic connection for administrators when standard connections to the server are not possible. This diagnostic connection allows an administrator to access SQL Server to execute diagnostic queries and troubleshoot problems even when SQL Server is not responding to standard connection requests. The DAC only allows changing the user context to another admin user. SQL Server makes every attempt to make DAC connect successfully, but under extreme situations it may not be successful. By default, the connection is only allowed from a client running on the server.
You would be able to see the connection name at the bottom of the query window as shown in the snippet below. Then, you would end up receiving below mentioned error message. Once you have established a successful DAC connection you can execute troubleshooting queries to identify the bottleneck and take action accordingly.
A dedicated administrator connections DAC is the last resort for a DBA when trying to fix an instance where no new connections are possible anymore because its resources are exhausted. For such situations, SQL server always retains a limited chunk of memory and other necessary resources to allow a last connection: the dedicated administrator connection. A DAC is also required to select from the system base tables.