The dedicated admin connection DAC can help you out of a sticky situation. This was built to help you connect to SQL Server and run basic queries in cases with critical performance problems. This works by telling SQL Server to reserve a thread specifically for processing your queries in an emergency. While it does reserve a connection for you, it is only one thread, there is no parallelism happening here, in fact, you will receive an error. Typically, what ends up happening and where it can help is when your SQL Server is screaming for resources, but there are none are available. In my experience, this has been related to long-running queries that are not optimized for data the end user wants to return or your SQL Server not allowing new connections. With the DAC you are now able to find the long-running query and kill that session or troubleshoot the issue.
What is the Dedicated Admin Connection (DAC) connection?
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. This feature can help you troubleshoot issue rather than just restarting SQL Services or rebooting servers remotely.
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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. The DAC is available and supported through the sqlcmd command-prompt utility using a special administrator switch -A. For more information about using sqlcmd , see Use sqlcmd with Scripting Variables. Because the DAC exists solely for diagnosing server problems in rare circumstances, there are some restrictions on the connection:.
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?