What is Dedicated Administrator Connection in SQL Server



           In SQL Server 2005, Microsoft introduced a new feature called Dedicated Administrator Connection (DAC). Using this feature a SQL Server Database Administrator can connect to a SQL Server Instance when the database engine is not responding to regular connections and troubleshoot and to kill any of the SQL Server Processes which are causing the issues.
WHAT CAN DAC (Dedicated Administrator Connection) DO FOR YOU?
Have a SQL Server that’s in crisis? The DAC can help you stage an intervention.

                    The Dedicated Admin Connection was built to help you connect and run basic troubleshooting queries in cases of serious performance problems. This is your opening to grab a ‘Get Out of Jail’ card, but since you don’t use this on a regular basis it’s easy to forget how to use it. It’s also easy to forget to enable access to the DAC remotely.
HOW THE DEDICATED ADMIN CONNECTION WORKS ??

                  
  •     The DAC uses a special reserved scheduler which has one thread for processing requests. 
  •     This is a backdoor open to processor resources available just for you.
  •     That one thread is just one thread— 
  •     There’s no parallelism for queries running on the DAC. 
  •     Using DAC for your maintenance jobs on busy systems can seriously heart your system. 
  •     The DAC was not designed for high performance.

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