Garry Bargsley forces down the door:
It’s 2 AM. Your phone is going off. Users can’t connect to the application, and when you open SSMS to investigate, the connection spinner just keeps spinning. SQL Server is alive; you can see the process running, but it’s too overwhelmed to let you in. You need to get in there and kill something, but you can’t get a connection to do it. This is exactly the scenario the Dedicated Admin Connection DAC was built for. And if you haven’t set it up yet, now is the time. Because when you need it, you really need it.
Because there is a preparatory step, it’s important to run that while the instance is in a healthy state. That way, it’ll be available to you when the instance is at the edge of failure.