Monday, October 4, 2010

What I learned today: No backup or restore with the DAC

For everything I teach, I learn something in return.

I began to train a new junior level DBA today. He’s someone that works with us already as a developer but he’s shown an interest in learning how to be a DBA so I’m beginning to teach him what I know. To start off we began by discussing his daily tasks, then we started to talk about backup and recovery, topics I gave him as an assignment last week. For this week’s homework I told him to read up on the Dedicated Administrator Connection (DAC) and to get familiar with SQLCMD. I mentioned how the DAC was useful in situations where the server was under a strain and you need a way to connect to troubleshoot the problem. Then I said you can use it for backups and recoveries.

I should have quit when I was ahead.

As my new colleague correctly pointed out, you can’t use the DAC for backups or recovery. If you try, you’ll get the following error:

Msg 3637, Level 16, State 3, Line 1
A parallel operation cannot be started from a DAC connection.
Msg 3013, Level 16, State 1, Line 1
BACKUP DATABASE is terminating abnormally.

And BOL can’t make it any clearer:

SQL Server prohibits running parallel queries or commands with the DAC. For example, error 3637 is generated if you execute either of the following statements with the DAC:



For some reason I’d forgotten that. It was something that I had read a long time ago and forgot since I don’t use the DAC very often, never in production other than to confirm that it was set up through the firewall. I had assumed that backing up a database in an emergency would be one of the reasons I’d use the DAC. Luckily I was reminded before a situation arose where I relied on it.

So I actually learned two things today. I was reminded about the restriction of the DAC, and I learned to be more careful about what I tell others when it comes to SQL, not to assume facts and to do my own homework. I hope I remember both lessons equally.