Tag: SQL Server Concept

Facts about Dedicated Administrator Connection (DAC)

Have you ever thought how Dedicated Administrator Connection (DAC) allow you to connect to SQL Server even when it cannot accept standard connection requests?
DAC is a special type of diagnostic connection, introduced by Microsoft as a new feature in SQL Server 2005.
SQL Server reserves a minimum amount of memory and a private scheduler using which it allows you to connect in situations when you need it.
You need to use a login which is a member of the SQL Server sysadmin role to use DAC and for obvious reasons only one such session is allowed at any point of time.

By default DAC is enabled locally except in SQL Express edition. That means if you are able to RDP to the server and want to use DAC to connect to the local SQL Instance, you will be able to do so even when DAC is not enabled explicitly.
Go ahead and watch this video showing what I am talking about:

In SQL Express you need to add “-T7806” to the startup parameters and restart SQL Services. Also SQL Browser Service should be running.

Then why Microsoft has provided the option to enable DAC? In situations OS can be completely exhausted of resources to handle any further user connections which prevents using DAC locally.
That is why you have the configuration settings “remote admin connections” to allow DAC from a remote machine.

There are two ways to enable DAC:
  1. Using Query
  2. GUI (Management Studio)

1. Using Query to set the configuration  settings:

EXEC sp_configure 'remote admin connections', 1
GO
RECONFIGURE
GO

The message says “Run the RECONFIGURE statement to install”. This is little misleading as we have run that already. No need to run it again.

You can verify by executing the following query:

EXEC sp_configure 'remote admin connections'

As you can see in the above screen shot, both config_value and run_value has been set to 1. Otherwise run_value would have been ‘0’.

2. Using Management Studio:

Right Click on SQL Server Instance > Facets as shown below.

Choose Surface Area Configuration and make “RemoteDacEnabled” to True.

Now your SQL Instance is ready to allow DAC from remote machine.

There are two ways you can connect to SQL Instance using DAC:
    1. Command Prompt
    2. GUI (SQL Server Management Studio)

1. Command prompt:

Commands to connect using DAC:

a. To use SQL Authentication
sqlcmd -S [ServerName] -U [ltloginname] -P [Password] –A

b. To use Windows Authentication which will use the login id you are connected with:
sqlcmd –S [ServerName] -A

Examples:

To connect to default SQL instance using windows authentication:
sqlcmd -S (local) –A

Or you can just use a (.) dot:

To connect named instance using SQL Authentication:

Note: In the above example the last letter is in lower case (-p), to prompt for password. If you want to mention the password in the command line itself then if is upper case (-P).

2. GUI (SQL Server Management Studio):

Go to File > New > Database Engine Query as shown below

Alternatively you can click on the icon as shown below:

This is important to use Database Engine Query, otherwise you will get error as explained later in this article.

Type Admin:SQLInstanceName for the Server Name field name as shown below:

You can use either SQL Authentication or Windows Authentication.

If you do not pay attention to use “Database Engine Query” you will get the below error message:

In this article you have learned what DAC is and how it allows the special connection, different ways to enable DAC and different ways to connect.

I would love to hear from you if you have any questions and yes I hope you have gone through the topic of Myth around Remote Query Timeout Option

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Remote Query Timeout Option – Common SQL Server Misconception

I have seen whenever customer complains about Query Time Out issue, there is a tendency to play around with this settings. This does not work the way you might think it does.

I would like to quote from TechNet:

https://technet.microsoft.com/en-us/library/ms177457(v=sql.105).aspx

Use the remote query timeout option to specify how long, in seconds, a remote operation can take before Microsoft SQL Server times out. The default is 600, which allows a 10-minute wait. This value applies to an outgoing connection initiated by the Database Engine as a remote query. This value has no effect on queries received by the Database Engine.

It clearly says, this applies to only outgoing connection and does not have any effect on incoming queries. The misunderstood part here is, what is outgoing and what is incoming query for SQL Server. This should get clarified once you read through this article.

Here is my LAB setup for the Demo:

As you can see in the above screen shot, I have set the remote query timeout setting for the default instance to 5 sec(s).

You can execute the below query to set ‘Remote Query Timeout’ settings:

sp_configure 'Remote Query Timeout', 5
GO
RECONFIGURE
GO

Kept the default settings for the Linked Server SQL Instance (SQL_Named). i.e. 600 Sec(s)

Now if I run a remote query to linked server that would be an outgoing query for the default instance and incoming for Linked Server.

In Linked server I am keeping an open transaction so that the lock on the table [AdventureWorks2014].[Production].[Product] does not get released.

Initiate a remote query from the default instance (Outgoing Query).

If you can see that the query got timed out exactly after 5 sec(s). So, it proves that the remote query time out settings applies for remote query (i.e. for outgoing query).

Now lets see how it works for incoming query. Keep and open transaction on the same table. This time executing it in the default instance.

Open a different session for the default instance and run select on the same table. Both the queries are incoming for the default instance.

This time query is still waiting even though it crossed 12 sec(s) without getting timed out. This was an incoming query for the database engine of default instance and hence Remote Query Timeout Settings (which is set to 5 sec(s)) did not apply.

I hope this clarifies how exactly “Remote Query Timeout” option works.

More over if you like video training check the demo here:

You may also like to spend couple of minutes to check the below articles:

how to delete tempdb data file without restarting SQL Server.
SQL Database Restore using NetBackup
SSRS Encryption Key Backup Information
User Defined Profiler Template Location
Remove or delete Tempdb data file without restarting SQL Server
Fact about Dedicated Administrator Connection (DAC)

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