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SQL backup fails with warning "A parallel operation cannot be started from a DAC connection."

Article ID: 129
Last updated: 08 Jun, 2012

Symptom

Using the SQL plug-in the following warnings are seen in the backup log and fails to dump the databases:

Message: 04:02:06 [MSSQL Server Plugin] Exporting database tempdb
Warning: 04:02:06 [MSSQL Server Plugin] Unable to export database tempdb: A parallel operation cannot be started from a DAC connection.
Message: 04:02:06 [MSSQL Server Plugin] Exporting database model
Warning: 04:02:06 [MSSQL Server Plugin] Unable to export database model: A parallel operation cannot be started from a DAC connection.

Cause

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. Because the DAC exists solely for diagnosing server problems in rare circumstances, there are some restrictions on the connection:

  • 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:
    • RESTORE
    • BACKUP

(Ref: http://technet.microsoft.com/en-us/library/ms189595.aspx)

Solution

On a default setup, SQL connects to port 1433 and the SQL server will automatically assign port 1434 for the DAC.
The quickest way to determine which port is used for the DAC, is to inspect the SQL Error Logs:

Server is listening on [ 'any' <ipv4> 1433]. This is the port that will be used by users and the plug-in.
Server local connection provider is ready to accept connection on [ \\.\pipe\SQLLocal\MSSQLSERVER ].

Server named pipe provider is ready to accept connection on [ \\.\pipe\sql\query ].

Server is listening on [ 127.0.0.1 <ipv4> 1434].

Dedicated admin connection support was established for listening locally on port 1434.
This is the DAC port that does not support Restore/Backup commands

SQL Server is now ready for client connections. This is an informational message; no use

Once you have determined the correct port as shown above (1433), open the SQL plug-in and supply the correct port.
Make sure you can view and select the databases in the plugin. You are now ready to backup.

Article ID: 129
Last updated: 08 Jun, 2012
Revision: 1
Views: 3586
Comments: 0
Posted: 01 Mar, 2012 by Van Rensburg J.
Updated: 08 Jun, 2012 by Van Rensburg J.
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