Last week, I wrote a blog post about a trace flag that can help resolve backup failures in SQL Server. In the comments section, a reader asked whether the trace flag needs to be enabled every time SQL Server is restarted. This question reminded me that not everyone is familiar with trace flags and how they work. So, in this blog post, I will cover the basics of trace flags in SQL Server.
What is a Trace Flag?
A trace flag in SQL Server is a configuration option that allows you to change certain behaviors of the server. You can think of trace flags as “IF” conditions in SQL Server. When a trace flag is enabled, it triggers a specific behavior or action within the server.
For example, one commonly used trace flag is 1222, which is used for printing deadlock graphs in the error log. When this trace flag is enabled, SQL Server will print the deadlock graph whenever a deadlock occurs.
Enabling Trace Flags
There are two ways to enable trace flags in SQL Server:
- Startup Parameter: You can add the trace flag as a startup parameter in the SQL Server Configuration Manager. For versions prior to SQL Server 2012, you need to add “;-T” (semicolon hyphen capital T) before the trace flag number.
- DBCC TRACEON Statement: You can use the DBCC TRACEON statement to enable trace flags. Many trace flags can be enabled and disabled using this statement. For example, to enable trace flag 1222, you can use the following command:
DBCC TRACEON(1222, -1). The second parameter defines the scope of the trace flag. If the value is 0 or not supplied, the trace flag is enabled only for the session where the query was run. If the value is specified as -1, the trace flag is enabled for all sessions on the SQL instance.
Checking Enabled Trace Flags
You can use the DBCC TRACESTATUS command to check which trace flags are currently enabled in SQL Server. This command will provide you with a list of enabled trace flags and their corresponding status.
Disabling Trace Flags
If you want to disable a trace flag that is currently enabled, you can use the DBCC TRACEOFF command. For example, to disable trace flag 1222, you can use the following command: DBCC TRACEOFF(1222, -1).
Conclusion
Trace flags are a powerful tool in SQL Server that allow you to modify the behavior of the server. They can be enabled as startup parameters or using the DBCC TRACEON statement. You can check which trace flags are enabled using the DBCC TRACESTATUS command, and disable them using the DBCC TRACEOFF command.
Which other trace flags have you used for production purposes? Please leave a comment below, and I will compile the answers and create a separate blog post.