Trace Flags (Transact-SQL)
Trace flags are used to temporarily set specific server characteristics or to switch off a particular behavior. For example, if trace flag 3205 is set when an instance of SQL Server starts, hardware compression for tape drivers is disabled. Trace flags are frequently used to diagnose performance issues or to debug stored procedures or complex computer systems.
The following table lists and describes the trace flags that are available in SQL Server.
Note
Trace flag behavior may not be supported in future releases of SQL Server.
Trace flag |
Description |
|---|---|
260 |
Prints versioning information about extended stored procedure dynamic-link libraries (DLLs). For more information about __GetXpVersion(), see Creating Extended Stored Procedures. Scope: global or session |
1204 |
Returns the resources and types of locks participating in a deadlock and also the current command affected. Scope: global only |
1211 |
Disables lock escalation based on memory pressure, or based on number of locks. The SQL Server Database Engine will not escalate row or page locks to table locks. Using this trace flag can generate excessive numbers of locks. This can slow the performance of the Database Engine, or cause 1204 errors (unable to allocate lock resource) because of insufficient memory. For more information, see Lock Escalation (Database Engine). If both trace flag 1211 and 1224 are set, 1211 takes precedence over 1224. However, because trace flag 1211 prevents escalation in every case, even under memory pressure, we recommend that you use 1224. This helps avoid "out-of-locks" errors when many locks are being used. Scope: global or session |
1222 |
Returns the resources and types of locks that are participating in a deadlock and also the current command affected, in an XML format that does not comply with any XSD schema. Scope:global only |
1224 |
Disables lock escalation based on the number of locks. However, memory pressure can still activate lock escalation. The Database Engine escalates row or page locks to table (or partition) locks if the amount of memory used by lock objects exceeds one of the following conditions:
If both trace flag 1211 and 1224 are set, 1211 takes precedence over 1224. However, because trace flag 1211 prevents escalation in every case, even under memory pressure, we recommend that you use 1224. This helps avoid "out-of-locks" errors when many locks are being used. .gif "Note") Note
Lock escalation to the table- or HoBT-level granularity can also be controlled by using the LOCK_ESCALATION option of the ALTER TABLE statement.
Scope: global or session |
2528 |
Disables parallel checking of objects by DBCC CHECKDB, DBCC CHECKFILEGROUP, and DBCC CHECKTABLE. By default, the degree of parallelism is automatically determined by the query processor. The maximum degree of parallelism is configured just like that of parallel queries. For more information, see max degree of parallelism Option. Parallel DBCC should typically be left enabled. For DBCC CHECKDB, the query processor reevaluates and automatically adjusts parallelism with each table or batch of tables checked. Sometimes, checking may start when the server is almost idle. An administrator who knows that the load will increase before checking is complete may want to manually decrease or disable parallelism. Disabling parallel checking of DBCC can cause DBCC to take much longer to complete and if DBCC is run with the TABLOCK feature enabled and parallelism set off, tables may be locked for longer periods of time. Scope: global or session |
3205 |
By default, if a tape drive supports hardware compression, either the DUMP or BACKUP statement uses it. With this trace flag, you can disable hardware compression for tape drivers. This is useful when you want to exchange tapes with other sites or tape drives that do not support compression. Scope: global or session |
3226 |
By default, every successful backup operation adds an entry in the SQL Server error log and in the system event log. If you create very frequent log backups, these success messages accumulate quickly, resulting in huge error logs in which finding other messages is problematic. With this trace flag, you can suppress these log entries. This is useful if you are running frequent log backups and if none of your scripts depend on those entries. |
3608 |
Prevents SQL Server from automatically starting and recovering any database except the master database. Databases will be started and recovered when accessed. Some features, such as snapshot isolation and read committed snapshot, might not work. Use for Moving System Databases and Moving User Databases. Do not use during normal operation. |
3625 |
Limits the amount of information returned in error messages. For more information, see Metadata Visibility Configuration. Scope: global only |
4616 |
Makes server-level metadata visible to application roles. In SQL Server, an application role cannot access metadata outside its own database because application roles are not associated with a server-level principal. This is a change of behavior from earlier versions of SQL Server. Setting this global flag disables the new restrictions, and allows for application roles to access server-level metadata. Scope: global only |
6527 |
Disables generation of a memory dump on the first occurrence of an out-of-memory exception in CLR integration. By default, SQL Server generates a small memory dump on the first occurrence of an out-of-memory exception in the CLR. The behavior of the trace flag is as follows:
Scope: global only |
7806 |
Enables a dedicated administrator connection (DAC) on SQL Server Express. By default, no DAC resources are reserved on SQL Server Express. For more information, see Using a Dedicated Administrator Connection. Scope: global only |
Remarks
In SQL Server, there are two types of trace flags: session and global. Session trace flags are active for a connection and are visible only to that connection. Global trace flags are set at the server level and are visible to every connection on the server. Some flags can only be enabled as global, and some can be enabled at either global or session scope.
The following rules apply:
A global trace flag must be enabled globally. Otherwise, the trace flag has no effect. We recommend that you enable global trace flags at startup, by using the -T command line option.
If a trace flag has either global or session scope, it can be enabled with the appropriate scope. A trace flag that is enabled at the session level never affects another session, and the effect of the trace flag is lost when the SPID that opened the session logs out.
Trace flags are set on or off by using either of the following methods:
Using the DBCC TRACEON and DBCC TRACEOFF commands.
For example, DBCC TRACEON 2528: To enable the trace flag globally, use DBCC TRACEON with the -1 argument: DBCC TRACEON (2528, -1). To turn off a global trace flag, use DBCC TRACEOFF with the -1 argument.
Using the -T startup option to specify that the trace flag be set on during startup.
The -T startup option enables a trace flag globally. You cannot enable a session-level trace flag by using a startup option. For more information about startup options, see Using the SQL Server Service Startup Options.
Use the DBCC TRACESTATUS command to determine which trace flags are currently active.
Behavior Changes
In SQL Server 2000, a simple DBCC TRACEON (1204) is enough to enable deadlock reporting to the error log. In SQL Server 2008, you must enable the flag globally because the session-level flag is not visible to the deadlock monitor thread.
For more information about changes in behavior, see Breaking Changes to Database Engine Features in SQL Server 2008.
Examples
The following example sets trace flag 3205 on by using DBCC TRACEON.
DBCC TRACEON (3205,-1)