A transaction is waiting to acquire a schema modification lock. This lock prevents access to a table while a table data definition language (DDL) operation takes place. These operations include dropping tables and adding or deleting columns. It’s also used for certain data manipulation language operations, such as truncating a table. All other operations are blocked until the lock is released. 

The SQL Server Database Engine uses locking to manage the way multiple users access the same data at the same time. Every transaction must first request a lock on the resource it wants to modify. This lock stops other transactions from modifying the same resource in a way that might cause conflict. The lock is only released once the transaction no longer needs it. For more information, including details of lock type compatibility, see: SQL Server Transaction Locking and Row Versioning Guide (TechNet).

Lock waits commonly occur on busy servers where concurrent transactions demand the same resource, resulting in poor performance. A high number of locking waits may indicate blocking problems and should be investigated.

Investigating

• Break long transactions down into shorter ones. See: Managing Long-Running Transactions (TechNet).

• Check the isolation levels for your transactions and update the locking and row versioning behavior if necessary. See: Set Transaction Isolation Level (TechNet).

• Check whether lock escalation is causing blocking problems and resolve if necessary. The Average wait time tells you whether you’re suffering from many short blocks or several long blocks. To troubleshoot blocking, see: How to resolve blocking problems that are caused by lock escalation in SQL Server (Microsoft Support). 

• Check the affected queries and the Top queries table. Tune queries so they run faster and require fewer locks. See: Query tuning (TechNet).

• On the Analysis page, check these metrics for additional details on locking behavior:

• Lock timeouts/sec

• Lock timeouts/sec

• Avg. lock wait time

• On the Analysis page, check these metrics to see whether memory problems or I/O bottlenecks are causing locks to be held for longer than usual:

• Machine: memory used

• Memory pages/sec

• Disk avg. read time

• Disk avg. write time

• Buffer cache hit ratio

• Buffer page life expectancy

See also: Investigating I/O bottlenecks.

• Check the sys.dm_trans_locks DMV for resources associated with lock requests. See: sys.dm_tran_locks (TechNet).

• Consider using partitions to split single lock resource into multiple resources (only available if you’re using more than 16 CPUs). See: http://technet.microsoft.com/en-us/library/ms187504(v=sql.105).aspx.