Wait Type LCK_M_SCH_M_LOW_PRIORITY

Wait statistics, in the context of SQL Server, refer to the amount of time that a query spends waiting to access data in the database. When a client application requests data from the database, the request is placed in a queue and the client application must wait for its turn to access the data. The time that the query spends waiting is called a "wait" and is tracked by SQL Server. This information can be used to identify potential performance bottlenecks and optimize the performance of the database. Wait statistics are commonly used by database administrators to diagnose and troubleshoot performance issues in SQL Server.

SQL Server uses wait types to help administrators identify where processes might be delayed. One such wait type, LCK_M_SCH_M_LOW_PRIORITY, occurs in relation to locks, specifically when a transaction is waiting for a schema modification (SCH-M) lock in low-priority mode. This lock is typically used when making changes to the structure of a database object, such as altering a table or index. Let’s explore what this wait type means, when it happens, and how to address it effectively.

What Is the LCK_M_SCH_M_LOW_PRIORITY Wait Type?

The LCK_M_SCH_M_LOW_PRIORITY wait type appears when a process is trying to acquire a schema modification lock but is running in low-priority mode. The low-priority mode allows the transaction to wait for the lock without interrupting higher-priority operations. This is often used for operations like online index builds or schema modifications to minimize the impact on other running queries.

Instead of forcing other queries to stop, the low-priority approach ensures that these changes wait their turn, avoiding unnecessary disruptions.

When Does LCK_M_SCH_M_LOW_PRIORITY Appear?

Some common scenarios where this wait type occurs include:

• Online Index Operations – When rebuilding or creating indexes with the ONLINE=ON and WAIT_AT_LOW_PRIORITY options.
• Schema Modifications – During operations that alter the structure of database objects, such as changing table schemas.
• High-Concurrency Environments – When many queries are accessing the same object, causing lock contention.

Why LCK_M_SCH_M_LOW_PRIORITY Waits Matter

While this wait type is expected in certain operations, excessive or prolonged waits can indicate performance bottlenecks. Common causes include:

• Blocking Transactions – Active queries holding locks on the target object can delay low-priority operations.
• Large Workloads – Heavy traffic on the database object increases contention for schema modification locks.
• Long-Running Transactions – Queries or updates that take a long time to complete may block schema modifications.
• Resource Constraints – Insufficient CPU, memory, or disk resources can exacerbate wait times.

How to Address LCK_M_SCH_M_LOW_PRIORITY Waits

If LCK_M_SCH_M_LOW_PRIORITY waits are affecting performance, try these strategies:

• Optimize Queries – Ensure queries accessing the target object are efficient and do not hold locks longer than necessary.
• Schedule Schema Modifications – Perform schema changes during periods of low database activity to reduce contention.
• Reduce Transaction Scope – Limit the number of rows or objects affected by transactions to minimize lock contention.
• Monitor Blocking Processes – Use tools like sys.dm_tran_locks or Activity Monitor to identify and address blocking queries.
• Increase Resources – Allocate additional CPU, memory, or disk resources if the system is under heavy load.

Monitoring LCK_M_SCH_M_LOW_PRIORITY Waits

To monitor this wait type, use SQL Server’s sys.dm_os_wait_stats to track occurrences and durations. Additionally, tools like sys.dm_exec_requests can help identify current processes experiencing these waits. For a broader view, tools like Database Health Monitor provide detailed insights into wait statistics and lock contention, making it easier to manage performance and resolve issues.

Stedman Solutions offers expert SQL Server performance tuning and managed services to help you optimize your database environment. Whether you need assistance addressing wait types like LCK_M_SCH_M_LOW_PRIORITY, improving query performance, or managing database workloads, we’re here to help. Our Database Health Monitor tool provides real-time analysis of wait types and system health, ensuring your SQL Server environment runs efficiently. Contact us today to learn how we can support your SQL Server needs!

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Applies to

• SQL Server 2019
• SQL Server 2022

Related Waits

See Also

• More About LCK_M_SCH_M_LOW_PRIORITY