Wait Type LCK_M_RS_U_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.

In SQL Server, locks are essential for maintaining data consistency and integrity in multi-user environments. However, when multiple transactions try to access the same data simultaneously, some may need to wait for a lock to be released. One such wait type is LCK_M_RS_U_LOW_PRIORITY, which indicates that a process is waiting for a shared lock with an update intent (RS-U) to be granted, but it is doing so at a low-priority level. In this post, we’ll explore what this wait type means, why it occurs, and how to address it.

What Is the LCK_M_RS_U_LOW_PRIORITY Wait Type?

The LCK_M_RS_U_LOW_PRIORITY wait type appears when a transaction is attempting to acquire a low-priority shared lock with update intent (RS-U). This type of lock is commonly used in operations such as ALTER TABLE or partition switching, where SQL Server needs to ensure that the data being updated remains consistent while still allowing other queries to read the data.

The "low priority" designation means that the transaction is waiting for the lock without escalating contention or blocking higher-priority operations. This is often used to minimize the impact on other active queries, especially in systems with high concurrency. However, if the requested lock is held by other processes for an extended period, the transaction may experience longer wait times.

When Does LCK_M_RS_U_LOW_PRIORITY Appear?

This wait type typically occurs during operations that use low-priority locking to avoid significant disruptions to other queries. Common scenarios include:

• Partition Switching – When switching partitions in a table, SQL Server uses RS-U locks to ensure consistency without fully blocking access to the data.
• Schema Modification – Operations like ALTER TABLE or modifying an index may involve this lock to maintain consistency while allowing reads.
• High Concurrency Workloads – In environments with many concurrent queries, low-priority locking can be used to reduce contention and improve overall throughput.

While the low-priority approach minimizes contention, it can also lead to longer wait times if higher-priority transactions are holding the lock.

Why LCK_M_RS_U_LOW_PRIORITY Waits Matter

Although the LCK_M_RS_U_LOW_PRIORITY wait type helps maintain smooth operation in busy systems, excessive waits may indicate contention or blocking issues. Prolonged waits can delay schema changes or partition operations, potentially impacting maintenance tasks or batch jobs. Common causes of high LCK_M_RS_U_LOW_PRIORITY waits include:

• Long-Running Queries – If a high-priority transaction holds a lock for an extended period, low-priority requests may wait longer to acquire the lock.
• High Concurrency – Systems with many simultaneous read and write operations can lead to more contention, increasing wait times.
• Resource Bottlenecks – Slow disk or memory performance may exacerbate contention and extend lock wait times.

How to Address LCK_M_RS_U_LOW_PRIORITY Waits

If LCK_M_RS_U_LOW_PRIORITY waits are impacting your SQL Server environment, consider these strategies to mitigate them:

• Optimize Queries – Review and tune long-running queries that may be holding locks for extended periods, as these can delay low-priority lock requests.
• Schedule Maintenance During Off-Peak Hours – Operations that use RS-U locks, such as partition switching or schema changes, should be scheduled during times of low activity to minimize contention.
• Monitor Blocking – Use SQL Server tools to identify and address blocking chains that may be causing delays for low-priority locks.
• Partition Large Tables – Partitioning tables can reduce contention by isolating operations to specific partitions, improving overall performance.
• Increase Resource Availability – Ensuring sufficient CPU, memory, and disk performance can help reduce contention and speed up locking operations.

Conclusion

The LCK_M_RS_U_LOW_PRIORITY wait type in SQL Server indicates that a transaction is waiting for a shared lock with update intent at a low priority. While this approach helps minimize the impact on other queries, excessive waits can delay schema modifications or partition operations. By optimizing queries, scheduling maintenance tasks appropriately, and monitoring blocking, you can reduce LCK_M_RS_U_LOW_PRIORITY waits and improve SQL Server performance in high-concurrency environments.

For expert help with SQL Server performance tuning, locking strategies, or query optimization, Stedman Solutions offers managed services to ensure your SQL Server environment runs efficiently and reliably.

Watch on YouTube

Find out more about our SQL Server Managed Services

Applies to

Related Waits

See Also

• More About LCK_M_RS_U_LOW_PRIORITY