The Locks, Blocks, and Deadlocks of SQL Server: Unraveling the Knots

by Dec 7, 2023


This blog post was authored by Pinal Dave from SQLAuthority.


Concurrency control is a fundamental part of any database management system, and SQL Server is no exception. However, it often leads to three common phenomena – locks, blocks, and deadlocks. These phenomena can significantly impact performance, affecting transactions and potentially causing slowdowns in business operations. In this post, we’ll delve into these issues, uncover native solutions for identification and resolution, and highlight how SQL Diagnostic Manager (SQL DM) offers a more comprehensive real-time and historical session analysis for easy problem detection and resolution.

Understanding Locks, Blocks, and Deadlocks

Locks are essential for maintaining the integrity of data while allowing concurrent access to the database. However, inappropriate lock usage can lead to blocking, where one process holds a lock and prevents other processes from proceeding. This can slow down operations significantly, for example, a long-running transaction holding a lock can delay other transactions, impacting business operations like report generation, order processing, etc.

A deadlock is a more severe form of blocking where two or more processes are each waiting for the other to release a resource, creating an endless loop that can’t be resolved without external intervention. Deadlocks can halt operations, causing significant business impact. For instance, if a deadlock occurs during an e-commerce transaction, it could result in order failure, leading to customer dissatisfaction and potential loss of sale.

Built-in SQL Solutions

SQL Server offers several native tools and techniques to identify and resolve locks, blocks, and deadlocks.

Identifying Locks with sp_lock

To identify locks, you can use the ‘sp_lock’ system stored procedure. This procedure shows all the locks currently held in the system and the processes that are requesting them. Here is a simple example:
EXEC sp_lock;
This command will return a table that includes information like the process ID, the type of lock, the object the lock is on, and more. This can be particularly useful when you’re experiencing performance issues and suspect locking might be the issue.

Detecting Blocks with sp_who2

To detect blocks, you can use the ‘sp_who2’ stored procedure. It returns information about current users, sessions, and processes, including any that are blocked or are causing a block. Here’s an example:

EXEC sp_who2 ‘active’;

This command will return a list of active sessions, along with information like the session ID, status, login, and more. By looking at the BlkBy column, you can see which sessions are being blocked by others.

Resolving Deadlocks with Deadlock Graph

SQL Server provides a Deadlock Graph via SQL Server Profiler, which can help you identify and resolve deadlocks. It captures a graphical representation of processes involved in a deadlock and the resources they’re waiting for.

To access the Deadlock Graph, you need to launch SQL Server Profiler, start a new trace, and select the Deadlock graph event in the Locks event category. When a deadlock occurs, a new row will be added to the trace, and you can view the details of the deadlock in the TextData column.

SQL DM: Enhancing Session Analysis

While SQL Server’s native tools are helpful, SQL Diagnostic Manager takes it a step further by providing a more comprehensive, user-friendly solution.

SQL DM provides real-time session monitoring. It displays an easy-to-understand graphical view of active user sessions, making it easier to spot and diagnose problems related to locks and blocks.

With SQL DM, you can also perform historical analysis. It captures and stores session data over time, allowing you to analyze past events, identify patterns, and proactively address potential issues before they become critical.

Moreover, SQL DM offers superior deadlock monitoring. It captures deadlock data and deciphers it into a more readable format, making it easier to understand and resolve deadlocks. It even notifies you in real time when a deadlock occurs, ensuring you can take immediate action.


The phenomena of locks, blocks, and deadlocks are common in SQL Server. They can significantly impact your SQL Server’s performance and, if not promptly identified and resolved, can disrupt business operations. While SQL Server’s native tools provide a starting point for addressing these issues, SQL Diagnostic Manager’s advanced real-time and historical session analysis capabilities ensure you can navigate these complexities efficiently. With SQL DM, you can ensure the optimal performance of your SQL Server, making lock, block, and deadlock issues a thing of the past.

Experience for yourself how SQL DM can help you unravel the knots of SQL Server locks, blocks, and deadlocks by downloading a free, 14-day trial (a credit card is not required).

About the Author

Pinal Dave is an SQL Server Performance Tuning Expert and independent consultant with over 21 years of hands-on experience. He holds a Masters of Science degree and numerous database certifications. Pinal has authored 13 SQL Server database books and 53 Pluralsight courses. To freely share his knowledge and help others build their expertise, Pinal has also written more than 5,600 database tech articles on his blog at SQLAuthority.