System resource consumption

Oct 13, 2025 by wbrum.org 80 views

Introduction

As a database administrator, you're likely no stranger to the importance of regular index maintenance. However, when it comes to SQL Server 2022, there's a peculiar issue that can arise during index rebuilds. This article delves into the world of HADR_SYNC_COMMIT sessions and "locking" queries, exploring the root causes and potential solutions to this frustrating problem.

Understanding HADR_SYNC_COMMIT Sessions

Before we dive into the nitty-gritty, let's take a step back and understand what HADR_SYNC_COMMIT sessions are. In SQL Server 2022, HADR (High Availability and Disaster Recovery) is a feature that enables database mirroring, log shipping, and other high-availability scenarios. When you run an index rebuild operation, the database engine creates a HADR_SYNC_COMMIT session to ensure that the changes are committed to the database log. This session is responsible for synchronizing the database log with the transaction log.

The Problem: Index Rebuilds and HADR_SYNC_COMMIT Sessions

Now, let's get to the crux of the issue. When you run an index rebuild job, it can lead to a surge in HADR_SYNC_COMMIT sessions. These sessions can consume a significant amount of system resources, causing other sessions from applications to hang or wait indefinitely. This can result in a "locking" scenario, where all other sessions are blocked, waiting for the HADR_SYNC_COMMIT sessions to complete.

Causes of the Issue

So, what's behind this peculiar behavior? There are several potential causes:

Index rebuild operations: As mentioned earlier, index rebuilds can lead to a high number of HADR_SYNC_COMMIT sessions.
Database mirroring: If you're using database mirroring, the HADR_SYNC_COMMIT sessions can be triggered by the mirroring process.
Log shipping: Log shipping can also cause HADR_SYNC_COMMIT sessions to be created.
System resources: If the system is under heavy load, the HADR_SYNC_COMMIT sessions can consume more resources, leading to a "locking" scenario.

Symptoms of the Issue

So, how do you know if you're experiencing this issue? Here are some common symptoms:

HADR_SYNC_COMMIT sessions: Check the SQL Server Activity Monitor or the sys.dm_exec_sessions dynamic management view to see if there are a high number of HADR_SYNC_COMMIT sessions.
"Locking" queries: If other sessions are hanging or waiting indefinitely, it's likely due to the HADR_SYNC_COMMIT sessions.
System resource consumption: If the system is under heavy load, it could be a sign that the HADR_SYNC_COMMIT sessions are consuming too many resources.

Solutions to the Issue

Now that we've identified the causes and symptoms, let's explore some potential solutions:

Run index rebuilds during maintenance windows: Schedule index rebuilds during maintenance windows when the system is under less load.
Use online index rebuilds: Online index rebuilds can reduce the impact on the system, as they don't require a table lock.
Adjust the HADR_SYNC_COMMIT threshold: You can adjust the HADR_SYNC_COMMIT threshold to reduce the number of sessions created.
Monitor system resources: Keep a close eye on system resources to ensure that the HADR_SYNC_COMMIT sessions aren't consuming too much.
Consider disabling HADR_SYNC_COMMIT: If you're not using database mirroring or log shipping, you can consider disabling HADR_SYNC_COMMIT to reduce the number of sessions created.

Best Practices for Index Maintenance

To avoid this issue in the future, follow these best practices for index maintenance:

Schedule index rebuilds regularly: Regular index rebuilds can help prevent fragmentation and reduce the impact on the system.
Use online index rebuilds: Online index rebuilds can reduce the impact on the system, as they don't require a table lock.
Monitor system resources: Keep a close eye on system resources to ensure that index rebuilds aren't consuming too much.
Adjust the HADR_SYNC_COMMIT threshold: Adjust the HADR_SYNC_COMMIT threshold to reduce the number of sessions created.

Conclusion

In conclusion, the issue of index rebuilds causing HADR_SYNC_COMMIT sessions and "locking" queries is a complex one. By understanding the causes and symptoms, you can take steps to mitigate the issue and ensure that your database remains healthy and performant. Remember to follow best practices for index maintenance, and don't hesitate to reach out to Microsoft support if you need further assistance.

Additional Resources

For more information on HADR_SYNC_COMMIT sessions and index rebuilds, check out the following resources:

Microsoft Documentation: HADR_SYNC_COMMIT
Microsoft Documentation: Index Rebuild
Microsoft Support: HADR_SYNC_COMMIT Sessions
Index Rebuild Causing HADR_SYNC_COMMIT and "Locking" Queries: A SQL Server 2022 Q&A =====================================================================================

Q&A Introduction

In our previous article, we explored the issue of index rebuilds causing HADR_SYNC_COMMIT sessions and "locking" queries in SQL Server 2022. In this article, we'll delve into a Q&A format to provide more insights and answers to common questions related to this issue.

Q1: What is the purpose of HADR_SYNC_COMMIT sessions?

A1: HADR_SYNC_COMMIT sessions are created to ensure that changes are committed to the database log. This session is responsible for synchronizing the database log with the transaction log.

Q2: Why do index rebuilds cause HADR_SYNC_COMMIT sessions?

A2: Index rebuilds can lead to a high number of HADR_SYNC_COMMIT sessions because they require the database engine to create a new transaction log and commit the changes.

Q3: What are the symptoms of this issue?

A3: The symptoms of this issue include:

HADR_SYNC_COMMIT sessions: Check the SQL Server Activity Monitor or the sys.dm_exec_sessions dynamic management view to see if there are a high number of HADR_SYNC_COMMIT sessions.
"Locking" queries: If other sessions are hanging or waiting indefinitely, it's likely due to the HADR_SYNC_COMMIT sessions.
System resource consumption: If the system is under heavy load, it could be a sign that the HADR_SYNC_COMMIT sessions are consuming too many resources.

Q4: How can I prevent this issue?

A4: To prevent this issue, follow these best practices:

Schedule index rebuilds regularly: Regular index rebuilds can help prevent fragmentation and reduce the impact on the system.
Use online index rebuilds: Online index rebuilds can reduce the impact on the system, as they don't require a table lock.
Monitor system resources: Keep a close eye on system resources to ensure that index rebuilds aren't consuming too much.
Adjust the HADR_SYNC_COMMIT threshold: Adjust the HADR_SYNC_COMMIT threshold to reduce the number of sessions created.

Q5: Can I disable HADR_SYNC_COMMIT?

A5: Yes, you can disable HADR_SYNC_COMMIT if you're not using database mirroring or log shipping. However, keep in mind that disabling HADR_SYNC_COMMIT may impact the availability of your database.

Q6: What are some common mistakes that can lead to this issue?

A6: Some common mistakes that can lead to this issue include:

Running index rebuilds during peak hours: Running index rebuilds during peak hours can lead to a high number of HADR_SYNC_COMMIT sessions.
Not monitoring system resources: Not monitoring system resources can lead to a "locking" scenario.
Not adjusting the HADR_SYNC_COMMIT threshold: Not adjusting the HADR_SYNC_COMMIT threshold can lead to a high number of sessions created.

Q7: How can I troubleshoot this issue?

A7: To troubleshoot this issue, follow these steps:

Check the SQL Server Activity Monitor or the sys.dm_exec_sessions dynamic management view to see if there are a high number of HADR_SYNC_COMMIT sessions.
Check the system resource consumption to see if it's impacting the system.
Adjust the HADR_SYNC_COMMIT threshold to reduce the number of sessions created.

Q8: Can I use SQL Server 2022 features to mitigate this issue?

A8: Yes, you can use SQL Server 2022 features to mitigate this issue. Some features that can help include:

Online index rebuilds: Online index rebuilds can reduce the impact on the system, as they don't require a table lock.
Partitioned tables: Partitioned tables can help reduce the impact of index rebuilds on the system.
Columnstore indexes: Columnstore indexes can help reduce the impact of index rebuilds on the system.

Conclusion

Additional Resources

For more information on HADR_SYNC_COMMIT sessions and index rebuilds, check out the following resources: