Wait Type HADR_DBSTATECHANGE_SYNC

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, wait types provide insight into where processes may be delayed. One wait type specific to Always On Availability Groups is HADR_DBSTATECHANGE_SYNC. This wait type appears when SQL Server is synchronizing database states across replicas in an Availability Group. In this post, we’ll break down what HADR_DBSTATECHANGE_SYNC means, when it typically appears, and how it may affect SQL Server performance.

What Is the HADR_DBSTATECHANGE_SYNC Wait Type?

The HADR_DBSTATECHANGE_SYNC wait type appears when SQL Server is waiting for a change in the database state to be synchronized across Availability Group replicas. An Availability Group is designed to provide high availability by keeping a copy of a database synchronized on a secondary server, so it can take over if the primary server goes down.

Sometimes, SQL Server needs to change the state of a database—for instance, if it’s transitioning from synchronized to synchronizing or vice versa. During this transition, SQL Server waits to make sure all replicas are updated and in the same state. This synchronization process ensures that data remains consistent across the primary and secondary replicas, which is essential for high availability.

When Does HADR_DBSTATECHANGE_SYNC Appear?

HADR_DBSTATECHANGE_SYNC waits occur in Always On Availability Group setups, particularly when SQL Server is updating or synchronizing the state of the database across replicas. Some common scenarios where this wait type may appear include:

• Failover Events – During a planned or automatic failover, SQL Server needs to synchronize the database state to make sure all replicas reflect the current role (primary or secondary) of each database.
• Adding or Removing Replicas – When a new replica is added or removed from an Availability Group, SQL Server updates the database state across replicas, leading to HADR_DBSTATECHANGE_SYNC waits.
• Changes in Synchronization Mode – If the synchronization mode is changed from asynchronous to synchronous or vice versa, SQL Server synchronizes the state of each replica to reflect this change.

These waits are generally expected in environments using Availability Groups and are part of SQL Server’s mechanisms to maintain high availability and data consistency.

Why HADR_DBSTATECHANGE_SYNC Waits Matter

While HADR_DBSTATECHANGE_SYNC waits are usually normal, they can sometimes signal potential issues, especially if they are frequent or prolonged. High HADR_DBSTATECHANGE_SYNC waits may indicate:

• Network Latency – If there is high latency between the primary and secondary replicas, SQL Server may experience delays in updating the state across replicas.
• Slow Replica Response – If a secondary replica is experiencing performance issues (such as limited CPU, memory, or disk speed), it may slow down the synchronization process, causing the primary replica to wait longer.
• Frequent Role Changes – Environments with frequent failovers or state changes may see more of these waits as SQL Server synchronizes each change across replicas.

While these waits are usually not a direct cause of application slowdowns, they can indicate underlying network or replica performance issues that may impact overall Availability Group performance.

How to Address HADR_DBSTATECHANGE_SYNC Waits

If you are seeing frequent or prolonged HADR_DBSTATECHANGE_SYNC waits, consider the following strategies to improve synchronization efficiency:

• Optimize Network Latency – Ensuring a low-latency and high-bandwidth network connection between replicas can help SQL Server synchronize states more quickly.
• Improve Replica Performance – Adding resources (like CPU or memory) to secondary replicas can help them respond faster to state updates, reducing wait times.
• Limit Unnecessary Failovers – Minimize unnecessary failovers or role changes where possible to reduce the frequency of synchronization events across replicas.
• Monitor for High Resource Usage – Regularly monitor resource usage on all replicas. Identifying and addressing high CPU, memory, or disk I/O usage on a replica can improve its synchronization performance.

Conclusion

The HADR_DBSTATECHANGE_SYNC wait type in SQL Server is an indicator that the system is synchronizing database states across replicas in an Availability Group. While these waits are expected during normal operations, frequent or prolonged waits can signal network latency or performance issues on a replica. By optimizing network connections, enhancing replica performance, and managing failovers effectively, you can help reduce HADR_DBSTATECHANGE_SYNC waits and maintain a more efficient Always On Availability Group environment.

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Applies to

Related Waits

See Also

• More About HADR_DBSTATECHANGE_SYNC