Wait Type HADR_DB_OP_COMPLETION_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.

SQL Server uses wait types to give insight into the performance and behavior of processes. The HADR_DB_OP_COMPLETION_SYNC wait type is specific to Always On Availability Groups. It occurs when SQL Server waits for database operations to complete in a synchronous commit mode. Understanding this wait type is crucial for maintaining high availability and ensuring that replicas remain synchronized efficiently.

What Is the HADR_DB_OP_COMPLETION_SYNC Wait Type?

The HADR_DB_OP_COMPLETION_SYNC wait type indicates that SQL Server is waiting for a database operation to complete as part of synchronization between the primary replica and the secondary replica(s). In synchronous commit mode, a transaction must be committed on both the primary and at least one secondary replica before it is considered complete. This ensures data consistency but can introduce waits if the synchronization process is delayed.

When Does HADR_DB_OP_COMPLETION_SYNC Appear?

This wait type can occur in the following scenarios:

• Synchronous Commit Mode – When the primary replica waits for acknowledgment from a secondary replica during transaction commit.
• Heavy Workloads – When a high volume of transactions is being processed, increasing the time required for synchronization.
• Network Latency – Delays in communication between the primary and secondary replicas can extend the wait.
• Resource Constraints – When the secondary replica experiences CPU, memory, or disk bottlenecks, causing delays in processing.

Why HADR_DB_OP_COMPLETION_SYNC Waits Matter

This wait type is a normal part of synchronous commit mode in Always On Availability Groups, but prolonged or frequent waits can impact performance and indicate issues. Some common causes include:

• Network Issues – High latency or unreliable connections between replicas can delay synchronization.
• Replica Overload – Secondary replicas under heavy workloads may take longer to process log records and respond to the primary replica.
• Large Transactions – Transactions involving significant data changes can take more time to synchronize across replicas.
• Disk Performance – Slow disk I/O on the secondary replica can hinder log writing and extend wait times.

How to Address HADR_DB_OP_COMPLETION_SYNC Waits

If this wait type appears frequently or lasts longer than expected, consider the following strategies:

• Optimize Network Performance – Ensure low-latency and high-bandwidth connections between replicas. Use dedicated network links for Always On traffic.
• Monitor Secondary Replica Load – Balance workloads across replicas to prevent bottlenecks on any single node.
• Tune Transactions – Break large transactions into smaller ones to reduce the impact on synchronization processes.
• Upgrade Hardware – Ensure the secondary replicas have sufficient CPU, memory, and fast disk storage to handle the workload.
• Adjust Availability Group Settings – If possible, consider switching to asynchronous commit mode for replicas that are not latency-sensitive.

Monitoring HADR_DB_OP_COMPLETION_SYNC Waits

To monitor this wait type effectively, you can use the Historic Waits Monitoring feature in Database Health Monitor. This tool helps track wait statistics over time, allowing you to identify trends and troubleshoot issues related to Always On synchronization. By monitoring waits in real-time and historically, you can take proactive steps to optimize your SQL Server environment.

Stedman Solutions specializes in SQL Server performance tuning and high availability optimization. If you need help addressing HADR_DB_OP_COMPLETION_SYNC waits or other performance issues, our Database Health Monitor tool provides powerful insights, and our managed services deliver expert support to keep your SQL Server environment running efficiently. Contact us today to learn how we can help with your SQL Server needs!

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Applies to

Related Waits

See Also

• More About HADR_DB_OP_COMPLETION_SYNC