Wait Type HADR_SYNCHRONIZING_THROTTLE

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 Always On Availability Groups ensure high availability and disaster recovery by synchronizing data between replicas. During this synchronization process, you may encounter the HADR_SYNCHRONIZING_THROTTLE wait type. Understanding this wait type can help you optimize your Always On environment and improve synchronization performance.

What is HADR_SYNCHRONIZING_THROTTLE?

The HADR_SYNCHRONIZING_THROTTLE wait type occurs when SQL Server is intentionally throttling the synchronization process to balance workloads or manage resource usage. This wait type often appears when the system tries to prevent excessive resource consumption while synchronizing data between the primary and secondary replicas.

In simpler terms, this wait indicates that SQL Server is pacing the synchronization process to ensure that other tasks and workloads continue running smoothly. While some occurrence of this wait type is normal, excessive or prolonged waits could indicate performance bottlenecks or misconfigurations.

Why Does HADR_SYNCHRONIZING_THROTTLE Happen?

Several factors can lead to HADR_SYNCHRONIZING_THROTTLE waits, including:

• High transaction log activity on the primary replica creating a large volume of data to synchronize.
• Insufficient bandwidth or network latency between the primary and secondary replicas.
• Resource constraints such as limited CPU, memory, or disk I/O on either the primary or secondary replicas.
• Contention between Always On synchronization and other high-priority workloads.
• Suboptimal Always On configurations or synchronization settings.

Identifying and addressing these factors can help reduce the impact of this wait type and improve performance.

How to Monitor HADR_SYNCHRONIZING_THROTTLE Waits

Monitoring HADR_SYNCHRONIZING_THROTTLE waits is key to diagnosing and resolving synchronization performance issues. The Database Health Monitor is an excellent tool for tracking this and other wait types. Its Historic Waits Monitoring feature allows you to identify when these waits occur, their frequency, and how they correlate with system events and workloads.

By using Database Health Monitor, you can gain insights into the causes of synchronization delays and determine whether adjustments to network, resources, or configuration are needed to optimize your Always On setup.

What Can You Do About HADR_SYNCHRONIZING_THROTTLE Waits?

If you experience frequent or prolonged HADR_SYNCHRONIZING_THROTTLE waits, consider these actions:

• Ensure the network connection between replicas is reliable and has sufficient bandwidth to handle synchronization traffic.
• Increase resources (CPU, memory, and disk I/O) on the primary and secondary replicas to support higher synchronization throughput.
• Optimize queries and transactions on the primary replica to reduce the volume of transaction log activity.
• Review Always On Availability Group configurations to ensure synchronization settings align with workload demands.
• Schedule heavy workloads during off-peak hours to reduce contention with synchronization processes.

These steps can help minimize synchronization throttling and ensure that your Always On Availability Group performs reliably and efficiently.

Why Use Database Health Monitor?

The Database Health Monitor is a powerful tool for monitoring SQL Server wait types, including HADR_SYNCHRONIZING_THROTTLE. Its Historic Waits Monitoring feature provides valuable insights into wait trends and performance bottlenecks, enabling you to take targeted action to optimize your environment. With Database Health Monitor, you can ensure your SQL Server and Always On Availability Groups are running at their best.

Start using Database Health Monitor today to gain visibility into your SQL Server’s performance and maintain a resilient and efficient database system!

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