Wait Type HADR_TRANSPORT_FLOW_CONTROL

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 rely on efficient data transport to keep replicas synchronized. During this process, you might encounter the HADR_TRANSPORT_FLOW_CONTROL wait type. Understanding this wait can help you troubleshoot synchronization issues and ensure your Always On setup operates smoothly.

What is HADR_TRANSPORT_FLOW_CONTROL?

The HADR_TRANSPORT_FLOW_CONTROL wait type occurs when SQL Server is managing the flow of data between the primary replica and secondary replicas. It specifically relates to situations where SQL Server temporarily pauses data transmission to avoid overloading the receiving replica or network. This wait type helps maintain balance and prevent excessive resource usage.

In simpler terms, this wait type indicates that SQL Server is pacing the flow of data to ensure stability and avoid congestion. While some waits are expected during normal operations, excessive or prolonged waits could signal underlying performance or configuration issues.

Why Does HADR_TRANSPORT_FLOW_CONTROL Happen?

Several factors can lead to HADR_TRANSPORT_FLOW_CONTROL waits, including:

• Network bandwidth limitations causing delays in data transmission between replicas.
• Resource constraints (CPU, memory, or disk I/O) on the secondary replica, slowing down log processing.
• High transaction log activity on the primary replica, generating more data than the system can handle efficiently.
• Suboptimal Always On configurations, such as improper synchronization modes.
• Contention or bottlenecks in the network infrastructure.

Addressing these factors can help reduce these waits and improve synchronization performance.

How to Monitor HADR_TRANSPORT_FLOW_CONTROL Waits

Monitoring HADR_TRANSPORT_FLOW_CONTROL waits is essential for identifying patterns and pinpointing issues in Always On Availability Groups. The Database Health Monitor provides an excellent way to track this wait type. Its Historic Waits Monitoring feature allows you to observe when these waits occur, their frequency, and their correlation with system events.

With Database Health Monitor, you can identify whether these waits stem from network issues, resource limitations, or workload imbalances and take corrective action to optimize your environment.

What Can You Do About HADR_TRANSPORT_FLOW_CONTROL Waits?

If you notice frequent or prolonged HADR_TRANSPORT_FLOW_CONTROL waits, consider taking these actions:

• Ensure your network infrastructure has sufficient bandwidth and low latency to handle Always On data transport.
• Allocate adequate CPU, memory, and disk I/O resources on both primary and secondary replicas to process data efficiently.
• Optimize workloads on the primary replica to reduce transaction log activity.
• Review and adjust Always On synchronization modes to match your workload needs (e.g., asynchronous for non-critical secondary replicas).
• Analyze network traffic to identify and resolve bottlenecks or contention.

These steps can help you reduce transport flow control waits and improve the overall performance of your Always On setup.

Why Use Database Health Monitor?

The Database Health Monitor is a comprehensive tool for tracking SQL Server wait types, including HADR_TRANSPORT_FLOW_CONTROL. Its Historic Waits Monitoring feature provides actionable insights into wait trends, helping you identify and address performance bottlenecks in your Always On Availability Groups. With Database Health Monitor, you can keep your SQL Server environment running efficiently and reliably.

Start using Database Health Monitor today to monitor and optimize your SQL Server’s performance, ensuring a resilient and high-performing database system!

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Applies to

Related Waits

See Also

• More About HADR_TRANSPORT_FLOW_CONTROL