Wait Type HADR_DBR_SUBSCRIBER

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 crucial insights into the performance and health of your environment. One such wait type, HADR_DBR_SUBSCRIBER, is specific to Always On Availability Groups. It occurs when a secondary replica is waiting for the data it needs to apply transaction log records sent by the primary replica. In this blog post, we’ll explain what the HADR_DBR_SUBSCRIBER wait type means, when it occurs, and how you can address it to maintain optimal performance.

What Is the HADR_DBR_SUBSCRIBER Wait Type?

The HADR_DBR_SUBSCRIBER wait type appears when a secondary replica is waiting for transaction log records to be sent from the primary replica. These logs are used by the secondary replica to stay synchronized with the primary database. During this wait, the secondary replica cannot apply changes until it receives the required log records.

This wait type typically indicates that the secondary replica is ready to process logs but is being delayed by the log shipping process, network latency, or primary replica performance.

When Does HADR_DBR_SUBSCRIBER Appear?

This wait type is common in the following scenarios:

• High Transaction Workloads – When the primary replica generates a large volume of transaction log records, the secondary replica may struggle to keep up.
• Network Latency – Delays in transmitting log records from the primary to the secondary replica can cause this wait.
• Resource Constraints – Limited CPU, memory, or storage resources on the primary or secondary replicas can delay log processing.

While some HADR_DBR_SUBSCRIBER waits are normal in Always On environments, frequent or prolonged waits may indicate performance issues that require attention.

Why HADR_DBR_SUBSCRIBER Waits Matter

Excessive or prolonged HADR_DBR_SUBSCRIBER waits can lead to increased latency between the primary and secondary replicas. This can delay failovers, reduce data availability, and negatively impact applications that rely on read-only access to the secondary replica. Common causes of these waits include:

• Slow Log Transmission – High network latency or limited bandwidth between replicas.
• Large Transactions – Large or complex transactions on the primary replica that generate significant log data.
• Slow Disk Performance – Disk I/O bottlenecks on the secondary replica delaying the application of logs.
• High Resource Utilization – Overloaded CPU or memory on either the primary or secondary replicas.

How to Address HADR_DBR_SUBSCRIBER Waits

If HADR_DBR_SUBSCRIBER waits are impacting performance, consider these strategies to address the issue:

• Optimize Network Performance – Ensure a fast and reliable network connection between the primary and secondary replicas. Consider dedicated links for Always On traffic.
• Monitor Workload on the Primary Replica – Analyze and optimize queries or transactions generating excessive log data.
• Upgrade Storage Performance – Use faster storage, such as SSDs, for transaction logs and data files on both primary and secondary replicas.
• Distribute Workload – Offload read-heavy workloads to the secondary replica to reduce resource contention on the primary replica.
• Use Asynchronous Commit – For less critical replicas, consider using asynchronous commit mode to reduce the synchronization overhead.

Monitoring HADR_DBR_SUBSCRIBER Waits

To monitor HADR_DBR_SUBSCRIBER waits, use SQL Server’s sys.dm_os_wait_stats to track wait statistics. Combine this with Always On-specific views like sys.dm_hadr_database_replica_states to understand the synchronization state of your replicas and identify potential bottlenecks.

Conclusion

The HADR_DBR_SUBSCRIBER wait type in SQL Server occurs when a secondary replica is waiting for transaction log records from the primary replica. While these waits are expected in Always On environments, excessive waits may indicate issues with network performance, resource availability, or workload optimization. By addressing these factors, you can reduce HADR_DBR_SUBSCRIBER waits and maintain efficient synchronization between replicas.

If you need expert help with Always On Availability Groups, SQL Server performance tuning, or resolving wait types, Stedman Solutions offers managed services to help you achieve optimal performance and reliability in your SQL Server environment.

Find out more about our SQL Server Managed Services

Applies to

Related Waits

See Also

• More About HADR_DBR_SUBSCRIBER