Wait Type HADR_FILESTREAM_BLOCK_FLUSH

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 help identify where resources are being delayed, offering valuable insights for performance tuning. One specific wait type, HADR_FILESTREAM_BLOCK_FLUSH, is associated with Always On Availability Groups and FileStream data. This wait type appears when SQL Server is waiting to flush FileStream blocks to disk while ensuring synchronization with secondary replicas in an Availability Group. In this post, we’ll explain what HADR_FILESTREAM_BLOCK_FLUSH means, when it occurs, and how it might affect performance.

What Is the HADR_FILESTREAM_BLOCK_FLUSH Wait Type?

The HADR_FILESTREAM_BLOCK_FLUSH wait type occurs when SQL Server is waiting to flush FileStream data to disk and synchronize it across Always On Availability Group replicas. FileStream is a feature that allows SQL Server to store large binary objects (such as images, videos, or documents) directly in the file system, while still being managed by the database. When used with Always On Availability Groups, SQL Server must ensure that these files are flushed to disk and synchronized with secondary replicas to maintain high availability and consistency.

This wait type specifically tracks the time SQL Server spends waiting for these blocks of FileStream data to be written to disk and replicated to the secondary replicas.

When Does HADR_FILESTREAM_BLOCK_FLUSH Appear?

This wait type typically appears in SQL Server environments where FileStream is enabled and Always On Availability Groups are used for high availability. Common scenarios include:

• Inserting or Updating Large FileStream Data – When large binary objects are added or modified, SQL Server must flush these changes to disk and synchronize them with replicas.
• High Transaction Volume on FileStream Data – Frequent updates or inserts to FileStream data can lead to increased wait times for block flushing.
• Slow Disk or Network Performance – If the disk subsystem or network between the primary and secondary replicas is slow, SQL Server may spend more time waiting to complete the block flush.

These waits are a normal part of SQL Server's operations when using FileStream with Availability Groups, but excessive waits can signal potential performance issues.

Why HADR_FILESTREAM_BLOCK_FLUSH Waits Matter

While some HADR_FILESTREAM_BLOCK_FLUSH waits are expected, frequent or prolonged waits can affect the performance of applications that rely on FileStream data. High wait times may indicate:

• Disk I/O Bottlenecks – If the primary or secondary replicas have slow storage, SQL Server may take longer to flush data to disk.
• Network Latency – High latency between Availability Group replicas can delay synchronization of FileStream data, increasing wait times.
• Large or Frequent FileStream Transactions – Systems with frequent FileStream operations may experience more waits due to the increased volume of data being synchronized.

Left unchecked, high HADR_FILESTREAM_BLOCK_FLUSH waits can slow down application performance, especially for workloads that depend on FileStream data being written and synchronized quickly.

How to Address HADR_FILESTREAM_BLOCK_FLUSH Waits

If you’re seeing high HADR_FILESTREAM_BLOCK_FLUSH waits, consider these strategies to reduce their impact:

• Optimize Disk Performance – Use faster storage, such as SSDs, for both the primary and secondary replicas to reduce the time it takes to flush FileStream blocks to disk.
• Improve Network Bandwidth – Ensure there’s sufficient bandwidth and low latency between Availability Group replicas to speed up data synchronization.
• Batch Large Transactions – When possible, batch FileStream operations to reduce the number of flushes needed during peak times.
• Monitor and Tune Always On Availability Group Configuration – Ensure that Availability Group settings are optimized for your workload, particularly for systems with heavy FileStream usage.
• Review FileStream Usage – If FileStream is being used extensively, consider whether some data can be moved to other storage methods to reduce the load.

Conclusion

The HADR_FILESTREAM_BLOCK_FLUSH wait type in SQL Server appears when the system is waiting to flush FileStream data to disk and synchronize it with secondary replicas in an Always On Availability Group. While some waits are expected in FileStream-enabled environments, prolonged or frequent waits may indicate performance bottlenecks in storage or network infrastructure. By optimizing disk and network performance, batching transactions, and monitoring FileStream usage, you can help reduce these waits and improve SQL Server performance.

For expert help with SQL Server performance tuning, Always On configuration, and FileStream optimization, Stedman Solutions offers managed services to ensure your SQL Server environment runs efficiently and reliably.

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Applies to

Related Waits

See Also

• More About HADR_FILESTREAM_BLOCK_FLUSH