ASYNC_NETWORK_IO SQL Server Wait Type

Wait Type ASYNC_NETWORK_IO

The wait type ASYNC_NETWORK_IO is ranked #7 by Stedman Solutions and Database Health Monitor.

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 wait types help identify where delays are occurring in your system. One commonly encountered wait type is ASYNC_NETWORK_IO. This wait type occurs when SQL Server is waiting to send data to a client application over the network. In this blog post, we’ll explain what ASYNC_NETWORK_IO means, when it occurs, and how to address it.

What Is the ASYNC_NETWORK_IO Wait Type?

The ASYNC_NETWORK_IO wait type appears when SQL Server is trying to send query results to a client application, but the client is not reading the data fast enough. Essentially, SQL Server is ready to send data, but it must wait for the client to process what it has already received before it can continue.

This wait type is often related to issues on the client side or network, rather than SQL Server itself.

When Does ASYNC_NETWORK_IO Appear?

ASYNC_NETWORK_IO waits commonly occur in the following scenarios:

  • Slow Client Processing – The client application is processing data slowly or is unable to keep up with SQL Server’s output.
  • Large Result Sets – Queries returning large amounts of data can overwhelm the client application or network, causing delays.
  • Network Bottlenecks – Limited bandwidth or high latency between SQL Server and the client can lead to delays in transmitting data.
  • Inappropriate Query Design – Inefficient queries that return more data than necessary can exacerbate the problem.

Why ASYNC_NETWORK_IO Waits Matter

Excessive or prolonged ASYNC_NETWORK_IO waits can indicate performance bottlenecks that may impact application responsiveness and overall database performance. These waits are typically a symptom of problems on the client side or network, rather than SQL Server itself. Common causes include:

  • Poorly Designed Applications – Applications that process data row by row instead of in batches can cause delays.
  • Large Result Sets – Queries returning unnecessary columns or rows add overhead and slow data transmission.
  • Network Congestion – Limited network resources or high traffic between the client and SQL Server can exacerbate waits.

How to Address ASYNC_NETWORK_IO Waits

To reduce ASYNC_NETWORK_IO waits, focus on optimizing client-side processing, query design, and network performance:

  • Optimize Queries – Ensure that queries return only the data needed by the application. Use proper indexing, and avoid SELECT * queries.
  • Batch Processing – Configure the client application to process data in batches rather than row by row to improve performance.
  • Limit Result Set Size – Apply filters or pagination to reduce the size of result sets returned by SQL Server.
  • Monitor Network Performance – Check network bandwidth, latency, and congestion between SQL Server and client applications. Consider upgrading network hardware if needed.
  • Improve Client Application Design – Ensure that the client is optimized for processing incoming data efficiently.

Monitoring ASYNC_NETWORK_IO Waits

To monitor ASYNC_NETWORK_IO waits, use SQL Server’s sys.dm_os_wait_stats to track wait statistics. Additionally, you can analyze queries and sessions contributing to these waits using tools like Activity Monitor or sys.dm_exec_requests.

Conclusion

The ASYNC_NETWORK_IO wait type in SQL Server occurs when the system is waiting for a client application to process data being sent over the network. While these waits are often caused by client-side or network issues, they can also be mitigated by optimizing queries, reducing result set sizes, and improving network performance. By addressing these factors, you can reduce ASYNC_NETWORK_IO waits and improve overall system responsiveness.

If you need expert assistance with SQL Server performance tuning, query optimization, or addressing network-related issues, Stedman Solutions offers managed services to help you achieve reliable and efficient database performance.

Joke

Why did the SQL Server query break up with the client application?

Because it got tired of waiting on ASYNC_NETWORK_IO while the client kept saying, "Hang on, I'm still chewing on the last batch!"


Watch on YouTube


Find out more about our SQL Server Managed Services

Applies to

Related Waits

ASYNC_DISKPOOL_LOCK
ASYNC_IO_COMPLETION
ASYNC_OP_COMPLETION

See Also


All Wait Types
ASYNC_NETWORK_IO SQL Server Wait Type