官方文档的定义，是指SQL Server 产生的结果集需要经过Network传递到Client，Network不能很快将结果集传输到Client，导致结果集仍然驻留在SQL Server的Session中，可能的原因是SQL Server返回的结果集非常大，或者Network带宽小，传输慢。
ASYNC_NETWORK_IO：Occurs on network writes when the task is blocked behind the network. Verify that the client is processing data from the server.
ASYNC_NETWORK_IO 等待状态出现在SQL Server已经把数据准备好，但是网络发送速度跟不上，导致SQLServer返回的数据集仍然驻留在Session中。
（1）出现这种情况一般不是数据库的问题，调整数据库配置不会有大的帮助。
（2）网络层的瓶颈当然是一个可能的原因：对此要考虑是否真有必要返回那么多数据？
（3）检查应用程序是否有必要向SQL Server申请这么大的结果集。
引用《Wait statistics, or please tell me where it hurts》
ASYNC_NETWORK_IO ： This is usually where SQL Server is waiting for a client to finish consuming data. It could be that the client has asked for a very large amount of data or just that it’s consuming reeeeeally slowly because of poor programming – I rarely see this being a network issue. Clients often process one row at a time – called RBAR or Row-By-Agonizing-Row – instead of caching the data on the client and acknowledging to SQL Server immediately.
使用Resource Monitor 查看Server的Network Activity
The ASYNC_NETWORK_IO wait indicates that one of two scenarios are happening. The first scenario is that the session (i.e., SPID) is waiting for the client application to process the result set and send a signal back to SQL Server that it is ready to process more data. The second is that there may be a network performance issue.
Reducing SQL Server waits / wait times
If there are significant wait times on ASYNC_NETWORK_IO you have the following options:
- Review the queries and identify large result sets. Verify that the client application is consuming data as efficiently as possible. For example, if the application is asking for a million rows of data but only processing one row at a time.
- Review that all rows being requested are necessary. Often times it is the case that you can reduce this wait by filtering the result set for only the rows that are needed. Using the TOP clause may be an option as well. Client applications such as Microsoft Access may benefit from querying a view instead of pulling data from an entire table.
If the above tuning tips are reviewed and applied, but the server is still encountering high wait times, then ensure there aren’t any network-related issues:
- Validate the network components between the application/clients and the SQL Server instance (router, for example).
- Look at your NIC configuration on the server to make sure there are no issues with the physical card. Also, check if autodetect is picking the fastest speed.
- Check network adapter bandwidth: 1 Gigabit is better than 100 megabits, and 100 megabits is better than 10 megabits.
Also worth mention is the common practice of performing data loads on the server. It is possible that you may be seeing the ASYNC_NETWORK_IO wait during the times that the data loads are occurring.If this is the case then make sure the shared memory protocol is enabled for the SQL Server instance and the session is connected using net_transport = ‘Shared memory’.
You can determine the net_transport for the connection by looking at the DMV – sys.dm_exec_connections.
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