Some SQL, some .NET, and whatever else


Slow SQL Server queries over a linked server from Windows Server 2008

We recently ran into an issue where linked server queries where sometimes taking significantly longer than they should have - queries that executed in under a second on the source server took two minutes when executed from a remote SQL Server over a linked server connection. We assumed it was something to do with our Windows Failover Cluster network connections, but it was happening on non-clustered servers as well.

After some digging, we discovered that it happened when the source server was Windows Server 2008 (or 2008 R2), regardless of what OS the remote linked server was running, and the issue wasn't tied to any particular version of SQL Server. As it turns out, it related to everybody's least favorite network setting, TCP Auto-Tuning - disabling it on the affected source servers resolved the problem immediately, and the queries returned instantly again.

To remedy the situation, follow these steps:

  1. On the source SQL Server (the one executing the query, not the linked server), open an elevated command prompt (Right click on CMD.EXE and click "Run as Administrator")
  2. To disable TCP Auto-tuning, run the following command:
    netsh int tcp set global autotuninglevel=disabled
  3. To disable Windows Scaling Heuristics (another feature that can speed up clients, but is no good for servers), run:
    netsh int tcp set heuristics disabled
  4. This will automatically take effect after Windows has the chance to refresh some network communications - you can either wait about an hour, or, if you'd prefer it take effect immediately, restart the SQL Instance. A reboot isn't necessary for this setting to take effect.

I'm not sure why these settings are on by default for servers - they seem to speed up internet browsing and other client-type activities (though there's no shortage of complaints online about them grinding Windows Vista/7 clients to a halt as well), but can cause big problems on servers.