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We have a case where we're accessing a RAID array (drive E:) on a Windows Server 2008 SP2 x86 box. (Recently installed, nothing other than SQL Server 2005 on the server.)

In one scenario, when directly accessing it (E:\folder\ we get 45MBps throughput to a video file.

If we access the same file on the same array, but through UNC path (\server\folder\ we get about 23MBps throughput with the exact same test.

Obviously the second test is going through more layers of the stack, but that's a major performance hit.

What tuning should we be looking at for making the UNC path be closer in performance to the direct access case?

Thanks, Kirk

(corrected: it is CIFS not SMB, but generalized title to 'file share'.)

(additional info: this happens during the read from a single file, not an issue across multiple connections. the file is on the local machine, but exposed via file share. so client and file server are both same Windows 2008 server.)

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migrated from Jan 15 '10 at 0:12

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Thought it was CIFS now, hadn't been SMB for a while – OMG Ponies Jan 14 '10 at 20:23
CIFS = SMB, it's the same thing renamed. What's the client OS and is it a newer machine? What about network structure, NICs, have you run a performance test on the network? I'd hesitate to "blame Windows"™ without testing the hardware first. – Chris S May 12 '10 at 2:07

You might want to read the Microsoft publication on that topic by Microsoft Engineering Dudes et al -- Performance Tuning Guidelines for Windows Server 2008 .

Try to understand how to setup and make use of perfmonoutput to determine what is the bottleneck in your setup.

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Excellent, excellent link. Thank you! – djangofan Jun 14 '12 at 16:28

You might want to look over this similar thread.

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I don't have an answer for you, but this behavior has existed since at least Windows 2000. I believe it has to do with Windows resolving the UNC for each request. Mapping a drive sort of caches this information.

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