Slow file write from Windows 2000 or Windows Server 2003 to Windows NT 4.0 Server (279282)

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256986 Description of the Microsoft Windows Registry

SYMPTOMS

The file-copy write process from a Microsoft Windows 2000 Professional-based computer or a Microsoft Windows Server 2003-based computer to a Microsoft Windows NT 4.0 Server-based computer may take up to four times longer than reading the same files from Windows NT Server.

CAUSE

The workstation redirector in Windows 2000 and Windows Server 2003 does not support RAW SMB mode. Instead, Windows 2000 and Windows Server 2003 use Large File (CAP_LARGE_FILE) support, which is just as fast. However, not all earlier operating systems implement Large File support. For example, Windows NT 4.0 has Large File Read support, but not Large File Write support.

RESOLUTION

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To work around this performance issue, make the following registry change on the Windows NT 4.0-based server:

Start Registry Editor (Regedt32.exe).
Locate and click the following key in the registry:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\lanmanserver\parameters
On the Edit menu, click Add Value, and then add the following registry value:
Value Name: SizReqBuf
Data Type: DWORD Value
Radix: Decimal
Value: 65535
Quit Registry Editor.
Restart the server.

After you make this change, the performance of the write process is approximately the same as a read operation between the two computers.

Note that the SizReqBuf value controls the buffer size for CORE SMB requests. Setting it to 64 KB has approximately the same effect as Large Write support, which uses 60-KB buffers.

NOTE: This adjustment causes the server services to use more memory.

MORE INFORMATION

Windows 2000 and Windows Server 2003 read operations are fast because they use the SMB Large Read capability of Windows NT Server (CAP_LARGE_READX). Windows NT Server does not support CAP_LARGE_WRITEX. For more information, see the "Common Internet File System (CIFS)" specification at the following Microsoft Web site:

http://msdn.microsoft.com/workshop/networking/cifs/default.asp

File-copy operations from Windows 2000 and Windows Server 2003 to Windows 2000 or Windows Server 2003 work quickly. They use CAP_LARGE_READX and CAP_LARGE_WRITEX capabilities to send files in 60-KB portions. Also, file-copy operations from Windows NT to Windows NT work quickly. They use CAP_RAW_MODE to send files in 64-KB portions. For additional information, click the following article numbers to view the articles in the Microsoft Knowledge Base:

99234 History of SizeReqBuf changes

223140 SMB block size negotiation when copying files with Windows NT Explorer