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Q223140: SMB Size Negotiation When Copying Files with Windows NT Explorer

Article: Q223140
Product(s): Microsoft Windows NT
Version(s): winnt:4.0
Operating System(s): 
Keyword(s): 
Last Modified: 11-JUN-2002

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The information in this article applies to:

- Microsoft Windows NT Server version 4.0 
- Microsoft Windows NT Workstation version 4.0 
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IMPORTANT: This article contains information about modifying the registry. Before you modify the registry, make sure to back it up and make sure that you understand how to restore the registry if a problem occurs. For information about how to back up, restore, and edit the registry, click the following article number to view the article in the Microsoft Knowledge Base:

  Q256986 Description of the Microsoft Windows Registry

SUMMARY
=======

When you copy or move a file, the negotiated Server Message Block (SMB) size is
different. The block size depends on a number of factors including:

- Whether Windows NT Explorer or an MS-DOS command prompt is used to issue the
  command.

- Which direction the file is being moved or copied to (that is, copying a file
  from your computer to another computer versus your computer copying a file
  from another computer back to your own computer.)

When you use Windows NT Explorer to copy a file from the client to a remote
computer, data is typically transfered in Core mode in 4 KB blocks.

When you use Windows NT Explorer to copy a file from the remote computer back to
the client, data is typically transfered in Raw mode in 60 KB blocks.

When you use an MS-DOS command prompt command to copy a file in either direction,
data is generally transfered in Raw mode. Because Raw mode typically uses a
60-64 KB transfer rate, and Core mode typically uses 4 KB, a Raw-mode transfer
is faster.

MORE INFORMATION
================

The factors listed below determine which transfer mode is chosen. However, in
this example, what's happening is not getting exclusive use of the Virtual
Circuit (VC), which is created when setting up the connection between the two
computers. When you cut or copy a file in Windows NT Explorer, make a folder, or
perform some other file system changes, Windows NT Explorer will show that
change quickly by updating the display. The update in the display is triggered
by a call called ChangeNotify.

When copying a file to a remote computer, a long distance ChangeNotify call is
triggered. This call goes over the network. This call also prevents getting
exclusive use of the VC to copy the file. When copying a file from a remote
computer back to the client (command issued locally), the ChangeNotify call is
still triggered, but it's now a local call. So the call is not sent over the
wire, therefore, the copy operation gets exclusive use of the VC.

Because ChangeNotify is not called for to copy or move a file when you use an
MS-DOS command prompt command, these transfers typically use Raw mode.

From the specifications for Core versus Raw mode data transfers:

  The Windows NT redirector has the following requirements for performing raw
  mode I/O, all of which must be satisfied for raw mode I/O to occur.

- The Server Message Block (SMB) protocol negotiated between the client and
  server must support Raw mode.

- The server must be configured to support Raw mode.

- The client redirector must be configured to allow Raw mode.

- No other SMB requests are pending on the same Virtual Circuit (VC).

- The I/O is not on a blocking named pipe.

- For a READ request: the requested data size is equal to or greater than 2
  times the currently configured request buffer size, and the read offset is
  not past the redirector's currently configured end-of-file.

- For a WRITE request: the requested data size is equal to or greater than 1.5
  times the currently configured request buffer size, and the write offset is
  not more than approximately 1 MB past the current end-of-file.

- The I/O is to take no longer than 5 seconds to complete."

Core Mode Data Transfer:

When writing data in Core mode, as in file copy to a remote server, size of data
blocks transferred is 4KBytes by default. This can be modified using the
following registry parameter on the remote server side.

WARNING: If you use Registry Editor incorrectly, you may cause serious problems
that may require you to reinstall your operating system. Microsoft cannot
guarantee that you can solve problems that result from using Registry Editor
incorrectly. Use Registry Editor at your own risk.
To increase the buffer size, edit the SizReqBuf registry value using the
following steps:

1. Start Registry Editor (Regedt32.exe or Regedit.exe).

2. Go to the following registry subkey:

  HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services \LanmanServer\Parameters
  NOTE: The above registry key is one path; it has been wrapped for
  readability.

3. Create the SizReqBuf value using the following information:

  Value Name: SizReqBuf
  Data Type : REG_DWORD
  Data      : 512 - 65536 (bytes in decimal)
  Default   : 4096

  NOTE: Small buffers use less memory and large buffers can improve performance.
  The exact value that works best in a given environment will depend on the
  specific configuration of that environment, but 14596 is a value that has
  been shown to work well in a fairly standard Ethernet environment. Setting
  this parameter to the maximum value will make server to transfer data in 60
  KB blocks.

4. Close Registry Editor and restart the Server service.

REFERENCES
==========

  Q152081 Use of Raw Data Transfer Mode Influenced by Application

Additional query words: data transfer raw core SMB negotitate slow

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Keywords          :  
Technology        : kbWinNTsearch kbWinNTWsearch kbWinNTW400 kbWinNTW400search kbWinNT400search kbWinNTSsearch kbWinNTS400search kbWinNTS400
Version           : winnt:4.0
Issue type        : kbinfo

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