SMB Block Size Negotiation When Copying Files with Windows NT Explorer

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.

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
3. Create the SizReqBuf value using the following information:

      Value Name: SizReqBuf
      Data Type: REG_DWORD
      Data: 512 - 65535
      Default: 4356 

   Note Small buffers use less memory; large buffers can improve performance. The exact value that works best in an environment depends on the specific configuration of that environment, but 14596 is a value that has been shown to work well in a reasonably standard Ethernet environment.
   
   By default, this setting is 4356 bytes on computers. On servers that have more than 512 MB of memory, this value is increased to 16 KB. A receive buffer that is larger can improve performance on query directory and similar commands, but at the price of more memory per work item.
4. Quit Registry Editor, and then restart the Server service. To restart the server service, use the following commands:
   net stop server
   net start server