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Increasing the TCP window size for CIFS

Increasing the TCP receive window size to its maximum setting on both the system and a CIFS client can improve performance for large transfers, provided that packet loss is not taking place and the client's send window is large.


The TCP window size controls the number of TCP messages that can be transmitted between the storage system and the CIFS client at one time. The default is 17,520 bytes. The number of bytes must be between 2,920 and 64,240, in multiples of 1,460 bytes. If you are using DOS, enter an NT4 value of 8,760.
Attention: You are strongly advised to call technical support before changing this value.


  1. Do one of the following:
    To maximize the TCP window size on a storage system running... Enter this command...
    CIFS options cifs.tcp_window_size 64240
    NFS options nfs.tcp.recvwindowsize 64240

    Note: The cifs.tcp_window_size and nfs.tcp.recvwindowsize options are invisible until you set them. After you set these invisible options, you can view them by entering the options cifs or the options nfs command.

  2. Change the window size in the Windows registry on a Windows NT client by adding the DWORD value \\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\TcpWindowSize and set it to 64,240 (0xFAF0 in hexadecimal).