VMS Help SET, FILE, Qualifiers, /GLOBAL_BUFFER  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /GLOBAL_BUFFER=n
       /NOGLOBAL_BUFFER

    Sets the OpenVMS Record Management Services (OpenVMS RMS)
    global buffer count (the number of buffers that can be shared
    by processes accessing the file) for the specified files. The
    value n must be an integer in the range from 0 to 32,767. A value
    of 0 disables buffer sharing.

    The /SHARE qualifier can be used to enable or disable global
    buffers on a file currently being accessed; however, any new
    global buffer settings will only be applied to new accessors of
    the file. If a file is already open with global buffers, any new
    number of global buffers will not take effect until the file is
    closed by all accessors of the file.