VMS Help Sys Parameters, SHADOW_SYS_DISK  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    A SHADOW_SYS_DISK parameter value of 1 enables shadowing of the
    system disk. A value of 0 disables shadowing of the system disk.
    The default value is 0.

    Also specify a system disk shadow set virtual unit number with
    the SHADOW_SYS_UNIT system parameter, unless the desired system
    disk unit number is DSA0.

    To enable minimerge on a system disk, add the value 4096 to
    your existing SHADOW_SYS_DISK value. For example, if you have
    SHADOW_SYS_DISK set to a value of 1, change it to 4097 to enable
    minimerge. Also, be sure to set the DUMPSTYLE parameter to dump
    off system disk, as described in the OpenVMS System Manager's
    Manual.