SHADOW_SYS_DISK

   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.

   A value of 4096 enables CI-based minimerge. To enable minimerge
   on a system disk, however, you must enable DOSD by setting the
   DUMPSTYLE parameter to dump off system disk, as described in the
   VSI OpenVMS System Manager's Manual. You can then 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.