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

    SHADOWING enables or disables shadowing and specifies the mode
    of shadowing operations that you want to enable. SHADOWING is
    a value that specifies the type of disk class driver that is
    loaded on the system: DUDRIVER, DSDRIVER, or SHDRIVER. See Volume
    Shadowing for OpenVMS for more information about setting system
    parameters for volume shadowing.

    Specify one of the following values:

    Value  Description

    0      No shadowing is enabled; SHDRIVER is not loaded. This is
           the default value.

    2      Phase II shadowing enabled. SHDRIVER is loaded. Phase II
           shadowing provides shadowing of all disks located on a
           standalone system or an OpenVMS Cluster system.

    Note that a parameter value of 1 represents Phase I, which is no
    longer supported. Instead, use Phase II shadowing.