(Alpha and Integrity servers) SHADOW_HBMM_RTC specifies, in
    seconds, how frequently each shadow set on this system has its
    modified block count compared with the reset threshold. If the
    modified block count exceeds the reset threshold, the bitmap for
    that shadow set is zeroed. This comparison is performed for all
    shadow sets mounted on the system that have HBMM bitmaps.

    The reset threshold is specified by the RESET_THRESHOLD keyword
    in the /POLICY qualifier of the SET SHADOW command.

    When the comparison is made, the modified block count might
    exceed the reset threshold by a small increment or by a much
    larger amount. The difference depends on the write activity to
    the volume and on the setting of this parameter.

    SHADOW_HBMM_RTC is a DYNAMIC parameter.