G_MIRROR_DEVICE_FLAG_MASK
if ((md->md_mflags & ~G_MIRROR_DEVICE_FLAG_MASK) != 0) {
sc->sc_flags &= ~G_MIRROR_DEVICE_FLAG_MASK;
sc->sc_flags |= (md->md_mflags & G_MIRROR_DEVICE_FLAG_MASK);
md->md_mflags = (sc->sc_flags & G_MIRROR_DEVICE_FLAG_MASK);