IMSM versus fakeRAID aka imsm last chance ;) Or how to get Intel VROC with little or no pain (not yet, sorry) ;)

mdadm --create --verbose /dev/md/imsm /dev/sd[ab] --raid-devices 2 --metadata=imsm
mdadm --create --verbose /dev/md/md128 /dev/md/imsm --raid-devices 2 --level 1

Some checkups below (CLI).