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).

# ls -la /dev/md/*
lrwxrwxrwx. 1 root root  8 Nov 11 19:00 /dev/md/imsm -> ../md127
lrwxrwxrwx. 1 root root  8 Nov 11 19:00 /dev/md/md128 -> ../md128
lrwxrwxrwx. 1 root root 10 Nov 11 19:00 /dev/md/md128p1 -> ../md128p1
lrwxrwxrwx. 1 root root 10 Nov 11 19:00 /dev/md/md128p2 -> ../md128p2
lrwxrwxrwx. 1 root root 10 Nov 11 19:00 /dev/md/md128p3 -> ../md128p3
# mdadm --detail /dev/md127
/dev/md127:
           Version : imsm
        Raid Level : container
     Total Devices : 2

   Working Devices : 2


              UUID : 1d1e824d:84ba14fb:534f364b:94ec042a
     Member Arrays : /dev/md/md128

    Number   Major   Minor   RaidDevice

       -       8        0        -        /dev/sda
       -       8       16        -        /dev/sdb
# mdadm --detail /dev/md128
/dev/md128:
         Container : /dev/md/imsm, member 0
        Raid Level : raid1
        Array Size : 1875368960 (1788.49 GiB 1920.38 GB)
     Used Dev Size : 1875368960 (1788.49 GiB 1920.38 GB)
      Raid Devices : 2
     Total Devices : 2

             State : clean
    Active Devices : 2
   Working Devices : 2
    Failed Devices : 0

Consistency Policy : resync


              UUID : 6f557a33:e2e40cac:c642aa56:2d08cac9
    Number   Major   Minor   RaidDevice State
       1       8       16        0      active sync   /dev/sdb
       0       8        0        1      active sync   /dev/sda