Autostart of md (level 1, Mirror) device not working in 11.3 x86_64


I tried to manually get an md mirror working on a system with two disks /dev/sda and /dev/sdb. I want to convert an existing linux installtion that has / and swap etc on a single device into a system that has all partitions on mirrors. The problem is, that I cannot get the mirror doing an autostart by the kernel so I can place the /-filesystem on it.

I created a mirror with
I set the partition type of /dev/sdb2 (one part of the mirror) to 0xfd (Raid autostart). Next I create the mirror with sdb2 and a missing device where / is currently on, which would be inserted later when the mirror is up and running:

mdadm -C /dev/md0 -l 1 -n 2 missing /dev/sdb2

After the mdadm command the mirror is running everything looks fine. I can create a filesystem, copy data etc…

Now I reboot the system expecting the the mirror will be automatically started by the kernel because of its partition type 0xfd. This automatic start is vital since after all I want to copy the /-filesystem on this mirror so I cannot rely on /etc/init.d/ to start a mirror because, … well this script is on the root filesystem which is on a md mirror device that first has to be startet to access this file.

Usually this used to work because the kernel would start all md devices with components that have the partition type 0xfd before everything else. However on 11.3 this no longer works. The mirror is not started automatically by the 11.3 desktop kernel.

How can this be done in 11.3?