I will try to keep this short.
I have a mdadm RAID 6 with 11 drives. This has for years "just worked" across reinstalls and such, without any issues. I have not even had the need for configuring mdadm.conf.
After the 12.04 release, i can no longer boot the machine without mdadm failing to mount the array, and then throwing me into initramfs shell. Under boot, mdadm takes 7 of the drives and starts them in /dev/md127, of course that will fail.
When booted, i stop it, reassemble all drives as md0, and it is all good again.
The question is, how do i make it behave even during boot?
After quite some testing, my current mdadm.conf is this: http://pastebin.com/SWx1nCNg
mdadm-status during boot: http://pastebin.com/Ua9W3aj4
Note that below on the paste above, is the command used for reassembly which works just fine.