Comment 17 for bug 1487365

There are multiple related issues here.

 - partman-md does not support assembling a RAID1 array out of a set of whole disks; it only supports assembling an array out of partitions. It *should* support using whole disks, so that you have the option of partitioning the RAID array and having those partitions show through on the underlying block devices in a way that is meaningful to the firmware.
 - in the absence of such support in partman-md, users are led down the garden path *thinking* they're creating arrays out of the disks when they're actually creating full-disk partitions on the component disks and RAIDing those. partman-prep should warn the user *early* (i.e., at the partitioning stage) that they've created themselves an impossible-to-boot partition layout, and guide them to go back to fix it.
 - grub-installer should support installing to multiple PReP partitions in parallel for proper full redundancy in the face of a disk failure. This can be achieved in one of two ways:
  - the user can create a RAID1 of two partitions on different disks (e.g. /dev/sda1+/dev/sdb1 -> /dev/md0), and grub-installer can support installing to this as a PReP partition; or
  - grub-installer can detect multiple PReP partitions (possibly requiring the user to manually specify each one that they want to use) and install the necessary boot record to each of them in turn.

The 2nd and 3rd issues should be relatively straightforward to fix. The first issue is going to require some redesign of partman-md and is not something we'll want to take on for 15.10.