Comment 24 for bug 1487365

There may be a way to make this work, but only for RAID mirrors (level 1), in a particular version of the mdadm format, since both disks should have the same layout and look identical to non-mirrored disks, but it needs more work.

In the meantime, it seems like the supported partition layout should be appropriately enforced by partman-prep on installs. So, indeed one will have to make sure not to install a prep on a RAID device, but there isn't an automated partitioning setup which allows this to be done automatically -- it always either involves a couple of steps to set up RAID and then the partitioning on top, or full automation via preseed (which would be something out of the control of the installer -- up to the user to fix).