We do CentOS installation on mdadm software RAID, which uses small 300MB
partitions at the beginning of the disks for /boot partition
So /dev/sda1 and /dev/sdb1 creates /dev/md0, and then /dev/md0 is
mounted as /boot. This is the setup we have been using for years.
However script CENTOS/40-insta
For Debian we use some code which handles all disks from a software
raid md array. Have a look at
https://github.com/faiproject/fai/blob/master/examples/simple/scripts/GRUB_PC/10-setup
BTW, with grub 2 a device.map is not needed any more. Currently I'm
testing the new code for GRUB-PC/10-setup on