Install GRUB onto multiple boot disks in Software RAID-1 (quick reference)

Here is an example where I have a 3-way RAID-1 array. The /boot partition is stored at /dev/md0. This installs GRUB to each disk, so that if one disk fails, you can boot off one of the other disks.

# grub
grub> find /grub/stage1
 (hd0,0)
 (hd1,0)
 (hd2,0)
grub> device (hd0) /dev/sda
grub> root (hd0,0)
grub> setup (hd0)
grub> device (hd0) /dev/sdb
grub> root (hd0,0)
grub> setup (hd0)
grub> device (hd0) /dev/sdc
grub> root (hd0,0)
grub> setup (hd0)
grub> quit

With this you should be able to now boot from any of the disks in the RAID-1 array, no matter what boot order you set in the BIOS.

For safety, I suggest using UUIDs in your /etc/fstab file for your /boot and / (root) partitions. This way the machine will boot off the UUIDs of the file systems, even if mdadm (software RAID) decides to renumber your /dev/md# devices. Note: This is the default behavior in RHEL 6 / CentOS 6.