No, you do not have to reinstall. You can migrate without too much horrible pain. Really its not that bad, though I would recommend practicing on a non production system. I've successfully done it several times, I don't have any notes handy but it goes something like this.
copy your partition table from your first disk to the second (btw I am assuming you initally installed on one disk then added a second)
sfdisk -d /dev/sda | sfdisk /dev/sdb
create the raid with sda missing so you don't lose your data
mdadm --create /dev/md0 --level 1 --raid-devices=2 missing /dev/sdb1
and so on for the rest of your partitions
mdadm --create /dev/md1 --level 1 --raid-devices=2 missing /dev/sdb2
make your file system
mkfs.ext3 /dev/md0
mount it
mount /dev/md0 /mnt
copy your data
rsync -avz / /mnt
now edit /mnt/etc/fstab and change your mounts to the appropriate /dev/mdx devices that correlate with your /dev/sdax devices
then edit /mnt/boot/grub/menu.lst (or whatever your grub.conf file is) make sure to replace /dev/sda with /dev/md
now you need to install grub on /dev/sdb so something like
grub-install /dev/sdb or do the manual grub install dance
reboot and pray
once the system comes up you just have to add in your missing devices (since we created the raid1 with sda missing)
mdadm --add /dev/md0 /dev/sda1
mdadm --add /dev/md1 /dev/sda2
and so on for your partition
then dont reboot until cat /proc/mdstat show that all your raid devices are properly synced
and so on for each of your partitions
dont forget to mkswap on one of those md devices
custangro was that process never covered in the RHCE? or am I just missing some obvious piece of information that makes it impossible to migrate to raid1?
Sorry for such a sloppy post its been a year or so since I've gone through the process and I dunno where my notes are. But unless there is some black magic to do with RHEL vs Centos, gentoo, or debian (i've done done migrations on all three) it is possible.
