Joel's Compendium of Total Knowledge

To set up RAID1 mirroring on an existing Linux system, I've done something like this in the past. It may need some modernisation - though maybe only because everyone uses grub and SATA drives now, and ext3 instead of ext2.

Boot from a rescue CD or floppy that has mdadm and resize2fs.

umount /mnt/sysimage/
# Set up RAID devices and superblocks on hda
mdadm --create --verbose /dev/md0 --level=1 --raid-devices=2 /dev/hda9 missing
mdadm --create --verbose /dev/md1 --level=1 --raid-devices=2 /dev/hda5 missing 
mdadm --create --verbose /dev/md2 --level=1 --raid-devices=2 /dev/hda6 missing
mdadm --create --verbose /dev/md3 --level=1 --raid-devices=2 /dev/hda1 missing 
mdadm --create --verbose /dev/md4 --level=1 --raid-devices=2 /dev/hda7 missing

# Fix summary information on disk. Don't abort on initial error.
fsck -f /dev/md0
fsck -f /dev/md1
fsck -f /dev/md2
fsck -f /dev/md3
fsck -f /dev/md4

# Resize the partitions to account for RAID superblock size.
resize2fs /dev/md0
resize2fs /dev/md1
resize2fs /dev/md2
resize2fs /dev/md3
resize2fs /dev/md4

mount /dev/md0 /mnt/sysimage/
vi /mnt/sysimage/etc/fstab

Using fdisk, change the partition types on both drives to RAID autodetect.
Edit the fstab to use the mdx nodes instead of hdax ones

reboot

Once the system comes back up:

# Hot-add the hdc partitions to the mirror sets

mdadm --manage -a /dev/md0 /dev/hdc9
mdadm --manage -a /dev/md1 /dev/hdc5
mdadm --manage -a /dev/md2 /dev/hdc6
mdadm --manage -a /dev/md3 /dev/hdc1
mdadm --manage -a /dev/md4 /dev/hdc7

# Create an mdadm.conf file (not really necessary, but prevents some
# start-up drive scanning rubbish from appearing on the console)

mdadm --detail --scan > /etc/mdadm.conf

Edit the mdadm.conf to include a DEVICE line
Change lilo to use a root of md0 instead of hda9
Done!

RAID Setup