So I played it by ear and I’m going to post how I did it so you won’t have to. You’ll need to be using mdadm. I’m going to assume the partitions you want raided are /dev/sda1 and /dev/sdb1 and that you are currently booted to /dev/sda1. A rescue disk will also be helpful.

We are going to get a mirror going with just /dev/sdb1 (empty I hope) in it. Then we’ll copy everything over to it so everything is the same after reboot when it does a resync. Makes your data a bit safer.

Here we go…

Have you tried to setup raid before on these drives? This checks them for superblocks.

If it spat up lines like “ARRAY blah blah UUID=34cd3:etc”, zero them. Don’t worry not dangerous to data.

OK now we want to copy the partition tables over:

Now we create the raid array in a degraded mode with the master drive missing and format it to your filesystem.

Have a look at your new RAID:

Now we need to make sure that the kernel modules for scsi and raid are built in to the kernel. I am not actually sure if this step is needed but it worked on mine.

Now you’ll be presented with the kernel modules config screen. Find Device Drivers then in SCSI Device Support. In this section we need to make sure a few things have the asterisk in the brackets (should mean built in on the legend). These things for me were RAID Transport Class, SCSI device support, SCSI disk support and SCSI generic support. Press spacebar on each of them to change the M or blank in the brackets to an asterisk. Then exit out of it saving your config as you leave.

Now the important part – creating the kernel image. This worked for me… at about the sixth time and exhaustive google searches:

Now you want to modify /boot/grub/grub.conf and /boot/grub/device.map.

Need to make sure we have both devices in there
/boot/grub/device.map:

Need to make a backup bootloader entry and a raided one too
/boot/grub/grub.conf:

That boot list boots off /dev/sdb before and loads into the system with the raid array active. This allows you to add /dev/sda1 into the array. If it booted off /dev/sda1 (e.g. hd0) then /dev/sda1 would be locked and you would not be able to do the mdadm command.

It also allows you to boot without the raid array if your raid setup failed to boot (i.e. kernel panic). The setting for root in fstab doesn’t matter if its set to /dev/md0, /dev/sda1 still becomes the root drive.

Now install grub on /dev/sdb:

Now change the root device in your fstab to be /dev/md0
/etc/fstab:

Now we need to copy everything over to the raid array, however it will probably crash out copying with the running system like mine did, but not to worry we are pretty much ready to go here.

Shut down and boot into a linux rescue disk (the first cd of RHEL5.4 has it), and get to the command line, don’t bother mounting filesystems or anything.
Try this:

If that doesn’t work then try this:

and push y if it asks you to create over another array.

Create mountpoints and mount your drives:

Now copy everything over:

Wait a while till that is done then reboot. Hopefully you don’t get a kernel panic, and it boots up on the degraded /dev/md0 array. If it does then add your /dev/sda1 in like follows:

Now you can watch the resync progress like this:

mdadm is pretty smart.

Here’s some other links that may help you out if you’re stuck:
http://wiki.clug.org.za/wiki/RAID-1_in_a_hurry_with_grub_and_mdadm
http://www.mail-archive.com/linux-raid@vger.kernel.org/msg09672.html
http://www.debian-administration.org/articles/238
http://unthought.net/Software-RAID.HOWTO/Software-RAID.HOWTO-7.html