Recover Failed RAID1 w/ MDADM

Preface

If you are here, you are already in trouble. Make sure you understand the situation before you move forward.

1) Was there a hardware failure?  If it was one of the drives in a RAID1, you can likely recover.
2) Was it a software or configuration failure?  This can be recovered, bu tmake sure you do it in the right order.

In any event, make sure you read carefully, and understand what the failure is.

I also wrote this primalially for myself. Your mileage may vary.

Diagnosis

Is your system still bootable and alive? If it is, then recovers is going to be easier. If it is not, then you will need to have a rescue disk or live CD to boot the system.

Bootable System

Since your system is still operable, you will likely have access to all the tools you need. Mostly, you will use mdadm, but you may also use sfdisk if you have to replace a disk.