A guide to mdadm

This page is an overview of mdadm. It is NOT intended as a replacement for the man pages - anything covered in detail there will be skimmed over here. This is meant to provide examples that you can adapt for yourselves.

Overview

mdadm has replaced all the previous tools for managing raid arrays. It manages nearly all the user space side of raid. There are a few things that need to be done by writing to the /proc filesystem, but not much.

Getting mdadm

This is a pretty standard part of any distro, so you should use your standard distro software management tool. If, however, you are having any problems it does help to be running the absolute latest version, which can be downloaded with

git clone git://neil.brown.name/mdadm

In the absence of any other preferences, it belongs in the /usr/local/src directory. As a linux-specific program there is none of this autoconf stuff - just follow the instructions as per the INSTALL file.

Modes

mdadm has seven modes. You will normally only use a few of them. They are as follows:-

Assemble

This is probably the mode that is used most, but you won't be using it much - it happens in the background. Every time the system is booted, this needs to run. It scans the drives, looking for superblocks, and rebuilds all the arrays for you. This is why you need an initramfs when booting off a raid array - because mdadm is a user-space program, if root is on an array then we have a catch-22 - we can't boot until we have root, and we can't have root until we've booted and can run mdadm.

Create

This is the first of the two modes you will use a lot. As the name implies, it creates arrays, and writes the superblocks for arrays that have them. It also fires off initialisation - making sure that the disks of a mirror are identical, or that on a parity array the parities are correct. This is why raids 5&6 are created in degraded mode - if they weren't then any check of the raid would spew errors for areas that hadn't been written to.

Grow

A bit of a misnomer, this mode takes care of all operations that changed the size of an array, such as changing the raid level, changing the number of active devices, etc.

Manage

Almost the same as "grow", this takes care of managing arrays, rather than managing one single array. It's used to add spares (which can be shared across multiple arrays), and as a matter of symmetry it is also used to remove (failed) devices.

Build

This is a relic of when superblocks didn't exist. It is used to (re)create an array, and should not be used unless you know exactly what you are doing. Because there are no superblocks, or indeed, any array metadata, the system just has to assume you got everything right because it has no way of checking up on you.

[TODO: Can you use this mode to create a temporary mirror, for the purpose of backing up your live data?]

 Misc

This contains all the bits that don't really fit anywhere else.