I run RAID1 on all of the machines I support. While such hard disk mirroring is not a replacement for having good working backups, it means that a single drive failure is not going to force me to have to spend lots of time rebuilding a machine.

The best possible time to set this up is of course when you first install the operating system. The Debian installer will set everything up for you if you choose that option and Ubuntu has alternate installation CDs which allow you to do the same.

This post documents the steps I followed to retrofit RAID1 into an existing Debian squeeze installation. Getting a mirrored setup after the fact.


Before you start, make sure the following packages are installed:

apt-get install mdadm rsync initramfs-tools

Then go through these steps:

  1. Partition the new drive.
  2. Create new degraded RAID arrays.
  3. Install GRUB2 on both drives.
  4. Copy existing data onto the new drive.
  5. Reboot using the RAIDed drive and test system.
  6. Wipe the original drive by adding it to the RAID array.
  7. Test booting off of the original drive.
  8. Resync drives.
  9. Test booting off of the new drive.
  10. Reboot with the two drives and resync the array.

(My instructions are mostly based on this old tutorial but also on this more recent one.)

1- Partition the new drive

Once you have connected the new drive (/dev/sdb), boot into your system and use one of cfdisk or fdisk to display the partition information for the existing drive (/dev/sda on my system).

The idea is to create partitions of the same size on the new drive. (If the new drive is bigger, leave the rest of the drive unpartitioned.)

Partition types should all be: fd (or "linux raid autodetect").

2- Create new degraded RAID arrays

The newly partioned drive, consisting of a root and a swap partition, can be added to new RAID1 arrays using mdadm:

mdadm --create /dev/md0 --level=1 --raid-devices=2 missing /dev/sdb1  
mdadm --create /dev/md1 --level=1 --raid-devices=2 missing /dev/sdb2

and formatted like this:

mkswap /dev/md1  
mkfs.ext4 /dev/md0

Specify these devices explicitly in /etc/mdadm/mdadm.conf:

DEVICE /dev/sda* /dev/sdb*

and append the RAID arrays to the end of that file:

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

You can check the status of your RAID arrays at any time by running this command:

cat /proc/mdstat

3- Install GRUB2 on both drives

The best way to ensure that GRUB2, the default bootloader in Debian and Ubuntu, is installed on both drives is to reconfigure its package:

dpkg-reconfigure grub-pc

and select both /dev/sda and /dev/sdb (but not /dev/md0) as installation targets.

This should cause the init ramdisk (/boot/initrd.img-2.6.32-5-amd64) and the grub menu (/boot/grub/grub.cfg) to be rebuilt with RAID support.

4- Copy existing data onto the new drive

Copy everything that's on the existing drive onto the new one using rsync:

mkdir /tmp/mntroot  
mount /dev/md0 /tmp/mntroot  
rsync -auHxv --exclude=/proc/* --exclude=/sys/* --exclude=/tmp/* /* /tmp/mntroot/

5- Reboot using the RAIDed drive and test system

Before rebooting, open /tmp/mntroot/etc/fstab, and change /dev/sda1 and /dev/sda2 to /dev/md0 and /dev/md1respectively.

Then reboot and from within the GRUB menu, hit "e" to enter edit mode and make sure that you will be booting off of the new disk:

set root='(md/0)'  
linux /boot/vmlinuz-2.6.32-5-amd64 root=/dev/md0 ro quiet

Once the system is up, you can check that the root partition is indeed using the RAID array by running mount and looking for something like:

/dev/md0 on / type ext4 (rw,noatime,errors=remount-ro)

6- Wipe the original drive by adding it to the RAID array

Once you have verified that everything is working on /dev/sdb, it's time to change the partition types on /dev/sda to fd and to add the original drive to the degraded RAID array:

mdadm /dev/md0 -a /dev/sda1  
mdadm /dev/md1 -a /dev/sda2

You'll have to wait until the two partitions are fully synchronized but you can check the sync status using:

watch -n1 cat /proc/mdstat

7- Test booting off of the original drive

Once the sync is finished, update the boot loader menu:


and shut the system down:

shutdown -h now

before physically disconnecting /dev/sdb and turning the machine back on to test booting with only /dev/sda present.

After a successful boot, shut the machine down and plug the second drive back in before powering it up again.

8- Resync drives

If everything works, you should see the following after running cat /proc/mdstat:

md0 : active raid1 sda1[1]  
280567040 blocks [2/1] [_U]

indicating that the RAID array is incomplete and that the second drive is not part of it.

To add the second drive back in and start the sync again:

mdadm /dev/md0 -a /dev/sdb1

9- Test booting off of the new drive

To complete the testing, shut the machine down, pull /dev/sda out and try booting with /dev/sdb only.

10- Reboot with the two drives and resync the array

Once you are satisfied that it works, reboot with both drives plugged in and re-add the first drive to the array:

mdadm /dev/md0 -a /dev/sda1

Your setup is now complete and fully tested.

Ongoing maintenance

I recommend making sure the two RAIDed drives stay in sync by enabling periodic RAID checks. The easiest way is to enable the checks that are built into the Debian package:

dpkg-reconfigure mdadm

but you can also create a weekly or monthly cronjob which does the following:

echo "check" > /sys/block/md0/md/sync_action

Something else you should seriously consider is to install the smartmontools package and run weekly SMART checks by putting something like this in your /etc/smartd.conf:

/dev/sda -a -d ata -o on -S on -s (S/../.././02|L/../../6/03)  
/dev/sdb -a -d ata -o on -S on -s (S/../.././02|L/../../6/03)

These checks, performed by the hard disk controllers directly, could warn you of imminent failures ahead of time. Personally, when I start seeing errors in the SMART log (smartctl -a /dev/sda), I order a new drive straight away.

Why do you use rsync, and don't directly setup the active mirror letting the RAID driver let the duplication ?
Comment by obergix
@obergix I tend to prefer not touching the original drive until I have been able to successfully boot into a RAID-enabled copy.
Comment by Fran├žois

Hello, thanks for this entry. I too run raid1 on all my Debian systems so I'll add comments from my experience:
If you don't set the metadata to 0.90 during raid creation you'd be better of with 1.* (I use 1.2), then using "fd" as filesystem type isn't needed anymore. All my systems run with 1.2 metadata and boot just fine without "fd" as file-system type.
If you want to have "md0" "non-partitionable" (not really) raid devices you could set --auto=md to avoid messing things up if you assemble raid arrays on another system (recovery, ...etc).
_Lastly using the --homehost and --name bits to identify the raid devices is coming in handy when juggling with many devices and switching systems. Easier to use than the array's UUID.

All the best.

Comment by Anonymous

Great how to! On Ubuntu I needed to create /etc/sys before it would boot using the degraded RAID, and substitute sd2 with sd5 everywhere. Thanks for the help!

FYI, an easy way to copy the partitions initially is:
sfdisk -d /dev/sda > partition.txt
sfdisk --force /dev/sdb < partition.txt

Comment by John Wesorick

I was having a bear of a time getting step 5 to work. (Kernel panics and/or /dev/md0 missing on boot and subsequently being dumped to the initramfs prompt.)

Turned out that step 4 was doing nothing raid related for me. Which I solved with the following 2 steps.

1) Kept the initrd generated during "dpkg-reconfigure mdadm".

2) "grub-install --modules='raid proc_msdos ext2'" to both drives so grub could find md0. (substituting proc_msdos and ext2 with current insmod entries in you grub's menu items)

Comment by Anonymous
On step 7 (Natty 64Bit) update-grub gives me the following error: /usr/sbin/grub-probe: error: no such disk. in reference to the /dev/md0
Comment by Yost

if you do that on Natty, make sure getting the grub2 common packages from oneric!!!
grub2 from natty has a bug, update grub2-pc packages and grub2-common
and install grub again to /dev/sdx /dev/sdy

Comment by Network23

Great guide, specially about how you play it safe, and fail check end the end.

But I had to supplement it with this guide: http://www.howtoforge.com/how-to-set-up-software-raid1-on-a-running-system-incl-grub2-configuration-ubuntu-10.04-p1

And I can add that, its also possible to make mdadm daemon send email about any fails if the happens. A nice warning system.

Comment by tjohansen

Awesome tutorial, thank you. I used this to migrate an old Ubuntu 10.04 system with minor changes:

  • Stick with metadata 0.90 I had problems with using 1.2
  • I didn't bother using partition type FD, I just flagged the ext4 partition as a RAID partition using gparted.
  • Install grub-pc first, I was using grub1 on my old machine
  • Had to fix a MAKEDEV bug
    ln -s /sbin/MAKEDEV /dev/MAKEDEV
  • Reconfiguring mdadm to boot degraded disks is important for the tests.
Comment by map7

On ubuntu 12.04:
...This should cause the init ramdisk (/boot/initrd.img-2.6.32-5-amd64) and the grub menu (/boot/grub/grub.cfg) to be rebuilt with RAID support....


Helped me with some steps from the older manual http://wiki.xtronics.com/index.php/Raid

# mount -o bind /dev /tmp/mntroot/dev
# mount -t proc none /tmp/mntroot/proc
# mount -t sysfs none /tmp/mntroot/sys
# chroot /mntroot
# /usr/share/mdadm/mkconf > /etc/mdadm/mdadm.conf
# update-initramfs -u -k all
# update-grub

updated /etc/fstab with the new UUID's (blkid /dev/md0) and installed grub first only on the raid disc (sdb) with
# grub-install /dev/sdb

Disconected the old disc (sda), the system booted from the degraded raid array.

Hope it helps someone.

Comment by Anonymous
if you're not worried about losing swap data in case of drive failure (ideally you won't be using any swap space for performance reasons) you can just tell linux to use a swap partition on each drive with same priority and it should automatically stripe the data to give you more swap space and performance and less overhead.
Comment by automaticgiant
needed polish
Agree w/ previous comment; Helped me to add ' --metadata=0.90' when creating arrays and rather than specify '/dev/mdX' in places (e.g. fstab or grub menu) better to use the UUID (i.e. obtained via 'sudo tune2fs -l /dev/md0'). Finally, I've enabled 'user_xattr' on my filesystem... thus it was wise to add -X to the rsync.
Comment by Anonymous
Changes needed for Ubuntu 12.04 (to avoid the message "no such disk")


first of all: Thank you for this manual.

In Step 5 (Reboot using the RAIDed drive and test system) there has to be an enhancement.

Before "set root=..:" you have to add the following two lines

insmod raid
insmod mdraid1x

so that the whole change is looking like

insmod raid
insmod mdraid1x
set root='(md/0)'  
linux /boot/vmlinuz-2.6.32-5-amd64 root=/dev/md0 ro quiet


Comment by Jörg Maschtaler