Convert a single drive system to RAID

From ArchWiki

This guide shows how to convert a functional single-drive system to a RAID 1 setup after adding a second drive, without the need to temporarily store the data on a third drive. The procedure can also be adapted, simplifying it, to the conversion of simple non-root partitions, and to other RAID levels.

Tip: You may consider using RaiderAUR, which can convert a single disk into a RAID system with a two-pass command.


This example assumes that the pre-existing disk is /dev/sda, which contains only one partition, /dev/sda1, used for the whole system. The newly-added disk is /dev/sdb.

Warning: Backup important data before proceeding.

Prepare the new disk

Partition the disk

The first step is creating the partition on the new disk, /dev/sdb1, that will be used as the mirror for the RAID array. In general, in this step it is not needed to recreate the exact partitioning scheme of the pre-existing drive; RAID can even be configured on whole disks, and partitions or logical volumes created later.

Make sure that the partition type is set as FD. See RAID#Prepare the devices and RAID#Partition the devices for more information.

Create the RAID device

Next, create the RAID array in a degraded state, using only the new disk. Note how the missing keyword is specified for the first device: this will be added later.

# mdadm --create /dev/md0 --level=1 --raid-devices=2 missing /dev/device
Tip: Run fdisk -l to find the disk you want to use

Replace device with the one you are wanting to use before running the command.

If you want to use Syslinux, then specify --metadata=1.0 (for the boot partition). As of Syslinux 6.03, mdadm 1.2 is not yet supported in Syslinux. See also Software RAID and LVM.

Make sure the array has been created correctly by checking /proc/mdstat:

# Personalities : [raid1]                                                                                                                                                                       
md0 : active raid1 sdb1[1]                                                                                                                                                                    
      2930034432 blocks super 1.2 [2/1] [_U]                                                                                                                                                  
      bitmap: 22/22 pages [88KB], 65536KB chunk                                                                                                                                               
unused devices: <none>

Make file system

Create the needed file system on the /dev/md0 device.

Copy the data on the array

Warning: It is recommended to copy the data from another system, such as a live image, to minimize the risk of the data changing in the middle of the copy. Alternatively, switch to single-user mode with systemctl isolate

Mount the array:

# mount --mkdir /dev/md0 /mnt/new-raid

Now copy the data from /dev/sda1 to /mnt/new-raid/, for example using rsync.

Boot on the new disk

Update the boot loader

Create a new entry in the boot loader to load the system from the RAID array in the new disk.

GRUB legacy

The factual accuracy of this article or section is disputed.

Reason: The following configuration has not been verified after the article has been reorganized on September 2015. (Discuss in Talk:Convert a single drive system to RAID)

Use your preferred text editor to open /mnt/new-raid/boot/grub/menu.lst.

--- SNIP ---
default   0
color light-blue/black light-cyan/blue

## fallback
fallback 1

# (0) Arch Linux
title  Arch Linux - Original Disc
root   (hd0,0)
kernel /vmlinuz-linux root=/dev/sda1

# (1) Arch Linux
title  Arch Linux - New RAID
root   (hd1,0)
#kernel /vmlinuz-linux root=/dev/sda1 ro
kernel /vmlinuz-linux root=/dev/md0 md=0,/dev/sda1,/dev/sdb1
--- SNIP ---

Notice we added the fallback line and duplicated the Arch Linux entry with a different root directive on the kernel line.

Also update the "kopt" and "groot" sections, as shown below, if they are in your /mnt/new-raid/boot/grub/menu.lst file, because it will make applying distribution kernel updates easier:

- # kopt=root=UUID=fbafab1a-18f5-4bb9-9e66-a71c1b00977e ro
+ # kopt=root=/dev/md0 ro md=0,/dev/sda1,/dev/sdb1

## default GRUB root device
## e.g. groot=(hd0,0)
- # groot=(hd0,0)
+ # groot=(hd0,1)

See GRUB Legacy for more information.


Please refer to GRUB#RAID.

To boot the system from your degraded array, you will need to (1) add the mdadm_udev hook to the HOOKS in /etc/mkinitcpio.conf (after the entry for block) and (2) regenerate the initramfs and generate a new configuration file. You can then add a menu entry in /boot/grub/grub.cfg pointing to the raid partitions for boot. This is complicated by the default generation options making use of a primary boot entry, and placing the remaining boot entries in sub-menus. To restore generation of a single entry per-line for each boot option, simply add:


to /etc/default/grub and regenerate grub.cfg. Now you can simply add an entry containing either the device files (e.g. /dev/md0, /dev/md1 or simply use the UUID for each of the raid filesystems. After having done so, the easiest way to add an entry to boot from the degraded arrays is simply to copy the "Arch Linux, with Linux linux" entry and change the UUID's to match your arrays as shown in /dev/disk/by-uuid.

Note: Another alternative that avoids the uuid-shuffle, is to chroot your degraded array after it has been created and file copied to each of the respective arrays (e.g. /, /boot, and /home). You can add the mdadm_udev hook to /etc/mkinitcpio.conf, regenerate the initramfs. install the bootloader and generate a grub.cfg within the chroot environment. If you then toggle the primary drive for boot, you can boot the degraded array.

Alter fstab

You need to tell fstab on the new disk where to find the new device. It is recommended to use Persistent block device naming.

/dev/md0    /    ext4     defaults   0 1

Rebuild the initramfs

Chroot into the RAID system

# mount --bind /sys /mnt/new-raid/sys
# mount --bind /proc /mnt/new-raid/proc
# mount --bind /dev /mnt/new-raid/dev
# chroot /mnt/new-raid/

If the chroot command gives you an error like chroot: failed to run command `/bin/zsh': No such file or directory, then use chroot /mnt/new-raid/ /bin/bash instead.

Record mdadm's config

Edit /etc/mdadm.conf and change the MAILADDR line to be your email address, if you want emailed alerts of problems with the RAID 1.

Then save the array configuration with UUIDs to make it easier for the system to find /dev/md0 at boot. If you do not do this, you can get an ALERT! /dev/md0 does not exist error when booting:

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

Rebuild initcpio

Follow RAID#Configure mkinitcpio.

Install the boot loader on the RAID array

This article or section needs expansion.

Reason: Support more boot loaders, simplify. (Discuss in Talk:Convert a single drive system to RAID)

GRUB Legacy

Start GRUB:

# grub --no-floppy

Then we find our two partitions - the current one (hd0,0) (I.e. first disk, first partition), and (hd1,1) (i.e. the partition we just added above, on the second partition of the second drive). Check you get two results here:

grub> find /boot/grub/stage1

Then we tell GRUB to assume the new second drive is (hd0), i.e. the first disk in the system (when it is not currently the case). If your first disk fails, however, and you remove it, or you change the order disks are detected in the BIOS so that you can boot from your second disk, then your second disk will become the first disk in the system. The MBR will then be correct, your new second drive will have become your first drive, and you will be able to boot from this disk.

grub> device (hd0) /dev/sdb

Then we install GRUB onto the MBR of our new second drive. Check that the "partition type" is detected as "0xfd", as shown below, to make sure you have the right partition:

grub> root (hd0,1)
 Filesystem type is ext2fs, partition type 0xfd
grub> setup (hd0)
 Checking if "/boot/grub/stage1" exists... yes
 Checking if "/boot/grub/stage2" exists... yes
 Checking if "/boot/grub/e2fs_stage1_5" exists... yes
 Running "embed /boot/grub/e2fs_stage1_5 (hd0)"...  16 sectors are embedded. succeeded
 Running "install /boot/grub/stage1 (hd0) (hd0)1+16 p (hd0,1)/boot/grub/stage2 /boot/grub/grub.conf"... succeeded
grub> quit

Verify success

Reboot the computer, making sure it boots from the new RAID disk (/dev/sdb) and not the original disk (/dev/sda). You may need to change the boot device priorities in your BIOS to do this.

Once the boot loader on the new disk loads, make sure you select to boot the new system entry you created earlier.

Verify you have booted from the RAID array by looking at the output of mount. Also check mdstat again only to confirm which disk is in the array.

# mount
 /dev/md0 on / type ext4 (rw)

The factual accuracy of this article or section is disputed.

Reason: The output of the following command has not been verified after the article has been reorganized on September 2015. (Discuss in Talk:Convert a single drive system to RAID)
# cat /proc/mdstat
 Personalities : [linear] [raid0] [raid1] [raid5] [multipath] [raid6] [raid10]
 md0 : active raid1 sdb1[1]
      40064 blocks [2/1] [_U]
 unused devices: <none>

If the system boots fine, and the output of the above commands is correct, then you are running off the degraded RAID array, as expected.

Add original disk to array

Partition original disk

Copy the partition table from /dev/sdb (newly implemented RAID disk) to /dev/sda (second disk we are adding to the array) so that both disks have exactly the same layout:

# sfdisk -d /dev/sdb | sfdisk /dev/sda

Alternative method: this will output the /dev/sdb partition layout to a file, then it is used as input for partitioning /dev/sda.

# sfdisk -d /dev/sdb > raidinfo-partitions.sdb
# sfdisk /dev/sda < raidinfo-partitions.sdb

Verify that the partitioning is identical:

# fdisk -l
Note: If you get an error when attempting to add the partition to the array:
mdadm: /dev/sda1 not large enough to join array
You might have seen an earlier warning message when partitioning this disk that the kernel still sees the old disk size: a reboot ought to fix this, then try adding again to the array.

Add disk partition to array

# mdadm /dev/md0 -a /dev/sda1
 mdadm: hot added /dev/sda1

Verify that the RAID array is being rebuilt:

# cat /proc/mdstat
 Personalities : [raid1] 
md0 : active raid1 sda1[2] sdb1[1]
      2930034432 blocks super 1.2 [2/1] [_U]
      [>....................]  recovery =  0.2% (5973824/2930034432) finish=332.5min speed=146528K/sec
      bitmap: 22/22 pages [88KB], 65536KB chunk

unused devices: <none>

See also