Convert a single drive system to RAID

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
raider
, which can convert a single disk into a RAID system with a two-pass command.
Scenario
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 rescue.target
.
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.
GRUB
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:
GRUB_DISABLE_SUBMENU=y
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 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
boot loader
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
.
/mnt/new-raid/etc/fstab
/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
(hd0,0)
(hd1,1)
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
 Done
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
Convert running system to RAID 5
— Example using RAID 5