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.
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
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.
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/sdb1
Note: If the above command causes mdadm to say "no such device /dev/sdb2", then reboot, and run the command again.
Make sure the array has been created correctly by checking
# Personalities : [raid1] md0 : active raid1 sdb1 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
Copy the data on the array
systemctl isolate rescue.target.
Mount the array:
# mount --mkdir /dev/md0 /mnt/new-raid
Now copy the data from
/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.
Use your preferred text editor to open
--- 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:
/etc/default/grub and regenerate
grub.cfg. Now you can simply add an entry containing either the device files (e.g.
/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
/home). You can add the
/etc/mkinitcpio.conf, regenerate the initramfs. install the bootloader and generate a
grub.cfgwithin the chroot environment. If you then toggle the primary drive for boot, you can boot the degraded array.
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
/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
Follow RAID#Configure mkinitcpio.
Install the boot loader on the RAID array
# 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
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.
/dev/md0 on / type ext4 (rw)
# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid5] [multipath] [raid6] [raid10] md0 : active raid1 sdb1 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
# sfdisk -d /dev/sdb > raidinfo-partitions.sdb # sfdisk /dev/sda < raidinfo-partitions.sdb
Verify that the partitioning is identical:
# fdisk -l
mdadm: /dev/sda1 not large enough to join arrayYou 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 sdb1 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>
- Convert running system to RAID 5 — Example using RAID 5