DMRAID MDADM Root Filesystem Linux (Debian)

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Moving your Linux root partition to software RAID

2008-10-01 19:32 UTC | 18 comments

One of the reasons I started this blog is to write about problems I’ve solved that I can’t find answers to already on the web. Today, let’s talk about moving your linux install to linux software raid (md raid / mdadm). This post assumes you are running Ubuntu Linux 8.04, but any Debian-based distro from the past two years, or most other distros, will have similar commands.

We start with an install on a single 80 GB SATA drive, partitioned as follows:

/dev/sda1 as /, 10GB, ext3, bootable
/dev/sda2 as swap, 1GB
/dev/sda3 as /home, 69GB, ext3

We want to add a second 80GB SATA drive and move the entire install to use RAID1 between the two drives. So the final configuration will appear:

/dev/md0 as /, 10GB, ext3
/dev/md1 as swap, 1GB
/dev/md2 as /home, 69GB, ext3

Where the raid arrays are:

md0 : active raid1 sda1[0] sdb1[1]
md1 : active raid1 sda2[0] sdb2[1]
md2 : active raid1 sda3[0] sdb3[1]

Here there be dragons. As always, back up your data first. If you don’t know how to use rsync, now is an excellent time to learn.

The general plan is:

   Partition the new drive
   Create RAID arrays and filesystems on the new drive
   Copy the data from drive 1 to the new RAID arrays on drive 2
   Install grub on drive 2
   Configure fstab and mdadm.conf, and rebuild initramfs images
   Reboot on the RAID arrays on drive 2
   Repartition drive 1 and add it to RAID

All commands are run as root. Use sudo if you prefer.

Step 1: Partition the new drive

Assuming you want to partition the second drive the same way as the first, this is easy. Just clone the partitions from /dev/sda to /dev/sdb:

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

You may want to format the new disk first to clear it of old data, especially if it previously had software RAID partitions on it. You may get unusual results if it has a similar partition structure or RAID setup as the original disk.

Now use parted to mark the partitions for software RAID, with the first partition to boot:

parted /dev/sdb
 (parted) toggle 1 raid
 (parted) toggle 2 raid
 (parted) toggle 3 raid
 (parted) toggle 1 boot
 (parted) print
Disk /dev/sdb: 80.0GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Number  Start   End     Size    Type     File system  Flags
 1      0.51kB  10.0GB  10.0GB  primary  ext3         boot, raid
 2      10.0GB  11.0GB  2000MB  primary  linux-swap   raid
 3      11.0GB  80.0GB  69.0GB  primary  ext3         raid
(parted) quit

Parted will show you a file system in each partition, but the reality is that they will be plain linux software raid partitions (type 0xfd):

fdisk -l /dev/sdb
Disk /dev/sdb: 80.0 GB, 80026361856 bytes
255 heads, 63 sectors/track, 9729 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0xa3181e57
  Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *           1        1216     9765625   fd  Linux RAID autodetect
/dev/sdb2            1216        1338      976562+  fd  Linux RAID autodetect
/dev/sdb3            1338        9730    67408556   fd  Linux RAID autodetect

Step 2: Create RAID arrays and filesystems on the new drive

Now we create RAID 1 arrays on each partition. These arrays will have just one member each when we create them, which isn’t normal for RAID 1. So we’ll have to force mdadm to let us:

apt-get install mdadm
mdadm --create /dev/md0 --level=1 --force --raid-devices=1 /dev/sdb1
mdadm --create /dev/md1 --level=1 --force --raid-devices=1 /dev/sdb2
mdadm --create /dev/md2 --level=1 --force --raid-devices=1 /dev/sdb3

Next, you’ll need to create filesystems on the new RAID devices. Assuming the same formats as your first partition:

mkfs.ext3 /dev/md0
mkswap /dev/md1
mkfs.ext3 /dev/md2

Step 3: Copy the data from drive 1 to the new raid arrays on drive 2

This is a job for rsync. First, there are some directories on a running Linux system that we do not want to copy, like /dev and /proc. We also want to ignore tempfs directories, like /var/run. The best way to avoid these is to make an excludes file. Create a file, /root/rsync-filter, with the following content:

- /dev/
- /lib/modules/*/volatile/
- /mnt/
- /proc/
- /sys/
- /var/lock/
- /var/run/

These lines define directories we will not copy over. You may wish to add /tmp, apt’s cache, etc, but if you do you must manually create the directories themselves on the new filesystem.

Mount the new RAID array:

mount /dev/md0 /mnt
mkdir /mnt/home
mount /dev/md2 /mnt/home

If you are using a different mount structure, just be sure to recreate it and mount it in the right places in the new filesystem under /mnt/.

And start the rsync copy:

rsync -avx --delete -n --exclude-from /root/rsync-filter / /mnt/
rsync -avx --delete -n /home/ /mnt/home/

You will see a list of files that will be changed, but nothing actually happens. This is the job of the -n argument, which performs a dry-run. Always do this before actually starting a copy. You WILL make a painful mistake with rsync some day, so learn to be cautious. Repeat the above commands without the -n when you are sure all is well.

The -x argument ensures that you will not cross filesystem boundaries, which is why you must copy /home separately, and any other mounted filesystems. If you omit this you only need one command. But make sure you have a good rsync filter file, and that you have nothing mounted like /media/cdrom that you don’t want an archive of.

Finally, create the directories that you skipped with your filter:

cd /mnt/
mkdir -p dev/ mnt/ proc/ sys/ var/lock/ var/run/
for i in /lib/modules/*/volatile ; do mkdir -p /mnt/$i ; done

Step 4: Install grub on drive 2

OK, we almost have a working RAID install on the second drive. But it won’t boot yet. Let’s use chroot to switch into it.

mount --rbind /dev /mnt/dev
mount --rbind /proc /mnt/proc
chroot /mnt

Now we have a working /dev and /proc inside the new RAID array, and by using chroot we are effectively in the root of the new array. Be absolutely sure you are in the chroot, and not the real root of drive 1. Here’s an easy trick: make sure nothing is in /sys:

ls /sys

If not, you’re in the new chroot.

Now we just need to edit grub’s and menu.list files. Edit /boot/grub/ and make sure both drives are listed:

(hd0)   /dev/sda
(hd1)   /dev/sdb

Now get the UUID of /dev/md0′s filesystem:

tune2fs -l /dev/md0 | grep UUID
Filesystem UUID:          8f4fe480-c7ab-404e-ade8-2012333855a6

Edit /boot/grub/menu.list and find the line like this:

Note: Prolly grub.cfg now. Replace all entries
# kopt=root=UUID=9e299378-de65-459e-b8b5-036637b7ba93 ro

Replace the UUID with the one you just found. Leave the line commented, and save the file. Now rebuild menu.lst to use the new UUID:


Double-check that each boot option at the bottom of menu.lst is using the right UUID. If not, edit them too. Finally, install grub on the second drive.

grub-install /dev/sdb

Step 5: Configure fstab and mdadm.conf, and rebuild initramfs images

We’re almost ready to reboot. But first, we need to build an initramfs that is capable of booting from a RAID array. Otherwise your boot process will hang at mounting the root partition. Still in the chroot, edit /etc/fstab and change the partition entries to the new filesystems or devices.

If your /etc/fstab has “UUID=” entries like the following, change them to the new entries:

proc            /proc           proc    defaults        0       0
UUID=8f4fe480-c7ab-404e-ade8-2012333855a6 /               ext3    relatime,errors=remount-ro 0       1
UUID=c1255394-4c42-430a-b9cc-aaddfb024445 none            swap    sw              0       0
UUID=94f0d5db-c525-4f52-bdce-1f93652bc0b1 /var/           ext3    relatime 0       1
/dev/scd0       /media/cdrom0   udf,iso9660 user,noauto,exec,utf8 0       0

In the above example, the first UUID cooresponds to /dev/md0, the second to /dev/md1, and so on. Find the UUIDs with:

/lib/udev/vol_id /dev/md0

If your distribution uses a newer udev, you may not have the vol_id command. Use:

/sbin/blkid /dev/md0

If your /etc/fstab has “/dev/sda1″ entries like it’s a bit easier. Just change them to /dev/md0 and so on:

proc            /proc           proc    defaults        0       0
/dev/md0        /               ext3    relatime,errors=remount-ro 0     1
/dev/md1        none            swap    sw              0       0
/dev/md2        /var/           ext3    relatime        0       1
/dev/scd0       /media/cdrom0   udf,iso9660 user,noauto,exec,utf8 0       0

Now, while still in the chroot, edit /etc/mdadm/mdadm.conf to list each RAID array:

DEVICE partitions
ARRAY /dev/md0 UUID=126a552f:57b18c5b:65241b86:4f9faf62
ARRAY /dev/md1 UUID=ca493308:7b075f97:08347266:5b696c99
ARRAY /dev/md2 UUID=a983a59d:181b32c1:bcbb2b25:39e64cfd

Find the UUID of each RAID array, which is not the same as the UUID of the filesystem on it (!), using mdadm:

mdadm --detail /dev/md0 | grep UUID

Now, rebuild your current kernel’s initramfs image:

update-initramfs -u

Or all of them:

update-initramfs -u -k all

Step 6: Reboot on the RAID arrays on drive 2

Now we’re ready to reboot. First, exit the chroot and power off the machine cleanly. You have three options:

   If your BIOS allows you to select which drive to boot from, elect to boot from drive 2.
   Swap drives 1 and 2 so drive 2 becomes /dev/sda, and restart
   Use a USB recovery stick to boot from drive 2

When the system restarts, you should reboot on the new RAID drive. Make sure:

df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/md0              9.2G  1.9G  6.9G  21% /
varrun                498M   56K  498M   1% /var/run
varlock               498M     0  498M   0% /var/lock
udev                  498M   68K  498M   1% /dev
devshm                498M     0  498M   0% /dev/shm
lrm                   498M   39M  459M   8% /lib/modules/2.6.24-19-generic/volatile
/dev/md2               63G  130M   60G   1% /home

Step 7: Repartition drive 1 and add it to RAID

Finally, we add the old drive into the array. Assuming you chose option 1 and didn’t swap the drive’s cables:

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

If you receive a warning message here, reboot now.

Re-reading the partition table ...
BLKRRPART: Device or resource busy
The command to re-read the partition table failed
Reboot your system now, before using mkfs

If not, continue on:

mdadm /dev/md0 --grow -n 2
mdadm /dev/md1 --grow -n 2
mdadm /dev/md2 --grow -n 2
mdadm /dev/md0 --add /dev/sda1
mdadm /dev/md1 --add /dev/sda2
mdadm /dev/md2 --add /dev/sda3

If you chose option 2 and your drives now have a different ordering, swap “/dev/sda” and “/dev/sdb” everywhere above.

The RAID array will now rebuild. To check its progress:

cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md0 : active raid1 sdb1[1] sda1[0]
      9767424 blocks [2/2] [UU]
md1 : active raid1 sdb1[1] sda1[0]
      1951808 blocks [2/2] [UU]
md2 : active raid1 sdb2[2] sda2[0]
      68693824 blocks [2/1] [U_]
      [>....................]  recovery =  0.4% (16320/68693824) finish=22.4min speed=8160K/sec
unused devices:

Once the array is finished rebuilding, reinstall grub on the new drive. We’ll do both, for good measure.

grub-install /dev/sda
grub-install /dev/sdb

You should now be able to reboot without either drive, and your system will come up cleanly. If you ever need to replace a failed drive, remove it, use step 7 above to clone the partition scheme to the new drive and add it to the array.


If you are having issues with your RAID setup, especially if you have an older RAID setup or older release already, you might need these packages:

apt-get install lvm2 evms dmsetup