How To Set Up Software RAID1 On A Running System (Incl. GRUB2 Configuration) (Debian Squeeze)

Version 1.0
Author: Falko Timme
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This guide explains how to set up software RAID1 on an already running Debian Squeeze system. The GRUB2 bootloader will be configured in such a way that the system will still be able to boot if one of the hard drives fails (no matter which one).

I do not issue any guarantee that this will work for you!

1 Preliminary Note

In this tutorial I'm using a Debian Squeeze system with two hard drives, /dev/sda and /dev/sdb which are identical in size. /dev/sdb is currently unused, and /dev/sda has the following partitions:

/dev/sda1: /boot partition, ext4;
/dev/sda2: swap;
/dev/sda3: / partition, ext4

In the end I want to have the following situation:

/dev/md0 (made up of /dev/sda1 and /dev/sdb1): /boot partition, ext4;
/dev/md1 (made up of /dev/sda2 and /dev/sdb2): swap;
/dev/md2 (made up of /dev/sda3 and /dev/sdb3): / partition, ext4

This is the current situation:

df -h

[email protected]:~# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda3             4.0G  712M  3.1G  19% /
tmpfs                 249M     0  249M   0% /lib/init/rw
udev                  244M  100K  244M   1% /dev
tmpfs                 249M     0  249M   0% /dev/shm
/dev/sda1             472M   25M  423M   6% /boot
[email protected]:~#

fdisk -l

[email protected]:~# fdisk -l

Disk /dev/sda: 5368 MB, 5368709120 bytes
255 heads, 63 sectors/track, 652 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000b0ecb

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          63      498688   83  Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2              63         125      499712   82  Linux swap / Solaris
Partition 2 does not end on cylinder boundary.
/dev/sda3             125         653     4242432   83  Linux
Partition 3 does not end on cylinder boundary.

Disk /dev/sdb: 5368 MB, 5368709120 bytes
255 heads, 63 sectors/track, 652 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

Disk /dev/sdb doesn't contain a valid partition table
[email protected]:~#

2 Installing mdadm

The most important tool for setting up RAID is mdadm. Let's install it like this:

apt-get install initramfs-tools mdadm

MD arrays needed for the root file system: <-- all

Afterwards, we load a few kernel modules (to avoid a reboot):

modprobe linear
modprobe multipath
modprobe raid0
modprobe raid1
modprobe raid5
modprobe raid6
modprobe raid10

Now run

cat /proc/mdstat

The output should look as follows:

[email protected]:~# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
unused devices: <none>
[email protected]:~#

3 Preparing /dev/sdb

To create a RAID1 array on our already running system, we must prepare the /dev/sdb hard drive for RAID1, then copy the contents of our /dev/sda hard drive to it, and finally add /dev/sda to the RAID1 array.

First, we copy the partition table from /dev/sda to /dev/sdb so that both disks have exactly the same layout:

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

The output should be as follows:

[email protected]:~# sfdisk -d /dev/sda | sfdisk --force /dev/sdb
Checking that no-one is using this disk right now ...
OK

Disk /dev/sdb: 652 cylinders, 255 heads, 63 sectors/track

sfdisk: ERROR: sector 0 does not have an msdos signature
/dev/sdb: unrecognized partition table type
Old situation:
No partitions found
New situation:
Units = sectors of 512 bytes, counting from 0

   Device Boot    Start       End   #sectors  Id  System
/dev/sdb1   *      2048    999423     997376  83  Linux
/dev/sdb2        999424   1998847     999424  82  Linux swap / Solaris
/dev/sdb3       1998848  10483711    8484864  83  Linux
/dev/sdb4             0         -          0   0  Empty
Warning: partition 1 does not end at a cylinder boundary
Successfully wrote the new partition table

Re-reading the partition table ...

If you created or changed a DOS partition, /dev/foo7, say, then use dd(1)
to zero the first 512 bytes:  dd if=/dev/zero of=/dev/foo7 bs=512 count=1
(See fdisk(8).)
[email protected]:~#

The command

fdisk -l

should now show that both HDDs have the same layout:

[email protected]:~# fdisk -l

Disk /dev/sda: 5368 MB, 5368709120 bytes
255 heads, 63 sectors/track, 652 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000b0ecb

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          63      498688   83  Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2              63         125      499712   82  Linux swap / Solaris
Partition 2 does not end on cylinder boundary.
/dev/sda3             125         653     4242432   83  Linux
Partition 3 does not end on cylinder boundary.

Disk /dev/sdb: 5368 MB, 5368709120 bytes
255 heads, 63 sectors/track, 652 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *           1          63      498688   83  Linux
Partition 1 does not end on cylinder boundary.
/dev/sdb2              63         125      499712   82  Linux swap / Solaris
Partition 2 does not end on cylinder boundary.
/dev/sdb3             125         653     4242432   83  Linux
Partition 3 does not end on cylinder boundary.
[email protected]:~#

Next we must change the partition type of our three partitions on /dev/sdb to Linux raid autodetect:

fdisk /dev/sdb

[email protected]:~# fdisk /dev/sdb

WARNING: DOS-compatible mode is deprecated. It's strongly recommended to
         switch off the mode (command 'c') and change display units to
         sectors (command 'u').

Command (m for help): <-- m
Command action
   a   toggle a bootable flag
   b   edit bsd disklabel
   c   toggle the dos compatibility flag
   d   delete a partition
   l   list known partition types
   m   print this menu
   n   add a new partition
   o   create a new empty DOS partition table
   p   print the partition table
   q   quit without saving changes
   s   create a new empty Sun disklabel
   t   change a partition's system id
   u   change display/entry units
   v   verify the partition table
   w   write table to disk and exit
   x   extra functionality (experts only)

Command (m for help): <-- t
Partition number (1-4): <-- 1
Hex code (type L to list codes): <-- L

0  Empty           24  NEC DOS         81  Minix / old Lin bf  Solaris
1  FAT12           39  Plan 9          82  Linux swap / So c1  DRDOS/sec (FAT-
2  XENIX root      3c  PartitionMagic  83  Linux           c4  DRDOS/sec (FAT-
3  XENIX usr       40  Venix 80286     84  OS/2 hidden C:  c6  DRDOS/sec (FAT-
4  FAT16 <32M      41  PPC PReP Boot   85  Linux extended  c7  Syrinx
5  Extended        42  SFS             86  NTFS volume set da  Non-FS data
6  FAT16           4d  QNX4.x          87  NTFS volume set db  CP/M / CTOS / .
7  HPFS/NTFS       4e  QNX4.x 2nd part 88  Linux plaintext de  Dell Utility
8  AIX             4f  QNX4.x 3rd part 8e  Linux LVM       df  BootIt
9  AIX bootable    50  OnTrack DM      93  Amoeba          e1  DOS access
a  OS/2 Boot Manag 51  OnTrack DM6 Aux 94  Amoeba BBT      e3  DOS R/O
b  W95 FAT32       52  CP/M            9f  BSD/OS          e4  SpeedStor
c  W95 FAT32 (LBA) 53  OnTrack DM6 Aux a0  IBM Thinkpad hi eb  BeOS fs
e  W95 FAT16 (LBA) 54  OnTrackDM6      a5  FreeBSD         ee  GPT
f  W95 Ext'd (LBA) 55  EZ-Drive        a6  OpenBSD         ef  EFI (FAT-12/16/
10  OPUS            56  Golden Bow      a7  NeXTSTEP        f0  Linux/PA-RISC b
11  Hidden FAT12    5c  Priam Edisk     a8  Darwin UFS      f1  SpeedStor
12  Compaq diagnost 61  SpeedStor       a9  NetBSD          f4  SpeedStor
14  Hidden FAT16 <3 63  GNU HURD or Sys ab  Darwin boot     f2  DOS secondary
16  Hidden FAT16    64  Novell Netware  af  HFS / HFS+      fb  VMware VMFS
17  Hidden HPFS/NTF 65  Novell Netware  b7  BSDI fs         fc  VMware VMKCORE
18  AST SmartSleep  70  DiskSecure Mult b8  BSDI swap       fd  Linux raid auto
1b  Hidden W95 FAT3 75  PC/IX           bb  Boot Wizard hid fe  LANstep
1c  Hidden W95 FAT3 80  Old Minix       be  Solaris boot    ff  BBT
1e  Hidden W95 FAT1
Hex code (type L to list codes): <-- fd
Changed system type of partition 1 to fd (Linux raid autodetect)

Command (m for help): <-- t
Partition number (1-4): <-- 2
Hex code (type L to list codes): <-- fd
Changed system type of partition 2 to fd (Linux raid autodetect)

Command (m for help): <-- t
Partition number (1-4): <-- 3
Hex code (type L to list codes): <-- fd
Changed system type of partition 3 to fd (Linux raid autodetect)

Command (m for help): <-- w
The partition table has been altered!

Calling ioctl() to re-read partition table.
Syncing disks.
[email protected]:~#

To make sure that there are no remains from previous RAID installations on /dev/sdb, we run the following commands:

mdadm --zero-superblock /dev/sdb1
mdadm --zero-superblock /dev/sdb2
mdadm --zero-superblock /dev/sdb3

If there are no remains from previous RAID installations, each of the above commands will throw an error like this one (which is nothing to worry about):

[email protected]:~# mdadm --zero-superblock /dev/sdb1
mdadm: Unrecognised md component device - /dev/sdb1
[email protected]:~#

Otherwise the commands will not display anything at all.

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By: al biheiri

Why would you modprobe multipath? I think that is only used for fiber cards. It shouldnt have anything to do with mdadm.

By: paul

Hey.

Great Tutorial!

Here are a couple of bumps you might run into, if you're trying to use this on Debian Unstable "Wheezy".

First:

If you want to use GPT enabled harddisks you'll have to create a small (1MB) BIOS Boot Partition or GRUB2 won't install on that drive.

Second:

If you ARE actually using GUID partition tables you won't be able to change your partitions using fdisk. You'll have to use gdisk (or parted) instead, which is its direct successor and fairly simple to use. Note however, that there's no direct way of duplicating the partition table from one disk to another. You'll have to use sgdisk for that, which installs with gdisk:


sgdisk --replicate=/dev/sdb /dev/sda

sgdisk --randomize-guids --move-second-header /dev/sdb

The second line is important to distinguish the disks in your system later on. In addition it ensures that your Secondary (Backup) GPT Header really resides on the very last blocks of your disk.

Third:

This one took me a while :-\

If you're creating the ''/etc/grub.d/09_swraid1_setup'' as mentioned you MUST rename the module GRUB2 is instructed to load from

insmod mdraid to insmod mdraid1x

(at least for a RAID1 or 10 configuration), as the old-named module does no longer exist. Furthermore, if you don't have a separated /boot/partition you need to fix the path to the kernel and ramdisk (probably "/boot/vmlinuz-2.6…" and "/boot/initrd.img-2.6…"

By: Anonymous

How can I do this on Debian 7? Now I've killed 6 VMs with this Turorial :D

By: Simon

Thanks for this tutorial. I had a much simpler task of just putting /home in raid 1. I followed this minus the grub stuff since I was not including a bootable partition in the array and it was very helpful.

By: timinski

This line is rights restricted on my running system:

vi /etc/mtab

I can open and edit the file but not save it. From what I understand, mtab is symlink of /proc/self/mounts. What are the steps required to save the changes on a live system?

By: Andrey

> Now I've killed 6 VMs with this Turorial

No wonder, really.

Tutorial is not only extremely badly written, even worse: it suggests making destructive changes to the system boot process.

All you need to migrate living system to the RAID is:

1. Add enough disks.

2. Create raid partitions on these newly added disks.

3. Assemble arrays with "missing" parts.

4. Format the arrays into chosen FS.

5. Copy (cp --preserve=all) everything to the respective arrays.

6. Install grub to raid. (grub-install --recheck --no-floppy /dev/md0)

Some operations may require to boot from LiveCD.

What to scatter through four pages? I don't see.

By: Raf

Based in your config and your failure scenario, is it possible to replace sda without put /dev/sdb in /dev/sda's place and connect the new HDD as /dev/sdb!)?.

For example, keeping your config (a closer to real life scenario): sda fails, you must reboot the system (for any reason) but you are not able to change sdb to sda connection and get a new replacement disk so you need to reboot with a faulty sda and a working sdb in a degradated md0. What's the best way to handle your grub config, considering it's pointing to sda (md/0 or hd0,1)

By: Edder

For Debian 8 I had to run dpkg-reconfigure mdadm to get RAID support into initrd.

By: rocky

Great tutorial. I got RAID working in a couple of days. What held me up was the outdated modulees. I loaded mdraid1x instead of raid and mdraid and it all just worked!

By: Alexander

I was able to use this to move Debian 10.4 to RAID1, with the following amendments:in grub menuentry had replace insmod raid insmod mdraidwith insmod mdraid1xand /etc/mtab were not save-able, but w/o editing it the guide still seem to workas for /boot/vmlinuzyou possibly did cp -dpRx /boot /mnt/md2which created /boot subfolder inside /bootcd /boot followed by cp -dpRx . /mnt/md2 is in the guide for that reason

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