How To Configure RAID 1 on a LVM System

 This article explains how to set up software RAID1 on an already running LVM  system (Linux Mint 13 Maya). 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). 

In this tutorial I'm using a Linux Mint 13 Maya 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 (this is the default Linux Mint LVM partitioning scheme

• /dev/sda1: /boot partition, ext4;
• /dev/sda2: extended, contains /dev/sda5;
• /dev/sda5: is used for LVM (volume group test-linuxhowto.in) and contains / (volume root) and swap (volume swap_1).

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/sda5 and /dev/sdb5): LVM (volume group test-linuxhowto.in), contains / (volume root) and swap (volume swap_1).

This is the current situation:

root@test-linuxhowto.in:~# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/ test-linuxhowto.in -root
                      4.5G  720M  3.6G  17% /
tmpfs                 249M     0  249M   0% /lib/init/rw
udev                  244M  104K  244M   1% /dev
tmpfs                 249M     0  249M   0% /dev/shm
/dev/sda1             228M   16M  201M   8% /boot

root@test-linuxhowto.in:~# 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: 0x0003717c

    Device Boot  Start  End   Blocks  Id  System
 /dev/sda1   *    1      32   248832  83  Linux 
Partition 1 does not end on cylinder boundary.
 /dev/sda2       32     653   4990977  5  Extended
Partition 2 does not end on cylinder boundary.
 /dev/sda5       32     653  4990976  8e  Linux LVM

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

Disk /dev/dm-0: 4844 MB, 4844421120 bytes
255 heads, 63 sectors/track, 588 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/dm-0 doesn't contain a valid partition table

Disk /dev/dm-1: 264 MB, 264241152 bytes
255 heads, 63 sectors/track, 32 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/dm-1 doesn't contain a valid partition table
root@test-linuxhowto.in:~#

root@test-linuxhowto.in:~# pvdisplay
  --- Physical volume ---
  PV Name               /dev/sda5
  VG Name               test-linuxhowto.in
  PV Size               4.76 GiB / not usable 2.00 MiB
  Allocatable           yes (but full)
  PE Size               4.00 MiB
  Total PE              1218
  Free PE               0
  Allocated PE          1218
  PV UUID               485485430-438954w8493-jfdjfsrefe-9949r3

 root@test-linuxhowto.in:~# vgdisplay
  --- Volume group ---
  VG Name               test-linuxhowto.in
  System ID
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  3
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                2
  Open LV               2
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               4.76 GiB
  PE Size               4.00 MiB
  Total PE              1218
  Alloc PE / Size       1218 / 4.76 GiB
  Free  PE / Size       0 / 0
  VG UUID               fajfdfj9-dae9a9dw92-394qerqjwdq-2rqe

 root@test-linuxhowto.in:~# lvdisplay
  --- Logical volume ---
  LV Name                /dev/test-linuxhowto.in/root
  VG Name                test-linuxhowto.in
  LV UUID                fdjfioasdfjsa9r-38e473q84qh-32e4qre-eqewr8q3
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                4.51 GiB
  Current LE             1155
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           254:0

  --- Logical volume ---
  LV Name                /dev/test-linuxhowto.in/swap_1
  VG Name                test-linuxhowto.in
  LV UUID                fadhfadhfsar-39qruq9rq-rqefjqefjq-r329rqr3
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                252.00 MiB
  Current LE             63
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           254:1

root@test-linuxhowto.in:~#

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

root@test-linuxhowto.in:~# cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
unused devices: <none>
root@test-linuxhowto.in:~#

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:

root@test-linuxhowto.in:~# 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
Warning: extended partition does not start at a cylinder boundary.
DOS and Linux will interpret the contents differently.
New situation:
Units = sectors of 512 bytes, counting from 0

   Device Boot    Start       End   #sectors  Id  System
/dev/sdb1   *      2048    499711     497664  83  Linux
/dev/sdb2        501758  10483711    9981954   5  Extended
/dev/sdb3             0         -          0   0  Empty
/dev/sdb4             0         -          0   0  Empty
/dev/sdb5        501760  10483711    9981952  8e  Linux LVM
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).)
root@test-linuxhowto.in:~#

The command fdisk -l should now show that both HDDs have the same layout:

root@test-linuxhowto.in:~# 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: 0x0003717c

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          32      248832   83  Linux
Partition 1 does not end on cylinder boundary.
/dev/sda2              32         653     4990977    5  Extended
Partition 2 does not end on cylinder boundary.
/dev/sda5              32         653     4990976   8e  Linux LVM

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          32      248832   83  Linux
Partition 1 does not end on cylinder boundary.
/dev/sdb2              32         653     4990977    5  Extended
Partition 2 does not end on cylinder boundary.
/dev/sdb5              32         653     4990976   8e  Linux LVM

Disk /dev/dm-0: 4844 MB, 4844421120 bytes
255 heads, 63 sectors/track, 588 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/dm-0 doesn't contain a valid partition table

Disk /dev/dm-1: 264 MB, 264241152 bytes
255 heads, 63 sectors/track, 32 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/dm-1 doesn't contain a valid partition table
root@test-linuxhowto.in:~#

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

root@test-linuxhowto.in:~# 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 (1-5): <-- 1
Hex code (type L to list codes): <-- L

  
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 (1-5): <-- 5
Hex code (type L to list codes): <-- fd
Changed system type of partition 5 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.
root@test-linuxhowto.in:~#

The command fdisk -l should now show that /dev/sdb1 and /dev/sdb5 are of the type Linux raid autodetect:

root@test-linuxhowto.in:~# 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: 0x0003717c

    Device Boot  Start    End      Blocks   Id  System
 /dev/sda1   *    1        32      248832   83  Linux
Partition 1 does not end on cylinder boundary.
 /dev/sda2        32      653     4990977    5  Extended
Partition 2 does not end on cylinder boundary.
 /dev/sda5        32      653     4990976   8e  Linux LVM

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          32      248832   fd  Linux raid autodetect
Partition 1 does not end on cylinder boundary.
/dev/sdb2              32         653     4990977    5  Extended
Partition 2 does not end on cylinder boundary.
 /dev/         32         653     4990976   fd  Linux raid autodetect

Disk /dev/dm-0: 4844 MB, 4844421120 bytes
255 heads, 63 sectors/track, 588 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/dm-0 doesn't contain a valid partition table

Disk /dev/dm-1: 264 MB, 264241152 bytes
255 heads, 63 sectors/track, 32 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/dm-1 doesn't contain a valid partition table
root@test-linuxhowto.in:~#

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/sdb5

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):

root@test-linuxhowto.in:~# mdadm --zero-superblock /dev/sdb1
mdadm: Unrecognised md component device - /dev/sdb1
root@test-linuxhowto.in:~#

Otherwise the commands will not display anything at all.

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