How To Set Up Software RAID1 On A Running System (Incl. GRUB Configuration) (CentOS 5.3)

Version 1.0
Author: Falko Timme
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This guide explains how to set up software RAID1 on an already running CentOS 5.3 system. The GRUB 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 CentOS 5.3 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, ext3;
  • /dev/sda2: swap;
  • /dev/sda3: / partition, ext3

In the end I want to have the following situation:

  • /dev/md0 (made up of /dev/sda1 and /dev/sdb1): /boot partition, ext3;
  • /dev/md1 (made up of /dev/sda2 and /dev/sdb2): swap;
  • /dev/md2 (made up of /dev/sda3 and /dev/sdb3): / partition, ext3

This is the current situation:

df -h

[[email protected] ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda3             9.1G  1.1G  7.6G  12% /
/dev/sda1             190M   12M  169M   7% /boot
tmpfs                 252M     0  252M   0% /dev/shm
[[email protected] ~]#

fdisk -l

[[email protected] ~]# fdisk -l

Disk /dev/sda: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          25      200781   83  Linux
/dev/sda2              26          90      522112+  82  Linux swap / Solaris
/dev/sda3              91        1305     9759487+  83  Linux

Disk /dev/sdb: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

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:

yum install mkinitrd mdadm

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 /dev/sdb

The output should be as follows:

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

Disk /dev/sdb: 1305 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   *        63    401624     401562  83  Linux
/dev/sdb2        401625   1445849    1044225  82  Linux swap / Solaris
/dev/sdb3       1445850  20964824   19518975  83  Linux
/dev/sdb4             0         -          0   0  Empty
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: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          25      200781   83  Linux
/dev/sda2              26          90      522112+  82  Linux swap / Solaris
/dev/sda3              91        1305     9759487+  83  Linux

Disk /dev/sdb: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *           1          25      200781   83  Linux
/dev/sdb2              26          90      522112+  82  Linux swap / Solaris
/dev/sdb3              91        1305     9759487+  83  Linux
[[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

The number of cylinders for this disk is set to 1305.
There is nothing wrong with that, but this is larger than 1024,
and could in certain setups cause problems with:
1) software that runs at boot time (e.g., old versions of LILO)
2) booting and partitioning software from other OSs
   (e.g., DOS FDISK, OS/2 FDISK)

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

Dear Sir,

Thanks for the tips, but i have 2 problem by follow the step on Centos 6.6:

1- I try to run modprobe multipath but theres an error FATAL: Module multipath not found. How can i fix it ?

2- My /dev/sda is:

/dev/sda1   *           1        1275    10240000   83  Linux/dev/sda2            1275       75198   593785856   83  Linux/dev/sda3           75198      100695   204800000   83  Linux/dev/sda4          100695      121602   167935704    5  Extended/dev/sda5          100695      108854    65536000   82  Linux swap / Solaris/dev/sda6          108854      115228    51200000   83  Linux/dev/sda7          115228      121602    51195904   83  Linux

So my question is, how to change partition type for /dev/sda4 (Extended) ?Please help. TQ