Configure Software Raid in Linux

The term RAID is an acronym for the phrase, Redundant Array of Independent Disks. RAID is a way of combining the storage available across multiple disks and supplying users a single, unified virtual device.
RAID can be used to provide:

• data integrity
• fault tolerance
• improved performance
• greater storage capacity

Configuring Software RAID

Configuring RAID using Linux requires a number of steps that need to be followed carefully. In the tutorial example, we will be configuring RAID 5 using a system with three pre-partitioned hard disks. The partitions to be used are:

/dev/hde1

/dev/hdf2
/dev/hdg1

RAID Partitioning

You first need to identify two or more partitions, each on a separate disk. If you are doing RAID 0 or RAID 5, the partitions should be of approximately the same size, as in this scenario. RAID limits the extent of data access on each partition to an area no larger than that of the smallest partition in the RAID set.

Determining Available Partitions

First use the fdisk -l command to view all the mounted and unmounted filesystems available on your system. You may then also want to use the df -k command, which shows only mounted filesystems but has the big advantage of giving you the mount points too.

These two commands should help you to easily identify the partitions you want to use. Here is some sample output of these commands.

[root@linuxbox tmp]# fdisk -l

Disk /dev/hda: 12.0 GB, 12072517632 bytes
255 heads, 63 sectors/track, 1467 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Disk /dev/hda: 12.0 GB, 12072517632 bytes
255 heads, 63 sectors/track, 1467 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot	Start	End	Blocks	Id	System
/dev/hda1	1	13	104391	83	Linux
/dev/hda2	14	144	1052257+	83	Linux
/dev/hda3	145	209	522112+	82	Linux swap
/dev/hda4	210	1467	10104885	5	Extended
/dev/hda5	210	655	3582463+	83	Linux
...
...
/dev/hda15	1455	1467	104391
[root@linuxbox tmp# df -k
Filesystem	1K-blocks	Used	Available	Use%	Mounted on
/dev/hda2	1035692	163916	819164	17%	/
/dev/hda1	101086	8357	87510	9%	/boot
/dev/hda15	101086	4127	91740	5%	/data1
...
...
...
/dev/hda7	5336664	464228	4601344	10%	/var

Unmount the Partitions

You don't want anyone else accessing these partitions while you are creating the RAID set, so you need to make sure they are unmounted.

[root@linuxbox tmp]# umount /dev/hde1
[root@linuxbox tmp]# umount /dev/hdf2
[root@linuxbox tmp]# umount /dev/hdg1

Prepare The Partitions With FDISK
You have to change each partition in the RAID set to be of type FD (Linux raid autodetect), and you can do this with fdisk. Here is an example using /dev/hde1.

[root@linuxbox tmp]# fdisk /dev/hde
The number of cylinders for this disk is set to 8355.
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):

Use FDISK Help

Now use the fdisk m command to get some help:
Command (m for help): m
...
...
p   print the partition table
q   quit without saving changes
s   create a new empty Sun disklabel
t   change a partition's system id
...
...
Command (m for help):