How to create a Raid Device using madadm in linux rhel5 with level 5, 0 and 1

The following article explains  what is Raid, what are important levels and how to install and configure a raid device in a linux system using the software mdadm. This is tested in Redhat rhel5 and also works with other distributions as fedora, centos etc. 


What is RAID?
RAID is redundant array of independent or inexpensive disks. It is mainly used for data protection. It protects our data storage from failures and data loss. All storage units now use  raid technology. 

It has following uses.

1. Data protection
2. Increasing performance

Types of RAIDs:

There are alot of levels of RAIDs. But the main levels are

1. Level 0 or Striping
2. Level 1 or Mirroring
3. Level 5 or Striping + Parity

Level 0:
It is also known as striping. You know hard disk is a block device. Datas are read and written from/to it by blocks. 
Suppose we have data block as below

1 0 1 1

Suppose each bit takes one cpu clock cycles for writing  into a disk. Total, it will take 4 cpu clock cycles.

With stripping:
In striping we use "N" number of hard disks. RAID devides the data block by "N" and writes each part to each disk in parallel.

If we have 4 hard disks, It'll take only one cpu clock  cycle if we use Level 0 RAID.

Raid 0 is best for where more writing take place than reading. But is insecure as there is no recovery mechanisms.

Level 1:
Also known as Mirroring. One is the exact replica of other. Whatever writing to master disk will be written in to mirror disk also. Reading can be performed fro each disk simultaneously, thus increasing the read performance.

But can be utilize only 50% of the total size.

Level 5:
It is a combination of striping and parity. Need at least three hard disks. Both parity and data are distributed in all. If one hard disk fails, data on that can be regenerated by the data and parity information in the other two hard disks.


###RAID###

Raid 5 :need 3 disks
Raid 0 :need 2 disks
Raid 1 :need 2 disks
first partition disks

***RAID 5***

Here we'll show how to create a Level 5 raid device. Here we use three partitions /dev/sda5 /dev/sda6 /dev/sda7. Keep in mind that, in real industry it'll be three different hard disks.

This following command will create a raid device /dev/md0 with level 5
#mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sda{5,6,7}

Formatting the raid device
#mke2fs -j /dev/md0

Creating mount point
#mkdir /raid5

Mounting the raid device to the created mount point
#mount /dev/md0 /raid5

Making the mount permanent by adding it in fstab
#vi /etc/fstab

/dev/md0 /raid5 ext3 defaults 0 0
:wq

#mount -a

Test read and write permissions. And if you want you can check whether the raid is working by failing or deleting any partition it consists of.
touch something in /raid5 and delete one of partitions of sda5,sda6,sda7

This is how we can create a Raid device with level 1
***RAID 1***
#mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sda{5,6}

This is how we can create a Raid device with level 0
***RAID 0***
#mdadm --create /dev/md0 --level=0 --raid-devices=2 /dev/sda{5,6}


Stopping mdadm
*Unmount the md0 before stopping mdadm

#mdadm --stop /dev/md0

If you want to create additional devices[ie there exists a /dev/md0] you may need to add an "-a yes" option to the mdadm command.

For example,
#mdadm --create /dev/md1 -a yes --level=0 --raid-devices=2 /dev/sda{5,6}