= Adding Software RAID under Linux =
'''Note:''' The following instructions and examples are for taking 2 hard drives and creating 2 RAID-1 arrays. Please adjust your command accordingly.

 1. Make sure that the Linux can see the disks
 1. Install mdadm
 {{{
yum install mdadm
}}}

 1. Partition each disk into the required sizes. That is, if you plan to have the array create 2 arrays, you will need to partition EACH drive into two. Make sure the drive partitions are of equal size.
 {{{
# fdisk /dev/sda
# fdisk /dev/sdb
}}}

 1. Change the system type (a.k.a. system id) of each partition to be 0xFD (Linux RAID auto-detect). Make sure you write the changes to the partition table
 {{{
# fdisk /dev/sda
# fdisk /dev/sdb}}}

 1. Create the arrays.
 {{{
# mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sda1 /dev/sdb1
# mdadm --create /dev/md1 --level=1 --raid-devices=2 /dev/sda2 /dev/sdb2}}}

 1. Create the filesystems.
 {{{
# mke2fs /dev/md0
# mke2fs /dev/md1}}}

 1. Reboot the system to check that the arrays are started at boot-time.

= Restoring an array after it fails =

 1. Boot up the system and check the arrays are up and running (in degraded mode)
 {{{
# cat /proc/mdstat
Personalities : [raid1] 
md1 : active raid1 sda3[0]
      466890624 blocks [2/1] [U_]
      
md0 : active raid1 sda1[0]
      19534912 blocks [2/1] [U_]
      
unused devices: <none>}}}
 1. Make sure the new drive is detected (sdb in this case)
 {{{
# fdisk -l

Disk /dev/sda: 500.1 GB, 500107862016 bytes
255 heads, 32 sectors/track, 119702 cylinders
Units = cylinders of 8160 * 512 = 4177920 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1        4788    19535024   fd  Linux raid autodetect
/dev/sda2            4789        5268     1958400   82  Linux swap / Solaris
/dev/sda3            5269      119702   466890720   fd  Linux raid autodetect

Disk /dev/sdb: 500.1 GB, 500107862016 bytes
255 heads, 32 sectors/track, 119702 cylinders
Units = cylinders of 8160 * 512 = 4177920 bytes

Disk /dev/sdb doesn't contain a valid partition table

Disk /dev/md0: 20.0 GB, 20003749888 bytes
2 heads, 4 sectors/track, 4883728 cylinders
Units = cylinders of 8 * 512 = 4096 bytes

Disk /dev/md0 doesn't contain a valid partition table

Disk /dev/md1: 478.0 GB, 478095998976 bytes
2 heads, 4 sectors/track, 116722656 cylinders
Units = cylinders of 8 * 512 = 4096 bytes

Disk /dev/md1 doesn't contain a valid partition table}}}
 1. Copy the partition table to the new drive (we will copy sda to sdb in this example)
 {{{
# dd if=/dev/sda of=/dev/sdb bs=1024 count=1
# partprobe /dev/sdb}}}
 1. Re-add all partitions for the new drive
 {{{
# mdadm --add /dev/md0 /dev/sdb1
# mdadm --add /dev/md1 /dev/sdb3}}}
 1. Make sure that the drive re-syncs:
 {{{
# cat /proc/mdstat}}}