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
2. Install mdadm

   yum install mdadm

3. 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

4. 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

5. 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

6. Create the filesystems.

   # mke2fs /dev/md0
   # mke2fs /dev/md1

7. 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>

2. 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

3. 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

4. Re-add all partitions for the new drive

   # mdadm --add /dev/md0 /dev/sdb1
   # mdadm --add /dev/md1 /dev/sdb3

5. Make sure that the drive re-syncs:

   # cat /proc/mdstat