The World According To Vissie

cat /proc/mdstat
cat /proc/partitions
sudo fdisk -l

To get started, find the identifiers for the raw disks that you will be using:

lsblk -o NAME,SIZE,FSTYPE,TYPE,MOUNTPOIN

To create a RAID 5 array with these components, pass them in to the mdadm –create command. You will have to specify the device name you wish to create (/dev/md0 in our case), the RAID level, and the number of devices:

sudo mdadm --create --verbose /dev/md0 --level=5 --raid-devices=4 /dev/sda /dev/sdb /dev/sdc /dev/sdd

The mdadm tool will start to configure the array (it actually uses the recovery process to build the array for performance reasons). This can take some time to complete, but the array can be used during this time. You can monitor the progress of the mirroring by checking the /proc/mdstat file:

cat /proc/mdstat

Output

  Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] 
  md0 : active raid5 sdc[3] sdb[1] sda[0]
        209584128 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/2] [UU_]
        [===>.................]  recovery = 15.6% (16362536/104792064) finish=7.3min speed=200808K/sec
 
  unused devices: <none>

As you can see in the first highlighted line, the /dev/md0 device has been created in the RAID 5 configuration using the /dev/sda, /dev/sdb and /dev/sdc devices. The second highlighted line shows the progress on the build. You can continue the guide while this process completes.

Next, create a filesystem on the array:

sudo mkfs.ext4 -F /dev/md0

Create a mount point to attach the new filesystem:

sudo mkdir -p /mnt/md0

You can mount the filesystem by typing:

sudo mount /dev/md0 /mnt/md0

Check whether the new space is available by typing:

df -h -x devtmpfs -x tmpfs

To make sure that the array is reassembled automatically at boot, we will have to adjust the /etc/mdadm/mdadm.conf file.

Before you adjust the configuration, check again to make sure the array has finished assembling. Because of the way that mdadm builds RAID 5 arrays, if the array is still building, the number of spares in the array will be inaccurately reported:

cat /proc/mdstat

Output

Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] 
md0 : active raid5 sdc[3] sdb[1] sda[0]
      209584128 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3] [UUU]

unused devices: <none>

The output above shows that the rebuild is complete. Now, we can automatically scan the active array and append the file by typing:

sudo mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf

Afterwards, you can update the initramfs, or initial RAM file system, so that the array will be available during the early boot process:

sudo update-initramfs -u

Add the new filesystem mount options to the /etc/fstab file for automatic mounting at boot:

echo '/dev/md0 /mnt/md0 ext4 defaults,nofail,discard 0 0' | sudo tee -a /etc/fstab

Your RAID 5 array should now automatically be assembled and mounted each boot.