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.