====== NAS - Build a Linux NAS - Create a Software RAID Array ======

===== Check the disk partitions to be used in the RAID =====

<code bash>
mdadm --examine /dev/sd[b-d]1

or

mdadm --examine /dev/sdb1 /dev/sdc1 /dev/sdd1
</code>

returns:

<code bash>
mdadm: No md superblock detected on /dev/sdb1
mdadm: No md superblock detected on /dev/sdc1
mdadm: No md superblock detected on /dev/sdd1
</code>

<WRAP info>
**NOTE:**  There is no any defined RAID on the sdb1, sdc1 and sdd1 drives so far, which is why no superblocks are detected.
</WRAP>

----

===== Create a Software RAID Array =====

<code bash>
sudo mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 /dev/sdb1 /dev/sdc1 /dev/sdd1

or 

sudo mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 --spare-devices=1 /dev/sd{b,c,d,e}1
</code>

<WRAP info>
**NOTE:**  This created a new block device **/dev/md0**.

  * This device represents the RAID 5 of /dev/sdb1, /dev/sdc1 and /dev/sdd1.
  * Obviously other RAID types could be used instead.

If the following message is seen: **Device or resource busy**, then reboot the OS.
</WRAP>

----

===== Ensure the RAID Array is reassembled automatically each time the system boots =====

<code bash>
sudo mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf

or

sudo mdadm --detail --scan --verbose | sudo tee -a /etc/mdadm/mdadm.conf
</code>

returns:

<code bash>
ARRAY /dev/md/0 level=raid5 num-devices=3 metadata=1.2 spares=1 name=xenial:0 UUID=c7a2743d:f1e0d872:b2ad29cd:e2bee48c
      devices=/dev/sdb1,/dev/sdc1,/dev/sdd1
</code>

<WRAP info>
**NOTE:**  This saves the RAID configuration in the **/etc/mdadm/mdadm.conf** file.

  * In the background, mdadm will sync the data of these disks.
  * This can take a very long time as it requires reading and writing a lot of data depending on the size of the disks.
</WRAP>

----

===== Check the RAID Details =====

<code bash>
sudo mdadm --detail /dev/md0
</code>

returns:

<code bash>
</code>

<WRAP info>
**NOTE:**  This also displays not only the health, but also the sync status.
</WRAP>


----

===== Check Raid Status =====

<code bash>
cat /proc/mdstat 
</code>

returns:

<code bash>
Personalities : [raid6] [raid5] [raid4] 
md0 : active raid5 sdc1[4] sdd1[3](S) sdb1[1] sda1[0]
      1953520640 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/2] [UU_]
      [>....................]  recovery =  4.3% (42161792/976760320) finish=215.1min speed=72409K/sec
</code>