Creating a RAID5 NAS with My Raspberry Pi 2B

My Raspberry Pi 2B has been unused since I bought it in 2015. 

After 4 years, I have also accumulated 3 x 2TB external SATA HDDs (2 of which are from someone who has moved on to using cloud storage, and 1 from my Alienware Aurora R5).

I have not given up on my pi and to satisfy my tinkering itch, I thought to myself: "Why not build a NAS?". And so I did. What follows are steps which I borrowed by sifting through information on the internet (as I am not the first one to do this). This link shows how the author had done it for 3 x 3TB SATA HDDs.

sudo apt-get update

sudo apt-get install mdadm

3 disks is the minimum required count for a RAID5. I'm not sure if having different sizes will work but fortunately, all my disks are 2TBs.

To identify the disks, one can do

cat /proc/partitions

or

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

My 3 disks were identified as sda1, sdb1, sdc2. Preparing them for RAID use requires some partitioning. Here's the sequence of commands:

sudo parted

select /dev/sda

mklabel gpt

mkpart primary ext4 1049KB 2TB

select /dev/sdb

mklabel gpt

mkpart primary ext4 1049KB 2TB

select /dev/sdc

mklabel gpt

mkpart primary ext4 1049KB 2TB

quit

After which, the RAID5 can now be created (note the double dashes):

sudo mdadm --create --verbose --force --assume-clean /dev/md0 --level=5 --raid-devices=3 /dev/sda1 /dev/sdb1 /dev/sdc1

To confirm that they are running (note that this command is also useful to know whether the RAID is active after a reboot):

cat /proc/mdstat

Create the ext4 filesystem with (it takes a while to complete as this formats the disks):

sudo mkfs.ext4 -F /dev/md0

Let's create a directory which will be used later as mountpoint:

sudo mkdir -p /mnt/raid5

And mount with:

sudo mount /dev/md0 /mnt/raid5

To check if everything is ok, issue the the following (note that if lost+found is missing, then there's something wrong)

ls -al /mnt/raid5

To check capacity:

df -h -x devtmpfs -x tmpfs

Save the configuration so it starts up at boot time automatically:

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

Raspberry pi uses ramdisk when booting up so we want to include the RAID:

sudo update-initramfs -u

Add the RAID to the filesystem table so it will be mounted automatically when booting up:

echo '/dev/md0 /mnt/raid5 ext4 defaults,nobootwait,nofail 0 0' | sudo tee -a /etc/fstab

Cross your fingers and issue:

sudo reboot

Log back to your pi and again check if lost+found is present:

ls /mnt/raid5

The best thing about experimentation is learning. What do you do when things go out of hand? How do you fix stuff?

(1) Check the state of your RAID:

cat /proc/mdstat

(2) To stop your RAID:

sudo mdadm --stop /dev/md0

(3.1) If your raid doesn't turn up one day first check the result of (take note most specially the /dev/md***):

sudo mdadm --examine --scan 

(3.2) Append the result to the config:

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

(3.3) Edit the entries in your /etc/fstab with the result of (3.1)
(3.4) Reassemble 

sudo mdadm --assemble --scan --force -v /dev/md** /dev/sda1 /dev/sdb1 /dev/sdc1

(4) Identify the disks you have:

cat /proc/partitions





QED