Linux - how to create RAID arrays in Linux using mdadm

Step 1: Install mdadm

Most Linux distributions don’t come with mdadm pre-installed.

sudo apt update           # Debian/Ubuntu
sudo apt install mdadm

sudo yum install mdadm    # RHEL/CentOS

Verify installation:

mdadm --version

Step 2: Check Available Disks

List all available disks and partitions:

lsblk
sudo fdisk -l

Important: Make sure the disks you plan to use do not have any important data, as creating RAID will erase them.

Step 3: Create Partitions (Optional)

You can use entire disks or create partitions. Example using fdisk:

sudo fdisk /dev/sdb

  • Type n → New partition

  • Type t → Change type → fd (Linux RAID autodetect)

  • Type w → Write changes

Repeat for each disk you want in the array.

Step 4: Create the RAID Array

The syntax for creating a RAID array:

sudo mdadm --create --verbose /dev/md0 --level=<RAID_LEVEL> --raid-devices=<NUM_DEVICES> /dev/sd[b-e]

Example: Create a RAID 5 array with 3 disks /dev/sdb, /dev/sdc, /dev/sdd:

sudo mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 /dev/sdb /dev/sdc /dev/sdd

RAID Levels:

  • 0 → Striping (no redundancy, fast)

  • 1 → Mirroring (full redundancy)

  • 5 → Striping with parity (needs ≥3 disks)

  • 6 → Striping with double parity (needs ≥4 disks)

  • 10 → Mirrored + striped (needs ≥4 disks, multiple of 2)

Step 5: Monitor RAID Creation

Check progress of array creation:

cat /proc/mdstat

You should see syncing status.

Step 6: Create a Filesystem

Once RAID is ready, format it:

sudo mkfs.ext4 /dev/md0

You can use other filesystems like xfs or btrfs.

Step 7: Mount the RAID Array

Create a mount point and mount the array:

sudo mkdir -p /mnt/raid
sudo mount /dev/md0 /mnt/raid

Check:

df -h | grep md0

Step 8: Make RAID Mount Persistent

Add entry in /etc/fstab:

sudo blkid /dev/md0

Copy the UUID, then edit fstab:

sudo nano /etc/fstab

Add:

UUID=<UUID_from_blkid> /mnt/raid ext4 defaults 0 0

Step 9: Save RAID Configuration

This ensures RAID automatically assembles on boot:

sudo mdadm --detail --scan >> /etc/mdadm/mdadm.conf

Step 10: Verify RAID Array

Check detailed status:

sudo mdadm --detail /dev/md0

You should see RAID level, devices, and state (active/syncing).