Basic usage of Software Raid (MDADM)

Creating and Using Software RAID with mdadm on Debian

Software RAID on Debian is commonly configured using the mdadm utility. This guide provides a step-by-step approach for creating, configuring, and managing a basic RAID array (RAID 0, RAID 1, or RAID 5), suitable for both virtual and dedicated servers.

Prerequisites

• Two or more storage devices (e.g., /dev/sdb, /dev/sdc)
• Root or sudo privileges
• Debian 10, 11, 12 or Ubuntu derivative
• Non-mounted and unformatted target disks

Step 1: Install mdadm

Install the RAID management tool:

<syntaxhighlight lang="bash"> sudo apt update sudo apt install mdadm -y </syntaxhighlight>

During installation, you may be prompted to configure mdadm to email alerts. You can skip or configure as preferred.

Step 2: Identify Available Disks

List all attached storage devices:

<syntaxhighlight lang="bash"> lsblk </syntaxhighlight>

Or use:

<syntaxhighlight lang="bash"> sudo fdisk -l </syntaxhighlight>

Ensure the devices are **unmounted** and **do not contain any important data**.

Step 3: Create the RAID Array

Examples:

RAID 1 (Mirroring, 2 Disks)

<syntaxhighlight lang="bash"> sudo mdadm --create --verbose /dev/md0 --level=1 --raid-devices=2 /dev/sdb /dev/sdc </syntaxhighlight>

RAID 0 (Striping, 2+ Disks)

<syntaxhighlight lang="bash"> sudo mdadm --create --verbose /dev/md0 --level=0 --raid-devices=2 /dev/sdb /dev/sdc </syntaxhighlight>

RAID 5 (Striped with Parity, 3+ Disks)

<syntaxhighlight lang="bash"> sudo mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 /dev/sdb /dev/sdc /dev/sdd </syntaxhighlight>

You may need to confirm with yes to proceed.

Step 4: Monitor the RAID Creation

Check array status:

<syntaxhighlight lang="bash"> cat /proc/mdstat </syntaxhighlight>

To view detailed status:

<syntaxhighlight lang="bash"> sudo mdadm --detail /dev/md0 </syntaxhighlight>

Step 5: Create Filesystem on RAID Array

After creation completes, format the RAID array:

<syntaxhighlight lang="bash"> sudo mkfs.ext4 /dev/md0 </syntaxhighlight>

Step 6: Mount the RAID Array

Create mount point and mount it:

<syntaxhighlight lang="bash"> sudo mkdir /mnt/raid sudo mount /dev/md0 /mnt/raid </syntaxhighlight>

Optional: Set up ownership or permissions:

<syntaxhighlight lang="bash"> sudo chown user:user /mnt/raid </syntaxhighlight>

Step 7: Save mdadm Configuration

Save the array details for boot persistence:

<syntaxhighlight lang="bash"> sudo mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf sudo update-initramfs -u </syntaxhighlight>

Step 8: Automount on Boot

Add entry to /etc/fstab:

<syntaxhighlight lang="bash"> sudo nano /etc/fstab </syntaxhighlight>

Add:

<syntaxhighlight lang="text"> /dev/md0 /mnt/raid ext4 defaults,nofail 0 0 </syntaxhighlight>

Save and close.

Optional: Assemble RAID After Reboot

If RAID is not automatically detected:

<syntaxhighlight lang="bash"> sudo mdadm --assemble --scan </syntaxhighlight>

Optional: Remove or Stop a RAID Array

To stop the array:

<syntaxhighlight lang="bash"> sudo umount /mnt/raid sudo mdadm --stop /dev/md0 </syntaxhighlight>

To remove the array:

<syntaxhighlight lang="bash"> sudo mdadm --remove /dev/md0 </syntaxhighlight>

To zero out superblocks:

<syntaxhighlight lang="bash"> sudo mdadm --zero-superblock /dev/sdb sudo mdadm --zero-superblock /dev/sdc </syntaxhighlight>

Summary Table

Task	Command	Description
Install mdadm	apt install mdadm	Install RAID tool
Create RAID 1	mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdX /dev/sdY	Create mirrored array
Monitor RAID	cat /proc/mdstat	View sync/progress
Format array	mkfs.ext4 /dev/md0	Prepare filesystem
Mount array	mount /dev/md0 /mnt/raid	Use RAID volume
Auto-mount	/etc/fstab	Persistent configuration
Save config	mdadm --detail --scan >> /etc/mdadm/mdadm.conf	Boot-time detection

See Also

• RAID
• mdadm
• Linux file systems
• Disk partitioning