mdadm (multiple device administration) — Software RAID configuration and management

Linux / File and disk management / mdadm.html

A tool for creating, growing and monitoring software RAID arrays with the Linux md driver.

Basic Information

What is this command?

mdadm combines several block devices into RAID 0, 1, 5, 6 or 10 arrays. Array status appears in /proc/mdstat, and mirrors or parity keep data available when one disk fails.

Array metadata is stored in /etc/mdadm/mdadm.conf so it can be assembled at boot. After replacing a disk, use --add and --grow to rebuild and expand.

What problems does it solve?

• You want to combine several disks into one volume
• You need a mirror that survives a disk failure
• You are replacing a failed disk and rebuilding
• You need array status and rebuild progress

Key features

Create an array

--create builds an array from a level and a device list.

• When to use: You are building a new RAID set
• What you get: An array device such as /dev/md0
• Limitations: Existing data on the devices is erased.

Monitor status

--detail and /proc/mdstat show members and health.

• When to use: During routine checks or incident response
• What you get: Per-device state and rebuild progress
• Limitations: Without a monitoring daemon you get no alerts.

Replace and grow

Mark a disk failed, remove it, add the new one, then grow if needed.

• When to use: Replacing a failed disk or adding capacity
• What you get: A rebuilt array with more capacity
• Limitations: Performance drops significantly during a rebuild.

Installation and usage

Check whether it is installed

mdadm --version

sudo apt install mdadm # Debian/Ubuntu sudo dnf install mdadm # Fedora

Real-world examples

Create a RAID 1 mirror

Combine two devices into a mirrored array.

sudo mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb1 /dev/sdc1

Check status

Show array details and kernel status.

sudo mdadm --detail /dev/md0
cat /proc/mdstat

Save configuration

Record the array so it is assembled automatically at boot.

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