mdadm Command Linux: Complete Guide to Managing Software RAID Arrays

The mdadm (Multiple Device Administrator) command is a powerful Linux utility for managing software RAID arrays. It provides comprehensive functionality for creating, assembling, monitoring, and managing RAID configurations without requiring specialized hardware controllers.

Understanding mdadm and Software RAID

Software RAID uses the operating system and CPU to manage redundancy and performance across multiple storage devices. The mdadm command serves as the primary interface for Linux’s MD (Multiple Device) driver, enabling administrators to implement various RAID levels including RAID 0, 1, 4, 5, 6, and 10.

Installation and Prerequisites

Most Linux distributions include mdadm by default. If not installed, use your package manager:

# Ubuntu/Debian
sudo apt update && sudo apt install mdadm

# CentOS/RHEL/Fedora
sudo yum install mdadm
# or
sudo dnf install mdadm

Basic mdadm Syntax

The general syntax for mdadm follows this pattern:

mdadm [mode] [options] <device> [component-devices...]

Common modes include:

• --create – Create a new RAID array
• --assemble – Assemble existing RAID components
• --manage – Manage existing arrays
• --monitor – Monitor RAID arrays
• --detail – Display array information

Creating RAID Arrays

RAID 0 (Striping)

RAID 0 combines multiple drives for improved performance but offers no redundancy:

sudo mdadm --create /dev/md0 --level=0 --raid-devices=2 /dev/sdb /dev/sdc

Expected Output:

mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md0 started.

RAID 1 (Mirroring)

RAID 1 mirrors data across drives for redundancy:

sudo mdadm --create /dev/md1 --level=1 --raid-devices=2 /dev/sdd /dev/sde

Expected Output:

mdadm: Note: this array has metadata at the start of the device.
mdadm: largest drive (/dev/sdd) exceeds size (1953383424K) by more than 1%
mdadm: continuing anyway.
mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md1 started.

RAID 5 (Striping with Parity)

RAID 5 requires at least three drives and provides both performance and redundancy:

sudo mdadm --create /dev/md2 --level=5 --raid-devices=3 /dev/sdf /dev/sdg /dev/sdh

Expected Output:

mdadm: Defaulting to version 1.2 metadata
mdadm: array /dev/md2 started.

RAID 10 (Nested RAID)

RAID 10 combines mirroring and striping, requiring at least four drives:

sudo mdadm --create /dev/md3 --level=10 --raid-devices=4 /dev/sdi /dev/sdj /dev/sdk /dev/sdl

Managing Existing RAID Arrays

Viewing Array Status

Check the status of all RAID arrays:

cat /proc/mdstat

Sample Output:

Personalities : [raid1] [raid6] [raid5] [raid4] [raid10] 
md1 : active raid1 sde[1] sdd[0]
      1953383424 blocks super 1.2 [2/2] [UU]
      
md0 : active raid0 sdc[1] sdb[0]
      3906766848 blocks super 1.2 512k chunks
      
unused devices: <none>

Detailed Array Information

Get comprehensive information about a specific array:

sudo mdadm --detail /dev/md1

Sample Output:

/dev/md1:
           Version : 1.2
     Creation Time : Mon Aug 25 10:15:32 2025
        Raid Level : raid1
        Array Size : 1953383424 (1.8 TiB 2.00 TB)
     Used Dev Size : 1953383424 (1.8 TiB 2.00 TB)
      Raid Devices : 2
     Total Devices : 2
       Persistence : Superblock is persistent

       Update Time : Mon Aug 25 10:20:15 2025
             State : clean
    Active Devices : 2
   Working Devices : 2
    Failed Devices : 0
     Spare Devices : 0

Consistency Policy : resync

              Name : server:1
              UUID : 12345678:90abcdef:12345678:90abcdef
            Events : 17

    Number   Major   Minor   RaidDevice State
       0       8       48        0      active sync   /dev/sdd
       1       8       64        1      active sync   /dev/sde

Adding Spare Drives

Add a spare drive to an existing array:

sudo mdadm --manage /dev/md1 --add /dev/sdz

Expected Output:

mdadm: added /dev/sdz

Removing Failed Drives

Mark a drive as failed and remove it:

sudo mdadm --manage /dev/md1 --fail /dev/sde --remove /dev/sde

Expected Output:

mdadm: set /dev/sde faulty in /dev/md1
mdadm: hot removed /dev/sde from /dev/md1

Array Assembly and Auto-Start

Manual Assembly

Assemble an existing RAID array:

sudo mdadm --assemble /dev/md1 /dev/sdd /dev/sde

Auto-Assembly Configuration

Create a configuration file for automatic assembly:

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

Sample Configuration Entry:

ARRAY /dev/md1 metadata=1.2 name=server:1 UUID=12345678:90abcdef:12345678:90abcdef

Monitoring and Maintenance

Real-time Monitoring

Monitor arrays for events and send email notifications:

sudo mdadm --monitor [email protected] --delay=1800 /dev/md1

Checking Array Health

Perform a consistency check on a RAID array:

echo check > /sys/block/md1/md/sync_action

Monitor the progress:

cat /sys/block/md1/md/sync_completed

Repair Operations

Initiate a repair operation:

echo repair > /sys/block/md1/md/sync_action

Advanced mdadm Operations

Growing Arrays

Add devices to expand an existing RAID 5 array:

sudo mdadm --grow /dev/md2 --raid-devices=4 --add /dev/sdn

Reshaping Arrays

Change the chunk size of a RAID array:

sudo mdadm --grow /dev/md0 --chunk=128

Converting RAID Levels

Convert RAID 1 to RAID 5 (requires adding devices first):

sudo mdadm --grow /dev/md1 --level=5 --raid-devices=3 --add /dev/sdx

Stopping and Removing Arrays

Stopping an Array

Unmount and stop a RAID array:

sudo umount /dev/md1
sudo mdadm --stop /dev/md1

Complete Removal

Zero the superblock to completely remove RAID metadata:

sudo mdadm --zero-superblock /dev/sdd /dev/sde

Troubleshooting Common Issues

Recovering from Boot Issues

If the system fails to boot due to RAID issues, use a rescue environment:

# Scan for arrays
sudo mdadm --assemble --scan

# Force assembly if degraded
sudo mdadm --assemble --force /dev/md1 /dev/sdd

Handling Dirty Arrays

Force assembly of a “dirty” array that wasn’t cleanly unmounted:

sudo mdadm --assemble --force /dev/md1

Replacing Failed Drives

Complete process for replacing a failed drive in RAID 1:

# Mark as failed and remove
sudo mdadm --manage /dev/md1 --fail /dev/sde --remove /dev/sde

# Add replacement drive
sudo mdadm --manage /dev/md1 --add /dev/sdx

# Monitor rebuild progress
watch cat /proc/mdstat

Performance Optimization

Chunk Size Considerations

Choose appropriate chunk sizes based on usage patterns:

• 64KB-128KB for database workloads
• 256KB-512KB for general file storage
• 1MB+ for large sequential files

Read-ahead Settings

Optimize read-ahead for better performance:

sudo blockdev --setra 4096 /dev/md1

Best Practices and Security

Regular Monitoring

Set up automated monitoring with cron jobs:

# Add to crontab
0 2 * * 0 /sbin/mdadm --monitor --oneshot --scan

Backup Configuration

Always maintain backups of your mdadm configuration:

sudo cp /etc/mdadm/mdadm.conf /etc/mdadm/mdadm.conf.backup

Testing Procedures

Regular testing ensures RAID reliability:

# Simulate drive failure
sudo mdadm --manage /dev/md1 --set-faulty /dev/sde

# Verify array continues operating
df -h /mount/point

# Replace and verify rebuild
sudo mdadm --manage /dev/md1 --remove /dev/sde --add /dev/sdx

Integration with File Systems

Creating File Systems

Format RAID arrays with appropriate file systems:

# For general use
sudo mkfs.ext4 /dev/md1

# For high-performance applications
sudo mkfs.xfs /dev/md1

Mount Configuration

Add RAID arrays to /etc/fstab for automatic mounting:

/dev/md1 /data ext4 defaults,noatime 0 2

The mdadm command provides robust software RAID functionality for Linux systems, offering enterprise-level storage management capabilities. Regular monitoring, proper configuration, and understanding of RAID principles ensure reliable data protection and optimal performance. Whether implementing simple mirroring or complex multi-level arrays, mdadm delivers the tools necessary for comprehensive storage management in Linux environments.