Linux Software RAID with mdadm

mdadm (Multiple Devices Admin) manages Linux software RAID through the kernel's MD driver. It combines two or more physical disks or partitions into virtual block devices (/dev/md0, /dev/md1, …), supporting RAID 0, 1, 5, 6, and 10 for redundancy and performance without dedicated hardware.

RAID improves availability but is not a substitute for backups. See the backup and disaster recovery guide for proper backup strategy.

RAID levels

RAID 10 layouts

The --layout option controls how mirror copies are distributed:

• near (default): copies placed consecutively, classic RAID 1+0
• far: copies placed far apart, improved sequential read performance, slight write penalty
• offset: copies striped consecutively, similar read characteristics to far without as much seeking

Creating arrays

# RAID 0
mdadm --create /dev/md0 --level=0 --raid-devices=2 /dev/sdb1 /dev/sdc1

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

# RAID 5
mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sdb1 /dev/sdc1 /dev/sdd1

# RAID 6
mdadm --create /dev/md0 --level=6 --raid-devices=4 /dev/sd[bcde]1

# RAID 10 with far layout
mdadm --create /dev/md0 --level=10 --layout=f2 --raid-devices=4 /dev/sd[bcde]1

# With hot spare
mdadm --create /dev/md0 --level=5 --raid-devices=3 --spare-devices=1 /dev/sd[bcde]1

# With custom chunk size (default 512 KiB)
mdadm --create /dev/md0 --level=5 --raid-devices=3 --chunk=64 /dev/sd[bcd]1

# With write-intent bitmap
mdadm --create /dev/md0 --level=5 --raid-devices=3 --bitmap=internal /dev/sd[bcd]1

Examining arrays

# Detailed view of an active array
mdadm --detail /dev/md0

# Brief view (suitable for mdadm.conf)
mdadm --detail --brief /dev/md0

# Scan all arrays and output ARRAY lines
mdadm --detail --scan

# Examine a component device's superblock
mdadm --examine /dev/sdb1

# Kernel view of active arrays
cat /proc/mdstat

Example /proc/mdstat output:

Personalities : [raid1] [raid5] [raid6] [raid10]
md0 : active raid5 sda1[0] sdb1[1] sdc1[2]
      209584128 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3] [UUU]
      bitmap: 0/1 pages [0KB], 65536KB chunk
unused devices: <none>

The [UUU] field shows device health: U = up, _ = down/failed.

Monitoring

# Start monitor daemon (scans mdadm.conf)
mdadm --monitor --scan --daemonize

# With email alerts
mdadm --monitor --scan --mail=admin@example.com --daemonize

# Send a test alert
mdadm --monitor --scan --test

# Via systemd
systemctl enable --now mdmonitor.service

For proactive disk health monitoring beyond mdadm's event alerts, see the SMART monitoring guide.

mdadm.conf

The configuration file (/etc/mdadm.conf or /etc/mdadm/mdadm.conf on Debian) tells the initramfs which arrays to assemble at boot. Generate it after every array change:

# Generate ARRAY lines from active arrays
mdadm --detail --scan >> /etc/mdadm.conf

# Update initramfs so boot can find the array
update-initramfs -u     # Debian/Ubuntu
dracut -f               # RHEL/Fedora

A typical file:

MAILADDR root
ARRAY /dev/md0 metadata=1.2 UUID=12345678:12345678:12345678:12345678 name=host:0

Adding, removing, and replacing disks

# Mark a disk as failed
mdadm /dev/md0 --fail /dev/sdb1

# Remove the failed disk
mdadm /dev/md0 --remove /dev/sdb1

# Add a new (or replacement) disk
mdadm /dev/md0 --add /dev/sdb1

# Re-add a previously removed disk
mdadm /dev/md0 --re-add /dev/sdb1

Complete disk replacement procedure

# 1. Identify the failed disk
mdadm --detail /dev/md0
cat /proc/mdstat
smartctl -a /dev/sdb | grep -i serial

# 2. Fail and remove
mdadm /dev/md0 --fail /dev/sdb1
mdadm /dev/md0 --remove /dev/sdb1

# 3. Physically replace the disk

# 4. Copy partition table from a healthy disk
sfdisk -d /dev/sdc | sfdisk /dev/sdb
# For GPT:
sgdisk -R /dev/sdb /dev/sdc
sgdisk -G /dev/sdb          # randomize the disk GUID

# 5. Add the new partition to the array
mdadm /dev/md0 --add /dev/sdb1

# 6. Watch the rebuild
watch cat /proc/mdstat

Growing arrays

# Add a disk, then grow to include it (RAID 5: 3 → 4 disks)
mdadm /dev/md0 --add /dev/sdd1
mdadm --grow /dev/md0 --raid-devices=4 --backup-file=/tmp/grow_backup

# RAID level migration (0 → 5)
mdadm --grow /dev/md0 --level=5 --raid-devices=3

# Change chunk size
mdadm --grow /dev/md0 --chunk=128

Stopping and assembling

# Stop an array
mdadm --stop /dev/md0

# Assemble from explicit devices
mdadm --assemble /dev/md0 /dev/sdb1 /dev/sdc1 /dev/sdd1

# Assemble all arrays from mdadm.conf
mdadm --assemble --scan

# Force assembly of a dirty/degraded array
mdadm --assemble --scan --force

Superblock versions

# Specify at creation time
mdadm --create /dev/md0 --level=1 --raid-devices=2 --metadata=1.0 /dev/sd[bc]1

# Zero out a superblock before reusing a disk
mdadm --zero-superblock /dev/sdb1

RAID 5/6 write hole

The write hole is a fundamental timing problem in parity RAID: updating a stripe requires writing both data and parity blocks, and these are separate writes. If power is lost between them, data and parity become inconsistent. On a degraded array, inconsistent parity cannot be recalculated, leading to silent data corruption during rebuild.

Mitigations

# Write-intent bitmap (tracks dirty chunks for fast recovery)
mdadm --grow /dev/md0 --bitmap=internal

# Write journal (dedicated journal device, RAID 4/5/6)
mdadm --create /dev/md0 --level=5 --raid-devices=3 \
  --write-journal=/dev/sdj1 /dev/sd[abc]1

# Consistency scrub
echo check > /sys/block/md0/md/sync_action
cat /sys/block/md0/md/mismatch_cnt
echo repair > /sys/block/md0/md/sync_action

Write-intent bitmaps

# Add internal bitmap
mdadm --grow /dev/md0 --bitmap=internal

# Add external bitmap on a separate file
mdadm --grow /dev/md0 --bitmap=/raidbitmap.bin

# Remove bitmap
mdadm --grow /dev/md0 --bitmap=none

Partitions vs whole disks

Recommendation: use partitions slightly smaller than full disk capacity (e.g., leave a 100 MB margin) to accommodate size variations between nominally identical disks from different manufacturers.

Many setups layer LVM on top of mdadm arrays for flexible volume management and snapshots. For TRIM support on RAID arrays, see the TRIM and discard guide.

Degraded boot

If the root filesystem is on RAID and a disk is missing at boot, the initramfs may refuse to start a dirty degraded array. Force it from the emergency shell:

# In initramfs emergency shell
mdadm --assemble --scan --force
mdadm --run /dev/md0

Kernel parameter to allow dirty degraded starts:

md-mod.start_dirty_degraded=1

Command quick reference