⚡ RAID Overview

# Install mdadm if not present
sudo dnf install -y mdadm     # RHEL/CentOS
sudo apt install -y mdadm     # Debian/Ubuntu

# Create RAID 1 (mirror) from two disks
sudo mdadm --create /dev/md0 \
    --level=1 \
    --raid-devices=2 \
    /dev/sdb /dev/sdc

# Create RAID 5 from four disks
sudo mdadm --create /dev/md1 \
    --level=5 \
    --raid-devices=4 \
    /dev/sdb /dev/sdc /dev/sdd /dev/sde

# Create RAID 6 — double parity
sudo mdadm --create /dev/md2 \
    --level=6 \
    --raid-devices=6 \
    /dev/sd{b,c,d,e,f,g}

# Create RAID 10 — four disks
sudo mdadm --create /dev/md3 \
    --level=10 \
    --raid-devices=4 \
    /dev/sdb /dev/sdc /dev/sdd /dev/sde

# Confirm creation started (sync in progress)
cat /proc/mdstat

# Format the RAID device like any block device
sudo mkfs.xfs /dev/md0
sudo mkfs.ext4 /dev/md1

# Mount it
sudo mkdir -p /data/raid
sudo mount /dev/md0 /data/raid

# Save mdadm configuration (essential for persistence)
sudo mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf
# or on RHEL:
sudo mdadm --detail --scan | sudo tee -a /etc/mdadm.conf

# Update initramfs so array assembles at boot
sudo update-initramfs -u          # Debian/Ubuntu
sudo dracut --force               # RHEL/CentOS

# Add to /etc/fstab for automatic mount
echo "/dev/md0  /data/raid  xfs  defaults  0 2" | sudo tee -a /etc/fstab

# Verify
sudo mount -a
df -h /data/raid

# Quick status of all arrays
cat /proc/mdstat

# Detailed status of one array
sudo mdadm --detail /dev/md0

# Status of all arrays
sudo mdadm --detail --scan

# Check individual disk status within array
sudo mdadm --examine /dev/sdb

# Watch sync/rebuild progress live
watch -n5 cat /proc/mdstat

# Enable email alerts for RAID events
sudo vi /etc/mdadm/mdadm.conf
# Add: MAILADDR admin@yourdomain.com

# Test email alerting
sudo mdadm --monitor --scan --test --oneshot

# Mark a disk as faulty (simulate failure for testing)
sudo mdadm /dev/md0 --fail /dev/sdb

# Check degraded status
cat /proc/mdstat
sudo mdadm --detail /dev/md0

# Remove the failed disk from the array
sudo mdadm /dev/md0 --remove /dev/sdb

# Physically replace the disk, then add the new disk
sudo mdadm /dev/md0 --add /dev/sdb

# Watch the rebuild
watch -n10 cat /proc/mdstat

A hot spare sits idle in the array and automatically begins rebuilding the moment a disk fails:

# Add a spare disk to an existing array
sudo mdadm /dev/md0 --add /dev/sdd

# Verify spare is listed
sudo mdadm --detail /dev/md0 | grep -A5 "Spare"

# Create array with spare included from the start
sudo mdadm --create /dev/md1 \
    --level=5 \
    --raid-devices=4 \
    --spare-devices=1 \
    /dev/sdb /dev/sdc /dev/sdd /dev/sde /dev/sdf

# Add a new disk to RAID 5 (increases array size)
sudo mdadm /dev/md1 --add /dev/sdf

# Grow the array to use the new disk
sudo mdadm --grow /dev/md1 --raid-devices=5

# Watch the reshape (can take hours on large arrays)
watch -n10 cat /proc/mdstat

# After reshape completes — resize the filesystem
# ext4:
sudo resize2fs /dev/md1

# XFS (must be mounted):
sudo xfs_growfs /data/raid

# Verify
df -h /data/raid

# Stop a RAID array (unmount first)
sudo umount /data/raid
sudo mdadm --stop /dev/md0

# Manually assemble an array from its member disks
sudo mdadm --assemble /dev/md0 /dev/sdb /dev/sdc

# Auto-assemble all arrays from config
sudo mdadm --assemble --scan

# Completely destroy an array (DESTRUCTIVE)
sudo umount /data/raid
sudo mdadm --stop /dev/md0
sudo mdadm --zero-superblock /dev/sdb
sudo mdadm --zero-superblock /dev/sdc
# Now disks are clean — no RAID metadata remains

# Run a periodic array check (checks parity consistency)
sudo echo check > /sys/block/md0/md/sync_action

# Monitor the check progress
cat /proc/mdstat

# Enable weekly automatic checks (already set up on most distros)
cat /etc/cron.d/mdadm        # Debian/Ubuntu
systemctl status mdmonitor   # RHEL/CentOS

# Start mdadm monitor daemon (alerts on failures)
sudo mdadm --monitor --scan --daemonize --mailaddr=admin@example.com

# Check SMART status of member disks
sudo smartctl -a /dev/sdb | grep -E "Health|Reallocated|Pending"
sudo smartctl -a /dev/sdc | grep -E "Health|Reallocated|Pending"

# Full health check script
for dev in /dev/sd{b,c,d,e}; do
    echo "=== $dev ==="
    sudo smartctl -H "$dev" | grep "overall-health"
done