Debian 9 – mdadm RAID array degraded after reboot – Fix & Prevention

📖 ~1 min read

Table of contents

1. Symptom & Impact
2. Environment & Reproduction
3. Root Cause Analysis
4. Quick Triage
5. Step-by-Step Diagnosis
6. Solution – Primary Fix
7. Solution – Alternative Approaches
8. Verification & Acceptance Criteria
9. Rollback Plan
10. Prevention & Hardening
11. Related Errors & Cross-Refs
12. References & Further Reading

Symptom & Impact

RAID array is degraded, increasing risk of data loss and reducing performance.

Environment & Reproduction

Appears after disk timeouts, cabling faults, or stale mdadm metadata.

cat /proc/mdstat
sudo mdadm --detail /dev/md0
dmesg | tail -n 120

Root Cause Analysis

One or more members marked faulty or missing during array assembly.

Quick Triage

Identify failed disk and check SMART health.

lsblk -o NAME,SIZE,TYPE,MOUNTPOINT
sudo mdadm --examine /dev/sd[a-z]* 2>/dev/null | head -n 120
sudo smartctl -H /dev/sda

Step-by-Step Diagnosis

Correlate kernel errors with mdadm member status.

journalctl -k -n 200 --no-pager
sudo mdadm --detail /dev/md0
sudo blkid

Solution – Primary Fix

Replace failed member and trigger controlled rebuild.

sudo mdadm /dev/md0 --fail /dev/sdb1 --remove /dev/sdb1
sudo mdadm /dev/md0 --add /dev/sdb1
watch -n 10 cat /proc/mdstat

Solution – Alternative Approaches

Reassemble array manually when member ordering changed.

sudo mdadm --assemble --scan
sudo update-initramfs -u

Verification & Acceptance Criteria

Array returns to clean state with all members active.

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

Rollback Plan

Stop rebuild and restore known-good backup if errors escalate.

sudo mdadm --stop /dev/md0
sudo mdadm --assemble --readonly /dev/md0 /dev/sd[ab]1

Prevention & Hardening

Enable SMART and RAID monitoring with alerting.

sudo systemctl enable --now smartd
sudo mdadm --monitor --scan --daemonise

Related Errors & Cross-Refs

Often tied to disk controller resets and power instability.

Related tutorial: View the step-by-step tutorial for Debian 9.

View all Debian 9 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Debian mdadm RAID administration documentation.