Introduction

RAID (Redundant Array of Independent Disks) provides fault tolerance and performance benefits, but even the best setups can experience failures. Understanding how RAID handles disk failures and how to recover from them is crucial for system administrators. In this guide, we will simulate RAID failures using mdadm, analyze failure scenarios, and practice recovery techniques for RAID 0, 1, 5, and 10.

Preparing a RAID Environment

Before simulating failures, ensure you have a working RAID setup. If you don’t already have a RAID array, create one using the guide from our previous article.

Simulating Failures in RAID

RAID 0 (Striping) – Single Disk Failure

RAID 0 offers performance benefits but no redundancy. A single disk failure leads to total data loss.

Failure Simulation:

sudo mdadm --fail /dev/md0 /dev/sdb

Check RAID Status:

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

Expected Outcome:

• The entire RAID array fails, making data recovery impossible.
• If this happens in production, data must be restored from external backups.

RAID 1 (Mirroring) – Single Disk Failure

RAID 1 duplicates data across two disks. If one fails, data remains accessible.

Failure Simulation:

sudo mdadm --fail /dev/md1 /dev/sdb
sudo mdadm --remove /dev/md1 /dev/sdb

Replacing the Failed Disk:

sudo mdadm --add /dev/md1 /dev/sdf  # Replace with a new disk

Expected Outcome:

• The RAID array remains operational.
• The new disk synchronizes automatically to restore redundancy.

RAID 5 (Striping with Parity) – Single Disk Failure

RAID 5 can tolerate one disk failure using parity data.

Failure Simulation:

sudo mdadm --fail /dev/md5 /dev/sdb
sudo mdadm --remove /dev/md5 /dev/sdb

Adding a Replacement Disk and Rebuilding:

sudo mdadm --add /dev/md5 /dev/sdf

Monitor Rebuild Progress:

cat /proc/mdstat

Expected Outcome:

• The RAID array continues to function in degraded mode.
• Once the new disk is added, RAID 5 rebuilds automatically.

RAID 10 (Mirrored Striping) – Multiple Disk Failures

RAID 10 combines RAID 1 and RAID 0, tolerating failures depending on which disks fail.

Failure Simulation:

sudo mdadm --fail /dev/md10 /dev/sdb
sudo mdadm --fail /dev/md10 /dev/sdc

Expected Outcome:

• If both failed disks belong to the same mirrored pair, data is lost.
• If failures occur in separate mirrored pairs, RAID remains operational.

Replacing Failed Disks and Rebuilding:

sudo mdadm --remove /dev/md10 /dev/sdb
sudo mdadm --remove /dev/md10 /dev/sdc
sudo mdadm --add /dev/md10 /dev/sdf
sudo mdadm --add /dev/md10 /dev/sdg

RAID Recovery Best Practices

• Regular Monitoring: Use mdadm --detail to check RAID health.
• SMART Checks: Monitor disk health with smartctl.
• Backups: Always maintain external backups in case of RAID failures.
• Scheduled Scrubbing: Detect errors early with:

  echo check > /sys/block/md0/md/sync_action
  

Conclusion

Simulating RAID failures is essential for understanding how to handle real-world issues. By practicing disk failures and recovery, system administrators can minimize downtime and prevent data loss. Always test in a non-production environment before applying these techniques in live systems.

Would you like to explore RAID performance tuning next? Let us know your thoughts!