Linux RAID Errors — mdadm Degraded Array and Disk Failure Recovery
About Linux RAID Errors
Fix Linux software RAID (mdadm) errors including degraded arrays, failed disk replacement, RAID rebuild procedures, and monitoring RAID health. This guide covers everything you need to know about this topic, including common causes, step-by-step solutions, and answers to frequently asked questions.
Here are the key things to understand: Linux software RAID uses mdadm to manage RAID arrays (RAID 0, 1, 5, 6, 10). A degraded RAID array has lost a disk but is still operational (RAID 1, 5, 6, 10). RAID 0 has no redundancy — any disk failure means total data loss. RAID rebuild (resync) can take hours to days depending on array size and I/O load. SMART monitoring can predict disk failures before they cause RAID degradation. Understanding these fundamentals will help you diagnose and resolve this issue more effectively.
The most common reasons this occurs include: Hard drive failure causing RAID array to enter degraded state. Disk removed or disconnected during operation. RAID rebuild interrupted by power failure or system crash. Disk experiencing bad sectors causing md to mark it as failed. SATA cable or controller fault causing intermittent disk disconnections. Identifying the root cause is the first step toward finding the right solution.
To resolve this, follow these recommended steps: Check RAID status: cat /proc/mdstat or mdadm --detail /dev/md0. Identify the failed disk: mdadm --detail /dev/md0 shows status of each member. Remove the failed disk: mdadm /dev/md0 --remove /dev/sdX. Replace the physical disk, then partition it identically to the other members. Add the new disk: mdadm /dev/md0 --add /dev/sdY — rebuild starts automatically. Monitor rebuild progress: watch cat /proc/mdstat. If these steps do not resolve the issue, consider consulting additional resources or a qualified professional.
This article is part of our Linux Error Codes collection on Error Codes Wiki. We provide comprehensive, up-to-date information to help you find solutions quickly.
Quick Answer
How long does a RAID rebuild take?
Depends on array size and activity. A 4TB drive can take 8-24 hours. During rebuild, the array is vulnerable — a second disk failure in RAID 5 means data loss. RAID 6 tolerates two failures.
Overview
Fix Linux software RAID (mdadm) errors including degraded arrays, failed disk replacement, RAID rebuild procedures, and monitoring RAID health.
Key Details
- Linux software RAID uses mdadm to manage RAID arrays (RAID 0, 1, 5, 6, 10)
- A degraded RAID array has lost a disk but is still operational (RAID 1, 5, 6, 10)
- RAID 0 has no redundancy — any disk failure means total data loss
- RAID rebuild (resync) can take hours to days depending on array size and I/O load
- SMART monitoring can predict disk failures before they cause RAID degradation
Common Causes
- Hard drive failure causing RAID array to enter degraded state
- Disk removed or disconnected during operation
- RAID rebuild interrupted by power failure or system crash
- Disk experiencing bad sectors causing md to mark it as failed
- SATA cable or controller fault causing intermittent disk disconnections
Steps
- 1Check RAID status: cat /proc/mdstat or mdadm --detail /dev/md0
- 2Identify the failed disk: mdadm --detail /dev/md0 shows status of each member
- 3Remove the failed disk: mdadm /dev/md0 --remove /dev/sdX
- 4Replace the physical disk, then partition it identically to the other members
- 5Add the new disk: mdadm /dev/md0 --add /dev/sdY — rebuild starts automatically
- 6Monitor rebuild progress: watch cat /proc/mdstat