RAID 5 Parity Failures, Rebuild Stalls, and Metadata Desynchronization
Your RAID 5 is offline.
One drive shows FAILED.
Nothing will mount. The server may not even boot.
Before anything else:
This is the most recoverable RAID 5 failure type — but only if you don’t touch the array.
Most RAID 5 disasters happen after the failure, not because of it.
1. What Actually Happened (Plain English)
RAID 5 can survive exactly one disk failure.
Once that drive dropped:
- The controller lost one parity leg
- The array could no longer verify data integrity
- The system chose offline instead of risking corrupted writes
- BIOS or controller utilities may show incomplete metadata
- Some controllers (PERC, MegaRAID, HP Smart Array) will not auto-preserve state
Your data is still recoverable — but only if the parity math isn’t overwritten.
2. The Critical Point Most Admins Miss
A single bad action can permanently destroy the one surviving parity path, including:
- Replacing the wrong drive
- Importing a “foreign config”
- Running a rebuild on the wrong disk
- Letting Windows or Linux initialize or “repair” the volume
- Adding a new disk that triggers automatic rebuild with mismatched metadata
- Running fsck / chkdsk / xfs_repair / btrfs-check on a degraded RAID 5
One wrong click destroys the only valid parity history the array has left.
3. Why the Array Went Offline Instead of Degraded
Controllers shut RAID 5 arrays offline when:
- The wrong disk is marked bad
- Metadata mismatches exist
- Power-loss corrupted the last parity stripe
- Two disks briefly appeared missing
- A drive reports “predictive failure,” but parity can’t be guaranteed
Offline is a protective move — not a death sentence.
4. What You Should Do Right Now (Safe Actions Only)
1. Power the system down cleanly
If it’s still running degraded, don’t stress the surviving disks.
2. Do NOT:
- Initialize
- Rebuild
- Force mount
- Import foreign configs
- Add or swap drives
- Run filesystem repairs
3. Run JeannieLite™
It will safely read:
- Which drive actually failed
- Logical block size & rotation order
- Parity position & stripe size
- Virtual disk metadata state
- Rebuild readiness
This tells you whether it’s a clean single-disk failure or a latent dual-condition.
4. If you need uptime, call ADR immediately
This is one of the few RAID failures where ADR can often recover:
- without imaging all disks, and
- without taking days offline
Because the parity domain is still mathematically intact as long as nothing overwrites it.
5. When This Becomes Dangerous (The Line You Don’t Want to Cross)
RAID 5 becomes unrecoverable if:
- Rebuilds start on the wrong disk
- A second drive drops during forced imports
- Parity gets overwritten by a failed recovery attempt
- Filesystem repair tools “fix” mismatched parity
If the array won’t mount, the safest path is always:
Clone → Analyze → Reconstruct → Export.
Never rebuild first.
6. What ADR Does to Recover RAID 5 Safely
Our engineering workflow:
- Clone all drives (or only the good ones if metadata is intact)
- Extract controller metadata from all disks
- Determine correct stripe rotation & parity order
- Validate last known parity domain
- Rebuild the array virtually — off the controller
- Export a clean file system image
This avoids destructive on-controller repairs completely.
Diagnostic Overview
- Device: RAID 5 array (single-parity stripe set)
- Observed State: Array offline or degraded after one drive reports failed; volumes may disappear, mount read-only, or show corrupted files
- Likely Cause: Metadata desynchronization, stale or partial parity, rebuild stalls, controller misinterpretation of the failed disk, or foreign-config conflicts
- Do NOT: Initialize disks, force-import foreign configs, attempt on-controller rebuilds, or swap additional drives without a diagnostic map
- Recommended Action: Capture controller state, image all disks, validate stripe order/rotation, reconstruct parity domains offline, and evaluate whether logical volumes can be rebuilt safely
RAID Triage Center – RAID 5 Triage – RAID 5 Technical Notes