RAID Triage Center — Real Help When RAID Goes Dark » Technical Note TN-R50-001

0. Purpose and Scope

This Technical Note documents real-world RAID 50 failure behaviors observed across enterprise controllers (Broadcom/LSI MegaRAID, Dell PERC, HPE Smart Array, Adaptec/Areca). It explains why RAID 50 arrays go offline when a single stripe-group is compromised, why rebuilds stall at 0%, and why foreign configurations diverge across groups. It also prescribes safe forensic triage procedures that prevent parity overwrite, stripe-group reinitialization, or irreversible metadata loss.

1. RAID 50 Primer — How Nested Parity + Striping Works

• Architecture: RAID 50 = (RAID 5 group A + RAID 5 group B + …) striped under RAID 0.
• Group Independence: Each RAID 5 subgroup manages its own parity block and rebuild logic.
• Upper-Layer Dependency: RAID 0 requires all groups to respond; one dead group = total array failure.
• Failure Reality: RAID 50 tolerates only one drive failure per subgroup at the same time.
• Stripe Alignment: Both read and parity math depend on groups having identical geometry and sequence.

2. Failure Surfaces That Break Stripe-Group Cohesion

RAID 50’s vulnerability is not “two drive failures” — it is which group loses redundancy first. The parity structure collapses when a subgroup is unreadable, even when all other groups remain healthy.

• Group Asymmetry: One RAID 5 group diverges in epoch, parity order, or block sequence.
• Silent Corruption: Latent sector errors on survivors inside one group stall cross-group reads.
• Cache Expiry / NVRAM Drift: Cache loss causes one group to revert to an older parity epoch.
• Unexpected Reindex: A controller event reorders one group’s metadata, breaking group alignment.
• Foreign Config Split-Brain: One group presents valid NVRAM identifiers while another mismatches.

3. Stripe-Group Loss — Why One Dead Group Takes the Array Offline

Even if only one drive has failed in a single RAID 5 subgroup, the upper RAID 0 layer becomes unreadable.

• Dependency Collapse: RAID 0 cannot synthesize missing sectors from a dead stripe-group.
• Parity Blindness: Upper grids do not know how to rebuild lower RAID 5 groups.
• Fragmented State: Controllers abort I/O when any group returns incomplete stripe data.
• “Healthy but Offline” Behavior: All disks may show “OK”; the array still won’t mount.

4. Cross-Group Rebuild Stalls (0%, 5%, or Immediate Abort)

Rebuild stalls in RAID 50 occur when the controller detects asymmetric metadata or inconsistent parity between subgroups.

• Parity Epoch Conflict: Group B is one epoch older than Group A after an unsafe shutdown.
• Geometry Mismatch: Stripe width or block numbering differs after hot-add events.
• Drive-Order Disagreement: Controllers detect different member orderings between groups.
• Bad Survivors: A “good” disk contains unreadable sectors; rebuild logic aborts.

Effect: Rebuild halts at 0% to prevent overwriting the only remaining valid parity data.

5. Metadata Drift — Why Groups Disagree After Restart

Metadata drift occurs when one RAID 5 subgroup updates parity, cache, or sequence information but the other subgroup(s) do not.

• NVRAM Epoch Drift: One group loses or rolls back cache journal entries.
• Foreign Config Divergence: Each group presents a different layout or epoch ID.
• Background Consistency Check Interruption: One group paused mid-verification.
• Unsafe Hot-Swap: Inconsistent foreign import states are created by staggered drive insertions.

6. “Online but Empty” — Why Data Vanishes After Rebuild

This behavior occurs when RAID 0 is rebuilt while one RAID 5 subgroup is incomplete or partially degraded.

• Virtual Array Rebuild Over Incomplete Group: Controller trusts last-known metadata, not actual parity.
• Incorrect Virtual Mapping: Stripe offsets resolve to blank sectors when one group has invalid blocks.
• Silent Zeroing: Some controllers zero incomplete segments for safety.
• Legacy Metadata Overwrite: Stale superblocks collide with new parity geometry.

Result: The array mounts but directories are empty, partial, or corrupt.

7. Forensic Triage — Safe Order of Operations

These steps preserve group identity, prevent parity overwrite, and ensure complete metadata capture before any rebuild or foreign import.

1. Document: Capture member order, WWN identifiers, and controller logs.
2. Freeze State: Do not import foreign configs; disable background initialization.
3. Clone Survivors: Image all disks including “healthy” members for latent errors.
4. Extract Metadata: Read virtual mapping, stripe width, epoch numbers, and group layouts.
5. Reconstruct Groups: Analyze each RAID 5 subgroup independently.
6. Rebuild Virtually: Combine groups offline in virtual RAID to verify reconstruction.
7. Validate: Confirm cross-group alignment before finalizing or committing any repair.