Author: ADR Data Recovery — Advanced RAID & Server Recovery Services
Program: InteliCore Logic™ — Human + Machine Diagnostic Synthesis
0. Purpose and Scope
This Technical Note explains why RAID 6 recoverability collapses once parity confidence is lost, even though two parity symbols exist per stripe.
1. Two-Drive Tolerance vs Two-Symbol Solvability
- Design claim: RAID 6 tolerates any two missing drives.
- Reality: Recovery requires two valid parity symbols per stripe.
- Constraint: Any unreadable survivor sector consumes a parity symbol.
2. How Parity Confidence Is Lost
- Unreadable survivor sectors (UNC)
- Partial parity writes during aborts
- Epoch drift between parity blocks
- Incorrect drive order or offsets
3. Why Controllers Stop Instead of Guessing
- Protection logic: Prevent writing unknown parity.
- Behavior: Rebuild stalls or disks drop offline.
- Misinterpretation: Seen as “controller failure.”
4. Recovery Implication
Once parity confidence collapses, RAID 6 no longer degrades gracefully. Reconstruction must proceed from preserved evidence, not controller rebuilds.