TN-R5-002 — Parity Confidence vs. Hardware Health
0. Purpose and Scope
This Technical Note explains why RAID 5 recovery outcomes are determined by parity confidence, not drive health indicators such as SMART status, controller “Optimal” reports, or surface-level diagnostics.
1. The False Signal of “Healthy Drives”
RAID controllers evaluate hardware health, not data trust.
Indicators such as:
- SMART pass/fail
- No predictive failures
- “Optimal” or “Degraded but OK” states
do not measure whether parity still maps correctly to original data.
2. What Parity Confidence Actually Means
Parity confidence refers to the certainty that:
- All surviving members contain correct data
- Stripe geometry is unchanged
- Parity blocks were written deterministically
Once any of these conditions is violated, redundancy exists mathematically but cannot be trusted logically.
3. Why Hardware Health Becomes Irrelevant
During degraded operation:
- Reads are reconstructed, not read directly
- Writes rewrite parity
- Errors compound silently
A “healthy” drive supplying incorrect data permanently corrupts parity just as effectively as a failed drive.
4. Diagnostic Mismatch
Controllers report:
- Hardware state
Recovery requires: - Logical certainty
These domains do not overlap.
5. Recovery Implication
RAID 5 recovery must begin by assessing parity confidence, not hardware appearance.
Proceeding based on health indicators alone leads to irreversible data loss.