TN-R5-004 — Why Parity Checks Become Destructive in Degraded RAID 5
0. Purpose and Scope
This note documents why consistency and parity checks destroy recoverability once parity confidence is compromised.
1. Assumptions Parity Checks Make
Parity checks assume:
- Correct stripe geometry
- Correct member order
- Correct data on surviving disks
These assumptions are often false after a failure.
2. What Parity Checks Actually Do
Parity checks:
- Compare computed parity to stored parity
- Rewrite parity to “fix” mismatches
They do not verify correctness — they enforce agreement.
3. Destructive Outcome
When incorrect data is treated as authoritative:
- Correct parity is overwritten
- Reconstruction paths are erased
- Data loss becomes permanent
4. Key Insight
Parity checks are write operations, not diagnostics.
Running them without parity certainty accelerates failure.