📘 11-04: Fault Tolerance & Recovery¶
📌 RAID (Storage Redundancy)¶
| Level | Method | Fault tolerance | Notes |
|---|---|---|---|
| RAID 0 | Striping | None | Speed only |
| RAID 1 | Mirroring | 1 drive | 50% usable capacity |
| RAID 5 | Striping + parity | 1 drive | Needs 3+ drives |
| RAID 6 | Double parity | 2 drives | Needs 4+ drives |
| RAID 10 | Mirror + stripe | Multiple | Speed + redundancy |
RAID protects against drive failure — it is not a backup.
📌 Power Protection¶
| Device | Role |
|---|---|
| UPS | Battery backup for short outages + power conditioning |
| Generator | Long-term power during extended outages |
| PDU | Distributes power in a rack |
📌 Backups¶
| Type | Backs up | Restore |
|---|---|---|
| Full | Everything | Just the full |
| Incremental | Changes since last backup | Full + all incrementals |
| Differential | Changes since last full | Full + latest differential |
3-2-1 rule: 3 copies, 2 media types, 1 offsite (offline/immutable). Test restores — an untested backup may be unrecoverable.
📌 Disaster Recovery Sites¶
| Site | Readiness | Cost | Recovery time |
|---|---|---|---|
| Hot | Fully running, current data | Highest | Near-instant |
| Warm | Equipped, needs data/activation | Medium | Hours |
| Cold | Space/power only | Lowest | Days |
RTO = target restore time; RPO = acceptable data loss.
Tip
Key idea — RAID survives drive loss (not a backup). Protect power with UPS/generator. Follow 3-2-1 backups and pick a DR site (hot/warm/cold) to meet your RTO/RPO.
See also: Policies, Physical Security & Continuity, High Availability & Redundancy
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