Backup & Disk Mirroring
Data can be lost due to hardware failure, human error, malware, or natural disasters. Backups create copies of data for recovery, while disk mirroring provides real-time redundancy to prevent downtime.
Learning Objectives
- 11.1.2.3 Explain data backup and disk mirroring
Conceptual Anchor
The Photocopier Analogy
A backup is like making a photocopy of an important document and storing it in a safe. If the original is lost, you can use the copy. Disk mirroring is like having two secretaries typing the same letter simultaneously — if one makes a mistake or stops, the other has an identical, up-to-date copy.
Rules & Theory
Types of Backup
| Type | What It Copies | Speed | Storage | Recovery |
|---|---|---|---|---|
| Full | All data every time | Slowest | Most space | Fastest — single restore |
| Incremental | Only data changed since last backup (any type) | Fastest | Least space | Slowest — needs full + all incrementals |
| Differential | Data changed since last full backup | Medium | Medium | Medium — needs full + latest differential |
Backup Schedule Example
Monday: Full backup (5 GB) → Tue–Fri: Incremental backups (0.2–0.5 GB each). Total weekly storage: ~7 GB instead of 25 GB for daily full backups. Recovery on Friday requires: Monday's full + Tue + Wed + Thu + Fri incrementals.
Backup Storage Locations
| Location | Advantages | Disadvantages |
|---|---|---|
| Local (external drive) | Fast, no internet needed | Vulnerable to same disaster (fire, flood) |
| Network (NAS/server) | Centralized, automated | Still on-site risk |
| Off-site / Cloud | Safe from local disasters | Slower, requires internet, monthly cost |
Disk Mirroring (RAID 1)
Disk mirroring writes identical data to two (or more) hard drives simultaneously. If one drive fails, the other continues working instantly — no data loss, no downtime.
| Feature | Backup | Disk Mirroring |
|---|---|---|
| When data is copied | Periodically (scheduled) | In real-time |
| Protection against | Data loss (all causes) | Hardware failure (disk crash) |
| Recovery time | Minutes to hours | Instant (automatic failover) |
| Virus protection | Can restore from before infection | No — virus mirrors to both disks |
| Cost | Low (one extra disk) | High (double storage needed) |
RAID Levels (Extra Knowledge)
RAID 0 — Striping (speed, no redundancy) · RAID 1 — Mirroring (redundancy) · RAID 5 — Striping + parity (balance of speed/redundancy) · RAID 10 — Mirror + stripe (high performance + redundancy).
Worked Examples
1 Backup Strategy for a School
Scenario: A school has 50 GB of student records, updated daily.
Recommended strategy:
- Weekly full backup to external NAS server (Sunday night)
- Daily incremental backups (Mon–Sat)
- Monthly off-site backup to cloud storage
- Keep at least 3 generations of full backups (Grandfather-Father-Son)
2 Recovery Scenario
Full backup on Monday. Incremental on Tue, Wed, Thu. System crashes Thursday afternoon.
Recovery steps:
- Restore Monday's full backup
- Apply Tuesday's incremental
- Apply Wednesday's incremental
- Apply Thursday's incremental
If using differential instead: Restore Monday full + Thursday differential only (2 steps).
Common Pitfalls
Mirroring ≠ Backup
Students often think disk mirroring replaces backups. It doesn't! Mirroring protects against hardware failure only. If you accidentally delete a file or get a virus, the deletion/virus is mirrored too. You still need backups.
Confusing Incremental and Differential
Incremental = changes since last backup (of any type). Differential = changes since last full backup. Differential files grow larger each day; incremental files stay small.
Tasks
Define full, incremental, and differential backup. State one advantage of each.
Explain why disk mirroring alone is not sufficient to protect data. What additional measure is needed?
Design a backup strategy for a small business with 20 GB of critical data. Specify backup type, frequency, and storage location. Justify your choices.
Compare the total storage needed and recovery time for a week of incremental vs differential backups given a 10 GB full backup and 500 MB of daily changes.
Self-Check Quiz
Q1: Which backup type copies only files changed since the last full backup?
Q2: What is RAID 1?
Q3: Why should backups be stored off-site?