Know and understand characteristics of backing storage

Backing (Secondary) Storage – Cambridge IGCSE 0417

Objective

Know and understand the characteristics of backing (secondary) storage, its uses, media, advantages/disadvantages and how it differs from internal (primary) memory.

1. What is Backing Storage and Why Is It Needed?

  • Backing storage (also called secondary storage) is non‑volatile memory that retains data when the computer is powered off.

  • It provides the large‑capacity, permanent repository for operating‑system files, applications, user data and backups that cannot be held in the limited, volatile primary memory (RAM).

  • In a typical system the hierarchy is: CPU → Cache → RAM (primary) → Backing storage (secondary) → Archive/Cloud.

2. Families of Backing Storage (Syllabus 1.3 – Section 3)

  • Magnetic storage – Hard‑disk drives (HDD), Magnetic tape (LTO)

  • Optical storage – CD, DVD, Blu‑ray

  • Solid‑state storage – Solid‑state drives (SSD), Flash memory (USB sticks, SD cards)

3. Key Characteristics – Direct Syllabus Mapping (Section 1.2)

  • Capacity – amount of data that can be stored (MB, GB, TB).

  • Access speed – how quickly data can be read or written (MB s⁻¹, IOPS).

  • Volatility – non‑volatile means data is retained without power; volatile means data is lost when power is removed.

  • Durability / reliability – resistance to shock, wear, magnetic fields and data corruption.

  • Cost per GB – economic factor influencing the choice of media.

  • Portability – ease of moving the device between computers.

  • Access method – random (any block can be accessed directly) vs. sequential (data must be read in order, e.g., tape).

4. Advantages & Disadvantages – Consolidated Table (Syllabus 3)

FamilyAdvantages (per syllabus)Disadvantages (per syllabus)
Magnetic

• High capacity, low cost per GB

• Well‑established technology, widely compatible

• Mechanical parts → wear, shock‑sensitive

• Slower access than solid‑state

• Vulnerable to magnetic fields

Optical

• Portable, inexpensive for small amounts

• Long shelf‑life (archival grade)

• Read‑only versions protect data from accidental overwrite

• Low capacity compared with HDD/SSD

• Slower read/write speeds

• Susceptible to scratches, heat and sunlight

Solid‑state

• Very fast random and sequential access

• No moving parts → shock‑resistant

• Low power consumption

• Higher cost per GB

• Limited write‑endurance (TBW)

• Data retention can degrade if left unpowered for many years

5. RAM vs. Backing Storage – Quick Comparison (Syllabus 1.3)

FeatureRAM (Primary)HDD (Magnetic)SSD (Solid‑state)Optical DiscMagnetic TapeFlash (USB/SD)
VolatilityVolatile – data lost when power offNon‑volatileNon‑volatileNon‑volatileNon‑volatileNon‑volatile
Access methodRandom, nanosecond latencyRandom, millisecond latency (mechanical)Random, microsecond latencyRandom (read‑only), slower than HDD/SSDSequential onlyRandom, similar to SSD but slower
Typical capacity4 GB – 128 GB500 GB – 8 TB120 GB – 4 TBCD ≈ 700 MB, DVD ≈ 4.7 GB, Blu‑ray ≈ 25 GB10 TB – 30 TB (uncompressed)16 GB – 1 TB
Typical speed10 GB s⁻¹+80–200 MB s⁻¹ (seq.)300–3 500 MB s⁻¹ (seq.), 50 k–100 k IOPS (rand.)5–30 MB s⁻¹ (read)≈ 300 MB s⁻¹ (streaming)100–400 MB s⁻¹
DurabilitySensitive to power loss; no moving partsMechanical wear, shock‑sensitiveNo moving parts, limited write‑enduranceResistant to shock if stored properly; can degrade over decadesVery durable for long‑term archive (30 + years)Portable, but easy to lose or damage
Cost per GB (approx.)~ $0.10 (high‑performance DDR4/5)~ $0.04~ $0.12~ \$0.20 (CD/DVD), ~ \$0.30 (Blu‑ray)~ $0.02~ $0.10
PortabilityFixed inside computerUsually internal; external versions existUsually internal; external versions existHighly portable (discs)Low portability (large cartridges)Very portable (sticks, cards)

6. Detailed Overview of Each Backing‑Storage Type (Syllabus 3)

6.1 Hard‑disk Drive (HDD) – Magnetic

  • Capacity: 500 GB – 8 TB (typical desktop/server)

  • Speed: 80–200 MB s⁻¹ sequential; random access limited by spindle rotation (5‑15 ms latency)

  • Durability: Mechanical wear, shock‑sensitive, vulnerable to magnetic fields

  • Typical uses: Main computer storage, routine backups, inexpensive bulk storage

  • Cost: ≈ $0.04 per GB (cheapest for large volumes)

  • Advantages/Disadvantages: See Table 4

6.2 Solid‑state Drive (SSD) – Solid‑state

  • Capacity: 120 GB – 4 TB

  • Speed: 300–3 500 MB s⁻¹ sequential; 50 k–100 k IOPS random

  • Durability: No moving parts → shock‑resistant; limited write‑endurance (TBW)

  • Typical uses: Operating‑system drive, high‑performance applications (gaming, video editing)

  • Cost: ≈ $0.12 per GB

  • Advantages/Disadvantages: See Table 4

6.3 Optical Discs – Optical

FormatTypical capacity (single‑layer)Read speedWrite speedTypical use
CD‑R / CD‑RW≈ 700 MB≈ 5 MB s⁻¹≈ 1 MB s⁻¹Software distribution, small archives
DVD‑R / DVD‑RW≈ 4.7 GB≈ 10 MB s⁻¹≈ 5 MB s⁻¹Video storage, medium‑size backups
Blu‑ray (BD‑R / BD‑RE)≈ 25 GB (single‑layer), 50 GB (dual‑layer)≈ 30 MB s⁻¹≈ 15 MB s⁻¹HD video, large archival projects

6.4 Magnetic Tape – Magnetic

  • Technology: Linear Tape‑Open (LTO); current generation LTO‑9 stores up to 30 TB (compressed) per cartridge.

  • Capacity: 10 TB – 30 TB (uncompressed)

  • Speed: Up to 300 MB s⁻¹ streaming; ideal for large sequential reads/writes.

  • Durability: Very long shelf‑life (30 + years) when kept cool and dry.

  • Typical uses: Enterprise backup, long‑term archival, disaster‑recovery sites.

  • Cost: ≈ $0.02 per GB (cheapest for massive archives).

  • Advantages/Disadvantages: See Table 4.

6.5 Flash Memory (USB sticks, SD cards) – Solid‑state

  • Capacity: 16 GB – 1 TB (common consumer sizes)

  • Speed: 100–400 MB s⁻¹ (USB 3.0/3.1)

  • Durability: No moving parts, but easy to lose or physically damage.

  • Typical uses: Portable data transfer, small backups, camera storage.

  • Cost: ≈ $0.10 per GB.

  • Advantages/Disadvantages: See Table 4.

7. Emerging Technologies – Impact (Syllabus 1.5)

Recent advances such as NVMe SSDs, cloud‑based object storage, and AI‑driven data deduplication are increasing the speed and efficiency of backing storage while reducing physical footprints. In schools and businesses these trends mean faster access to large data sets, more reliable automated backups, and a shift toward “storage as a service” rather than on‑site hardware.

8. Physical Safety of Storage Devices – Syllabus 8.1

  • Handle HDDs and SSDs by the edges; avoid dropping or exposing them to strong magnetic fields.

  • Store optical discs in protective cases, away from direct sunlight and extreme temperatures.

  • Keep tape cartridges upright in a climate‑controlled vault; never bend or crush the tape.

  • Before inserting or removing flash drives, discharge static electricity by touching a grounded metal object (static discharge can corrupt flash memory).

  • Keep all storage media away from fire sources; consider fire‑proof safes for critical backups.

9. Data Protection & e‑Safety – Syllabus 8.2

Key points for students

  • Encrypt sensitive data on removable media (e.g., BitLocker, VeraCrypt).

  • When a device is no longer required, securely erase it (overwrite, degauss magnetic media, manufacturer’s Secure Erase for SSDs).

  • Comply with data‑protection legislation (e.g., GDPR): store personal data only as long as necessary and keep it protected.

  • Maintain regular backups on a separate device or cloud service (the 3‑2‑1 rule).

Assessment Objective Checklist (AO) – This section covers:

  • AO1 – Recall: definitions of volatility, encryption, secure erase.

  • AO2 – Apply: choose appropriate protection measures for a given storage medium.

  • AO3 – Analyse: evaluate the adequacy of a backup strategy against data‑protection regulations.

10. Security Threats to Data – Syllabus 8.3

  • Malware / ransomware – encrypts files on HDDs, SSDs, NAS and removable media.

  • Physical theft – loss of laptops, external drives, USB sticks.

  • Accidental deletion / formatting – mitigated by versioned backups.

  • Data corruption – caused by power surges, magnetic interference, or flash‑cell wear.

Simple mitigation steps (AO2):

  • Install and keep anti‑virus/anti‑malware software up to date.

  • Use strong passwords and encryption for all removable media.

  • Store critical devices in locked cabinets or use cable locks.

  • Perform regular integrity checks and keep at least two independent backups (3‑2‑1 rule).

11. Storage in the Systems Life‑Cycle – Syllabus 7

  1. Analysis – Identify data volume, speed requirements and budget.

  2. Design – Specify type and capacity (e.g., “1 TB HDD for archive, 512 GB SSD for OS”).

  3. Implementation – Install devices, configure RAID or backup software.

  4. Testing – Verify read/write performance, run error‑checking, confirm restore from backup.

  5. Documentation – Record specifications, serial numbers, warranties, backup schedules.

  6. Maintenance – Monitor health (SMART data), replace ageing media, update security measures.

12. Choosing the Right Backing Storage – Decision Matrix

  • If speed is critical (OS, gaming, video editing) → SSD.

  • If a large, low‑cost archive is required → HDD or magnetic tape.

  • If data must be moved frequently between computers → USB/SD flash or external SSD.

  • If long‑term preservation with minimal degradation is needed → archival‑grade optical disc or magnetic tape stored in a controlled environment.

  • If network‑wide access is desired (school or business) → consider NAS or cloud storage (logical backing storage).

13. Summary

  • Backing storage provides non‑volatile, high‑capacity data retention that complements volatile RAM.

  • Key characteristics: capacity, speed, volatility, durability, cost, portability, access method.

  • Magnetic, optical and solid‑state families each balance these characteristics differently, giving distinct advantages and disadvantages.

  • Physical safety, data protection, and awareness of security threats are essential parts of responsible ICT practice.

  • Storage choices must be justified at each stage of the systems life‑cycle and documented clearly.

Suggested diagram: hierarchy of computer storage (CPU → Cache → RAM → SSD/HDD → Tape/Cloud) showing relative speeds and typical capacities.