Know and understand magnetic drives including magnetic hard disks, magnetic tape

Storage Devices and Media – Magnetic, Optical & Solid‑State Drives

Objective

Know and understand the three main families of backing‑store media covered in the Cambridge IGCSE ICT syllabus – magnetic (hard‑disk and tape), optical (CD/DVD/Blu‑ray) and solid‑state (SSD) – and be able to describe their components, data organisation, performance factors, error‑correction, security, environmental considerations and typical uses.

1. Where Magnetic, Optical and Solid‑State Storage Fit in the Computer Hierarchy

• Primary (volatile) storage – RAM, cache.
• Secondary (non‑volatile) storage – the three families below, used for data that must be retained when power is removed.
• Backup/archival media – magnetic tape is the dominant technology for long‑term offline storage, but optical media are also used for small‑scale archiving.

2. Magnetic Hard Disk (HDD)

2.1 Main Components

• Platters – one or more circular disks coated with a magnetic layer.
• Spindle motor – rotates the platters (typical speeds: 5400 rpm, 7200 rpm, 10 000 rpm).
• Read/Write heads – tiny electromagnets that change and sense magnetic polarity.
• Actuator arm & voice coil – moves the heads radially to the required track.
• Controller circuit – converts computer commands into head movements, encodes data and manages error‑correction.

2.2 Data Organisation

• Each platter is divided into concentric tracks.
• Tracks are split into sectors (most common sizes: 512 bytes or 4096 bytes).
• Bits are magnetised on the surface to represent 0 or 1.
• Zone‑bit recording – outer tracks contain more sectors than inner tracks, increasing total capacity.

2.3 Performance Factors

• Rotational latency – average wait for the required sector to come under the head (½ rev time).
• Seek time – time for the actuator to position the head over the correct track.
• Data‑transfer rate – bits moved per second once the head is over the correct sector; affected by linear density and interface (SATA, SAS, PCIe).
• File‑system impact – fragmentation can increase average seek time; defragmentation restores contiguous allocation.
• Error‑correction – CRC for detection + Reed‑Solomon/ECC for correction.
• Security – self‑encrypting drives (SED) use AES‑256 to protect data at rest.

2.4 Advantages & Disadvantages

Advantages	Disadvantages
High capacity (0.5 TB – 20 TB typical in 2025).	Mechanical parts wear; vulnerable to shock and vibration.
True random access – fast retrieval of any file.	Higher power consumption than SSDs (≈5–10 W idle).
Low cost per gigabyte.	Performance degrades with fragmentation; periodic defragmentation required.
Built‑in ECC and optional hardware encryption.	Limited write‑cycle lifespan under heavy, continuous write loads.

2.5 Typical Use‑Cases

• Operating system and applications on desktops & laptops.
• Primary data store for servers, NAS and external backup drives.
• Cost‑effective bulk storage where ultra‑fast random access is not critical.

3. Magnetic Tape

3.1 Main Components

• Cartridge or reel – houses the thin magnetic tape.
• Read/Write heads – either fixed (linear) or mounted on a rotating drum (helical‑scan).
• Transport mechanism – pulls the tape past the heads at a precisely controlled speed.
• Controller – performs encoding, compression, error‑correction and communicates with the host.

3.2 Data Organisation

• Data is written in longitudinal tracks that run the length of the tape.
• In helical‑scan systems (e.g., LTO) the tape wraps around a rotating drum; each drum rotation writes a diagonal track, dramatically increasing linear density.
• Tracks are divided into blocks (typically 64 KB – 256 KB) that include Reed‑Solomon ECC.

3.3 Performance Characteristics

• Access type – sequential; locating a specific file requires winding through preceding data.
• Capacity (2025) – LTO‑9: 18 TB native, up to 45 TB compressed (2.5:1 typical).
• Transfer rate – 400 MB/s native, ≈1 GB/s compressed.
• Error‑correction & compression – Reed‑Solomon ECC + built‑in hardware compression.
• Security – optional AES‑256 encryption performed by the drive controller.

3.4 Backup Strategies (IGCSE focus)

• Full backup – copies the entire data set; simplest to restore but uses most tape.
• Incremental backup – records only files changed since the last backup (full or incremental); minimises tape usage, but restore requires the last full plus each incremental.
• Differential backup – records changes since the last full backup; faster restore than incremental, uses more tape.

3.5 Environmental & Longevity Considerations

• Optimal storage temperature: 15 °C – 25 °C.
• Relative humidity: 40 % – 60 % (avoid condensation).
• Properly stored cartridges retain data for 30 years or more.
• Handle with care – avoid bending, dust, strong magnetic fields.

3.6 Advantages & Disadvantages

Advantages	Disadvantages
Very high capacity at low cost per GB.	Sequential access – slow retrieval of individual files.
Long shelf life (≥30 years under proper conditions).	Physical handling required; risk of tape wear or breakage.
Ideal for offline backup, archival and bulk data transfer.	Requires dedicated tape‑drive hardware and regular media rotation.
Built‑in ECC and optional hardware encryption.	Higher latency for restore operations compared with HDD/SSD.

4. Optical Storage

4.1 Principle of Operation

• Data is stored as a pattern of pits and lands on a reflective disc.
• A low‑power laser in the drive reads the pattern; a higher‑power laser can melt the dye (write) on recordable media (CD‑R, DVD‑R, BD‑R).
• Tracks are concentric circles; each track is divided into sectors (typically 2 KB for CD, 4 KB for DVD, 2 KB for Blu‑ray).

4.2 Common Formats (2025)

Media	Typical Capacity (single layer)	Typical Use
CD‑R / CD‑RW	700 MB	Music, small software distribution.
DVD‑R / DVD‑RW	4.7 GB (single layer)	Video, larger software, backups.
Blu‑ray (BD‑R / BD‑RE)	25 GB (single layer) / 50 GB (dual layer)	High‑definition video, archival of medium‑size data sets.

4.3 Performance & Reliability

• Access type – random within a track but overall slower than HDD/SSD because the laser must move to the correct track.
• Transfer rate – up to 36 MB/s for Blu‑ray (UHS‑II), much lower for CD/DVD.
• Error‑correction – Reed‑Solomon Product Code (CIRC for CD, EDC/ECC for DVD, LD‑PC for Blu‑ray).
• Durability – resistant to electromagnetic interference; susceptible to scratches, heat and UV light.
• Security – optional encryption software; hardware encryption is rare.

4.4 Advantages & Disadvantages

Advantages	Disadvantages
Portable, inexpensive, and immune to magnetic fields.	Limited capacity compared with HDD/SSD; slower random access.
Read‑only (CD‑ROM) provides a tamper‑evident archive.	Disc surface can be scratched; data degrades over many read/write cycles.
Widely supported by computers, consoles and media players.	Requires a dedicated optical drive; many modern laptops omit them.

5. Solid‑State Drives (SSD)

5.1 Architecture

• Based on NAND flash memory – arrays of memory cells (SLC, MLC, TLC, QLC) that store charge to represent bits.
• Cells are organised into pages (4 KB – 16 KB) and blocks (typically 256 KB – 2 MB). Writing occurs at page level; erasing occurs at block level.
• Controller includes a DRAM cache, wear‑leveling algorithm and ECC engine.

5.2 Data Organisation & File‑System Impact

• Because blocks must be erased before rewriting, SSDs use TRIM commands so the OS can inform the drive which pages are no longer in use.
• Fragmentation has little impact on performance; the controller can remap writes to any free page.
• File‑systems commonly used: NTFS, exFAT, ext4 – all support TRIM.

5.3 Performance Factors

• Random I/O performance – measured in IOPS; SSDs deliver 10 000 – 100 000 IOPS, far exceeding HDDs.
• Sequential transfer rate – SATA III up to 550 MB/s; PCIe NVMe (Gen 4) > 5 GB/s.
• Latency – typically <0.1 ms (vs. ~5 ms for HDD seek).
• Endurance – expressed as TBW (terabytes written) or DWPD (drive writes per day); modern consumer SSDs ≈150 TBW, enterprise models > 1 PBW.
• Error‑correction – BCH or LDPC codes built into the controller.
• Security – self‑encrypting SSDs (AES‑256) and optional password protection.

5.4 Advantages & Disadvantages

Advantages	Disadvantages
No moving parts – very low mechanical failure risk.	Higher cost per gigabyte than HDDs.
Random‑access speeds 5‑10× faster than HDDs; very low latency.	Limited write‑cycle life; endurance must be managed.
Low power consumption (≈0.5 W idle, 2–4 W active).	Data recovery after severe failure can be more complex.
Built‑in ECC, optional hardware encryption, and TRIM support.	Performance can degrade when the drive is near full capacity.

5.5 Typical Use‑Cases

• Operating‑system and application drives in laptops, desktops and servers.
• High‑performance databases and virtualisation environments.
• Cache or tiered‑storage in hybrid systems (SSD for hot data, HDD for cold data).

6. Side‑by‑Side Comparison of the Three Storage Families (2025)

Feature	Magnetic (HDD)	Magnetic (Tape)	Optical (CD/DVD/BD)	Solid‑State (SSD)
Access type	Random (fast locate)	Sequential (slow locate, fast streaming)	Random within track, slower overall	Random (very fast) & sequential
Typical capacity (single unit)	0.5 TB – 20 TB	18 TB – 45 TB (LTO‑9, native)	0.7 GB – 50 GB	0.25 TB – 8 TB (consumer), up to 30 TB (enterprise)
Typical transfer rate	150 MB/s – 250 MB/s (SATA/PCIe‑3)	400 MB/s native, ≈1 GB/s compressed	5 MB/s (CD) – 36 MB/s (BD‑UHS‑II)	550 MB/s (SATA III) – 5 GB/s (PCIe Gen 4 NVMe)
Cost per GB (approx.)	£0.03 – £0.07	£0.02 – £0.04	£0.10 – £0.30	£0.10 – £0.30 (consumer)
Power consumption (idle / active)	5–10 W / 6–12 W	≈2 W only when drive is active	≈0.5 W (drive only when in use)	0.5 W / 2–4 W
Reliability / lifespan	MTBF ≈ 1–2 M hrs; wear on bearings.	MTBF > 5 M hrs; 30 yr shelf life.	Susceptible to scratches, ~10 yr shelf life.	MTBF > 2 M hrs; endurance limited by TBW.
Error‑correction	CRC + Reed‑Solomon/ECC.	Reed‑Solomon ECC + compression.	Reed‑Solomon (CIRC, EDC/ECC, LD‑PC).	BCH / LDPC ECC in controller.
Security options	Hardware AES‑256 encryption (SED).	Optional AES‑256 encryption.	Software encryption only.	Self‑encrypting SSDs (AES‑256), password lock.
Typical use‑case	Primary storage for OS, apps, active data.	Offline backup, archival, bulk data transfer.	Media distribution, small‑scale archiving, portable data exchange.	Performance‑critical OS, applications, cache tier.

7. Summary

1. Magnetic hard disks provide fast random access, high capacity and low cost, making them the workhorse of everyday computing.
2. Magnetic tape offers the highest capacity per media and the longest shelf life, but only sequential access – ideal for backup and archival.
3. Optical media are portable, inexpensive and immune to magnetic fields, but have limited capacity and slower access; they are mainly used for distribution and small‑scale archiving.
4. Solid‑state drives deliver superior speed and reliability with no moving parts, at a higher price; they are preferred for operating systems, applications and any workload that benefits from low latency.
5. Choosing the right technology depends on a trade‑off between capacity, speed, cost, durability and the intended use (primary storage vs. backup vs. archival).
6. Remember the storage hierarchy: RAM → SSD → HDD → Tape**, with optical media sitting alongside HDD as an alternative secondary option.