Understand secondary storage

Secondary Storage – Overview

Primary storage – the fast, volatile memory that the CPU can access directly. Typical examples are RAM (Random‑Access Memory) for temporary data and ROM (Read‑Only Memory) for firmware. Primary storage loses its contents when power is removed.

Secondary storage – non‑volatile devices that retain data permanently (or for many years) even without power. It provides the large capacity required for programmes, documents, media files, backups and for extending primary memory when RAM is full.

Why Both Types Are Needed

• RAM offers nanosecond‑level access speeds but is limited in size and loses data when the computer is switched off.
• Secondary storage supplies the high capacity and permanence that RAM cannot, and it can also be used to store data that exceeds the available RAM.
• Portable secondary‑storage media (USB sticks, external drives, cloud accounts) enable easy transfer of data between computers.

Types of Secondary Storage

1. Magnetic Storage – Hard‑Disk Drive (HDD)

• Data is recorded on rotating magnetic platters. Each bit is represented by the direction of magnetic particles (north‑south polarity).
• A magnetic read/write head (often called a “magnetic head”) flies a few nanometres above the platter surface. It changes polarity to write data and senses polarity to read data.
• Typical capacities: 500 GB – 4 TB (7200 rpm) or larger.

2. Solid‑State Storage – SSD & USB Flash Drive

• Both SSDs and USB flash drives use NAND flash memory. Each memory cell contains a floating‑gate transistor that stores charge; the presence or absence of charge represents a binary 1 or 0.
• No moving parts → very low access times, high resistance to shock.
• SSDs are usually installed inside the computer (SATA, NVMe); USB flash drives are external and connect via a USB interface.

3. Hybrid Drive – SSHD (Solid‑State + Hard‑Disk)

• Combines a small SSD cache (usually 8–64 GB) with a conventional magnetic HDD.
• Frequently accessed data is stored in the SSD portion for fast access, while bulk data remains on the magnetic platters.
• Provides a compromise between speed and cost.

4. Optical Storage – CD, DVD, Blu‑ray

• Data is encoded as a series of pits (depressions) and lands (flat areas) on a reflective polymer layer.
• A laser beam is reflected off the surface; changes in reflected light are interpreted as 0s and 1s.
• Typical capacities: CD‑ROM ≈ 700 MB, DVD‑R ≈ 4.7 GB, Blu‑ray ≈ 25 GB (single‑layer).

5. Magnetic Tape – Linear Tape‑Open (LTO)

• Long strips of magnetic coating on a plastic backing. Data is written sequentially as the tape moves past a read/write head.
• Used mainly for high‑capacity archival backup because of very low cost per gigabyte.
• Typical capacity (LTO‑9, native): 18 TB per cartridge.

6. Virtual Memory – OS‑Managed Secondary Storage

• When RAM becomes full, the operating system moves less‑used pages of memory to a reserved area of secondary storage (a “swap file” or “page file”).
• This creates an *extension* of primary memory, allowing programs larger than the physical RAM to run, though at reduced speed.
• It is essentially “RAM overflow” onto the hard‑disk/SSD.

Diagram suggestion: flow‑chart showing RAM ↔ Swap file (on HDD/SSD) ↔ Disk with arrows indicating page‑in/page‑out operations.

7. Cloud Storage

• Data is stored on remote servers owned by a third‑party provider and accessed via the Internet.
• Advantages: access from any device with an Internet connection, automatic redundancy/back‑up, easy sharing.
• Disadvantages: depends on a reliable Internet link, ongoing subscription fees, and security/privacy concerns (encryption, data‑centre jurisdiction, provider trustworthiness).

Key Characteristics of Secondary Storage

Comparison of Common Devices

Calculating Storage Requirements

When estimating how much secondary storage is needed, add the sizes of all files, then include a safety margin (usually 10‑20%). Use binary units (KiB, MiB, GiB) as required by the Cambridge syllabus.

Example – Mixed Media Project

1. Documents: 5 MiB × 120 = 600 MiB
2. Images: 2 MiB × 300 = 600 MiB
3. Video clips: 700 MiB × 8 = 5 600 MiB
4. Total = 6 800 MiB ≈ 6.64 GiB (1 GiB = 1024 MiB)
5. Safety margin 15 %: 6.64 GiB × 1.15 ≈ 7.64 GiB

Result: a **8 GiB** USB flash drive would satisfy the requirement; a 16 GiB drive would allow future growth.

Image‑Size Formula

Image size (bytes) = width × height × colour‑depth / 8

Example: a 1920 × 1080 pixel image with 24‑bit colour depth:

• Pixels = 1920 × 1080 = 2 073 600
• Bits = 2 073 600 × 24 = 49 766 400 bits
• Bytes = 49 766 400 / 8 ≈ 6 220 800 ≈ 5.93 MiB

Sound‑File Size Formula

Sound size (bytes) = sample‑rate × sample‑resolution × channels × duration (seconds) / 8

Example: a 5‑minute stereo track recorded at 44.1 kHz, 16‑bit resolution:

• Samples = 44 100 samples/s × 5 min × 60 s = 13 230 000
• Bits = 13 230 000 × 16 bits × 2 channels = 424 320 000 bits
• Bytes = 424 320 000 / 8 ≈ 53 040 000 ≈ 50.6 MiB

Advantages & Disadvantages of Each Secondary‑Storage Type

Sample Comparison Prompt (Cambridge‑style)

“A school wants to edit a short documentary film (≈ 10 GB of raw footage) and then archive the final version for five years. Choose the most appropriate secondary‑storage solution and justify your choice by comparing at least two devices.”

Choosing the Right Secondary Storage

1. Required Capacity: Estimate total data size plus future growth.
2. Speed Needs: For frequent read/write (e.g., video editing) prefer SSDs or high‑speed external drives.
3. Portability: For transport between locations, use USB flash drives, external SSDs, or cloud services.
4. Budget: Balance cost per GB against performance and durability.
5. Longevity: For archival storage, consider magnetic tape or high‑quality optical discs.
6. Security: Decide whether encryption, password protection, or secure cloud providers are required.

Summary

Secondary storage provides permanent, high‑capacity data retention. The main technologies differ in the physical principle used to store bits – magnetic polarity (HDD, tape), pits and lands on a reflective surface (optical), or charge in flash cells (SSD, USB). Each device has distinct trade‑offs in capacity, speed, durability, cost and portability. Understanding these characteristics, together with concepts such as virtual memory, cloud storage, and data‑size calculations, equips students to select the most suitable storage solution for any computing task.