Know and understand characteristics of backing storage

Types and Components of Computer Systems

Objective

Know and understand the characteristics of backing (secondary) storage.

What is Backing Storage?

Backing storage, also called secondary storage, is the non‑volatile memory used to retain data permanently or for long periods, even when the computer is powered off. It complements primary storage (RAM) by providing large‑capacity, durable space for programs, files, and system data.

Common Types of Backing Storage

• Hard Disk Drives (HDD)
• Solid‑State Drives (SSD)
• Optical Discs (CD, D \cdot D, Blu‑ray)
• Magnetic Tape
• Flash Memory (USB sticks, SD cards)
• Network‑Attached Storage (NAS) and Cloud Storage (considered logical backing storage)

Key Characteristics to Compare

1. Capacity – Amount of data that can be stored, usually expressed in megabytes (MB), gigabytes (GB) or terabytes (TB).
2. Access Speed – How quickly data can be read from or written to the medium (measured in MB/s or IOPS).
3. Volatility – Whether the storage retains data without power (non‑volatile) or loses data when power is removed (volatile).
4. Durability / Reliability – Resistance to physical shock, wear, magnetic fields, and data corruption.
5. Cost per GB – Economic factor influencing choice for personal vs. enterprise use.
6. Portability – Ease of moving the storage device between computers.
7. Access Method – Sequential (e.g., tape) vs. random access (e.g., HDD, SSD).

Comparison of Common Backing Storage Devices

Storage Type	Typical Capacity	Access Speed	Volatility	Typical Use	Cost per GB (approx.)
Hard Disk Drive (HDD)	500 GB – 8 TB	80–200 MB/s (sequential)	Non‑volatile	Desktop storage, servers, backup	~$0.04
Solid‑State Drive (SSD)	120 GB – 4 TB	300–3500 MB/s (sequential), 50 k–100 k IOPS (random)	Non‑volatile	Operating system, high‑performance applications	~$0.12
Optical Disc (D \cdot D‑R)	4.7 GB (single‑layer)	\overline{5} MB/s (read)	Non‑volatile	Software distribution, archival	~$0.20
Magnetic Tape	10 TB – 30 TB (LTO‑9)	\overline{300} MB/s (streaming)	Non‑volatile	Enterprise backup, long‑term archiving	~$0.02
Flash Memory (USB/SD)	16 GB – 1 TB	100–400 MB/s	Non‑volatile	Portable data transfer, small backups	~$0.10

Detailed Discussion of Characteristics

Capacity

Capacity determines how much information can be stored. Modern HDDs and SSDs provide several terabytes, suitable for large media libraries and databases. Portable flash devices are limited by size and cost.

Access Speed

Speed influences how quickly a program can load data. SSDs use NAND flash and have no moving parts, giving them much higher random‑access performance than HDDs, which rely on rotating platters and moving read/write heads.

Volatility

All backing storage discussed is non‑volatile, meaning data remains after power loss. This contrasts with primary storage (RAM), which is volatile.

Durability and Reliability

• HDD: Sensitive to shocks and magnetic fields; mechanical wear over time.
• SSD: No moving parts, more shock‑resistant, but limited write‑endurance (measured in TBW – terabytes written).
• Optical & Tape: Good for long‑term archival if stored properly, but susceptible to physical damage and degradation.
• Flash Memory: Portable but can be lost or damaged easily.

Cost per GB

Cost is a major factor in selecting storage. Tape offers the lowest cost per GB for massive archives, while SSDs are the most expensive but provide speed advantages.

Portability

Devices such as USB sticks and external SSDs are designed for easy transport, whereas internal HDDs/SSDs are fixed within a computer chassis.

Access Method

Sequential access (tape) reads data in order, making it unsuitable for random‑access tasks. Random‑access devices (HDD, SSD, flash) allow any block to be read or written instantly.

Choosing the Right Backing Storage

When selecting backing storage for a particular application, consider the following decision matrix:

1. If speed is critical (e.g., operating system, gaming, video editing) → choose SSD.
2. If large capacity at low cost is required for archival → choose HDD or magnetic tape.
3. If data must be moved frequently between computers → choose portable flash (USB/SD).
4. If long‑term preservation with minimal degradation is needed → consider optical media or tape stored in controlled environments.

Summary

• Backing storage provides non‑volatile, high‑capacity data retention.
• Key characteristics: capacity, speed, volatility, durability, cost, portability, and access method.
• Different technologies balance these characteristics in various ways, making each suitable for specific tasks.