Study Notes: Secondary Storage (9645)

Welcome to the essential chapter on Secondary Storage! This is where your computer saves its long-term memories. Understanding secondary storage is crucial because it bridges the gap between the super-fast, temporary processing done by the CPU and the permanent, large-scale data you rely on every day. Let's dive into how data survives the power-off button!

3.7.5 The Need for Secondary Storage

Before we look at the devices themselves, we must understand why we need them.

Distinguishing Main Memory from Secondary Storage

In the "Computer organisation and architecture" section, you learned about the Processor and Main Memory (RAM).

  • Main Memory (RAM): This is fast, temporary storage used while the computer is running. It is volatile—meaning all data is lost when the power is switched off.
  • Secondary Storage: This is required for keeping data and programs permanently. It is non-volatile—meaning the data remains intact even when the computer is shut down.

Analogy Alert!
Think of RAM as your desk (fast access, temporary notes), and Secondary Storage as your filing cabinet (slower access, permanent documents).

The core purpose of secondary storage is to provide permanent, high-capacity storage for the operating system, applications, and user files.

Magnetic Hard Disk Drives (HDD)

The Magnetic Hard Disk Drive (HDD) is one of the oldest forms of mass storage still widely used, known for its high capacity and relatively low cost.

Principles of Operation (HDD)

An HDD stores data magnetically on spinning disks called platters.

  1. The drive contains several rigid, magnetically coated platters stacked on top of each other.
  2. These platters spin at high speeds (often 5400 or 7200 revolutions per minute, RPM).
  3. A set of read/write heads (one for each side of each platter) float just micro-millimetres above the surface.
  4. The heads move across the platters to access specific locations.
  5. Data is stored as tiny magnetic fields (North/South polarity representing 0s and 1s).
Characteristics and Suitability (HDD)
  • High Capacity: Can store terabytes (TB) of data cheaply.
  • Cost: Generally the cheapest storage option per gigabyte (GB).
  • Speed: Slower than SSDs because the read/write heads must physically move and wait for the platter to rotate to the correct position (this wait time is known as rotational latency).
  • Durability: Low durability; moving mechanical parts make it sensitive to physical shock.

Suitability: Great for large volume data storage, archival, and low-budget systems (e.g., storing security camera footage or huge media libraries).

Solid-State Drives (SSD)

Solid-State Drives (SSDs) represent modern storage technology, offering substantial speed improvements over HDDs.

Principles of Operation (SSD)

SSDs use flash memory (similar to the memory chips in a USB stick or smartphone) to store data.

  • Unlike HDDs, SSDs have no moving mechanical parts (hence "solid-state").
  • Data is stored in NAND flash memory chips using electrical charges trapped in floating-gate transistors.
  • Because there are no moving heads or spinning platters, access time is virtually instantaneous and consistent, regardless of where the data is stored on the chip.

Did you know? Because an SSD doesn't have to wait for a spinning disk, its access time is measured in microseconds (millionths of a second), whereas an HDD is measured in milliseconds (thousandths of a second).

Characteristics and Suitability (SSD)
  • Speed: Extremely fast read/write speeds, significantly boosting system boot-up times and application loading.
  • Durability: Very high durability; resistant to physical shock, quieter, and uses less power.
  • Cost: Traditionally much more expensive per GB than HDDs, though costs are dropping.
  • Lifespan: Flash memory cells have a finite number of read/write cycles (though modern controllers manage this well).

Suitability: Ideal for operating systems, high-performance computing, gaming, and portable devices where speed and durability are essential.

Quick Review: HDD vs SSD Comparison

Comparison based on suitability for different applications:
  • High Speed/Performance Required: Use SSD. (e.g., running the OS, professional video editing).
  • Mass Storage/Archive Required: Use HDD. (e.g., storing backups, large media files that are rarely accessed).
  • Portability/Ruggedness Required: Use SSD. (e.g., laptops, tablets, where shock resistance is vital).

Memory Aid:
HDDs have Heavy, Heaping capacity (cheap and big).
SSDs are Speedy and Silent (fast and durable).

Cloud Storage

So far, we have discussed local storage. But what if the data isn't physically attached to your computer?

Definition of Cloud Storage

Cloud storage is the storage of data on servers at a remote location that is accessed via the Internet.

Instead of saving files onto your local HDD or SSD, you upload them to a provider (like Google Drive, Microsoft OneDrive, or Dropbox). These companies maintain vast farms of servers where your data is stored.

Don't worry if this seems tricky at first! Cloud storage is just someone else's HDD/SSD that you rent access to over the network.

Advantages of Cloud Storage (vs Local Storage)
  • Accessibility: Data can be accessed from anywhere with an internet connection, across multiple devices.
  • Scalability: You can instantly increase or decrease your storage capacity without buying new hardware.
  • Disaster Recovery/Backup: If your local computer fails, your data is safe remotely. Providers usually handle backups and data redundancy.
  • Lower Local Requirements: You don't need to purchase or manage large, expensive local drives.
Disadvantages of Cloud Storage (vs Local Storage)
  • Internet Dependency: You must have a reliable internet connection to access or save your files.
  • Speed: Transfer speeds are limited by your internet bandwidth, which is always slower than accessing a local disk.
  • Security Risks: You hand over responsibility for your data security to a third-party provider, increasing the risk of data breaches or loss of control.
  • Recurring Costs: Local storage is a one-off purchase; cloud storage often involves ongoing subscription fees.

Key Takeaway for Secondary Storage: Always remember the three main types we covered (HDD, SSD, Cloud), their operating principles (magnetic vs. flash vs. remote server), and their respective trade-offs in terms of speed, cost, and mobility.