Storage Types
Why Secondary Storage Is Needed
Primary storage (RAM and Cache) is:
- Volatile, loses all data when the power is off
- Small in capacity relative to what a computer stores long-term
This is why secondary storage is needed, to hold programs and data permanently between power cycles.
Storage Tiers
, Primary, RAM, cache (fast, volatile, in use by CPU right now) , Secondary, HDD, SSD, optical, USB (non-volatile, long-term) , Tertiary, magnetic tape (archival/backup, rarely accessed)
The Three Types of Secondary Storage
1. Optical Storage
How it works: A laser reads data from a disc. Read-only discs (CD-ROM, DVD-ROM, Blu-ray) have permanent pits and lands pressed into the surface. Rewritable discs use a reversible chemical coating.
| Characteristic | Detail |
|---|---|
| Examples | CD, DVD, Blu-ray |
| Capacity | Low (CDs ~700 MB, Blu-ray ~50 GB) |
| Speed | Slow |
| Portability | High |
| Durability | Low, easily scratched or dirtied |
| Cost | Very cheap |
Best for: Distributing software or media cheaply; personal backups of small files.
2. Magnetic Storage
How it works: A read/write head moves over spinning magnetic platters. Data is stored by changing the magnetic polarity of particles on the platter surface. The orientation of each particle represents a 0 or a 1.
| Characteristic | Detail |
|---|---|
| Examples | HDD (Hard Disk Drive), magnetic tape |
| Capacity | Very high (HDDs: up to several TB) |
| Speed | Moderate (spinning + head movement adds latency) |
| Portability | Low (HDDs are fragile when moving) |
| Durability | Moderate, moving parts can fail |
| Cost | Very low per GB |
HDD Note: Fragmentation can slow access; benefits from defragmentation. Tape Note: Sequential access only, extremely slow but very high capacity; used for large-scale archival backups.
3. Solid-State Storage
How it works: Stores data by trapping electrons in floating gate transistors. No moving parts. A trapped electron = 1; no trapped electron = 0.
| Characteristic | Detail |
|---|---|
| Examples | SSD (Solid-State Drive), USB flash drive |
| Capacity | High (SSDs: up to several TB) |
| Speed | Very fast (no moving parts) |
| Portability | High |
| Durability | High, no moving parts to fail |
| Cost | Higher per GB than HDD |
Drawback: Limited number of write cycles before cells degrade.
Comparison Table
| Optical | Magnetic (HDD) | Solid-State (SSD) | |
|---|---|---|---|
| How it stores data | Pits & lands (laser) | Magnetic polarity | Trapped electrons |
| Moving parts | Yes (disc spins) | Yes (platters + head) | No |
| Speed | Slow | Moderate | Fast |
| Capacity | Low | Very high | High |
| Cost per GB | Very cheap | Cheap | More expensive |
| Durability | Low (scratches) | Moderate | High |
| Volatile? | No | No | No |
Exam Questions
Q: Why is secondary storage necessary?
Because primary storage (RAM) is volatile, it loses all data when the power is off. Secondary storage is non-volatile and keeps programs and data permanently.
Q: Why are SSDs faster and more reliable than HDDs?
SSDs have no moving parts, they access data electronically, so there is no physical movement to slow them down or break.
Q: What is the main trade-off with solid-state storage?
It has a higher cost per gigabyte and a limited number of write cycles compared to magnetic storage.
Exam Tip
Know the mechanism for each type: optical = laser + pits/lands; magnetic = polarity of particles; solid-state = trapped electrons. These are frequently asked.