💽 Disk Storage – Detailed Explanation
Disk storage refers to non-volatile storage devices that hold data permanently, even when power is turned off. The most common disk storage types are Hard Disk Drives (HDDs) and Solid State Drives (SSDs).
🔸 1. What Is Disk Storage?
Disk storage is a form of secondary storage that provides large capacity and persistent data storage. It is slower than main memory (RAM), but far cheaper and much more spacious.
Used for:
- Operating system storage
- Application software
- Files and user data
- Virtual memory (as swap space)
⚙️ 2. Components of a Hard Disk Drive (HDD)
An HDD consists of mechanical parts:
| Component | Description |
|---|---|
| Platters | Circular disks coated with magnetic material to store data |
| Spindle | Rotates the platters at high speed (e.g., 5400 RPM or 7200 RPM) |
| Read/Write Head | Reads from or writes to the disk surface |
| Actuator Arm | Moves the read/write head to the correct track |
| Controller | Manages data transfer between disk and system |
HDDs store data magnetically on tracks and sectors across platters.
📐 3. Disk Geometry – Logical View
A hard disk is logically divided as:
- Track: A circular path on the surface of a disk platter
- Sector: A section of a track (typically 512 bytes or 4 KB)
- Cylinder: Set of tracks located at the same position on all platters
📊 Example:
Disk = Multiple Platters
Each Platter = Multiple Tracks
Each Track = Multiple Sectors
🚀 4. Disk Access Time
Disk performance depends on access time, which is the total time to read/write data:
| Component | Description |
|---|---|
| Seek Time | Time to move the head to the right track |
| Rotational Latency | Time waiting for the desired sector to rotate under the head |
| Transfer Time | Time to transfer the data once head is in position |
Access Time ≈ Seek Time + Rotational Latency + Transfer Time
🧠 5. How the OS Manages Disk Storage
Operating Systems use several techniques to manage disk efficiently:
a) Disk Scheduling Algorithms
Optimize the order of read/write requests:
- FCFS (First-Come, First-Serve)
- SSTF (Shortest Seek Time First)
- SCAN (Elevator algorithm)
- LOOK, C-SCAN, C-LOOK
b) File System Management
- Maps logical files to physical sectors
- Manages free space and file allocation (e.g., FAT, NTFS, ext4)
- Ensures security and access permissions
c) Disk Caching and Buffering
- Uses memory buffers to reduce access latency
d) Virtual Memory
- Part of disk used as swap space to extend RAM
🧾 6. Solid State Drives (SSDs) – Modern Alternative
Unlike HDDs, SSDs use NAND flash memory (no moving parts). Benefits:
- Faster data access (low latency)
- More durable (no mechanical wear)
- Lower power consumption
Disadvantages:
- Higher cost per GB
- Limited write cycles (though improving)
🔁 7. Disk Formatting and Partitioning
- Low-Level Formatting: Prepares disk with tracks/sectors
- Partitioning: Divides disk into logical units (e.g., C:, D:)
- File System Formatting: Installs a file system in a partition (FAT, NTFS, ext4)
🔐 8. Data Integrity and Security
OS ensures:
- Access control (user permissions)
- Encryption for confidentiality
- Backups and journaling to prevent data loss during crashes
- Redundancy (RAID) for fault tolerance
📦 9. Storage Hierarchy
| Level | Device | Speed | Cost/GB | Volatility |
|---|---|---|---|---|
| CPU Registers | Internal | Fastest | Very high | Volatile |
| Cache | On-chip | Very fast | High | Volatile |
| Main Memory | RAM | Fast | Moderate | Volatile |
| Secondary Storage | HDD/SSD | Slow | Low | Non-volatile |
| Tertiary Storage | Optical/disks, tapes | Very slow | Very low | Non-volatile |
🧪 Real-Life Use Case
When you open a file:
- The OS requests the data from the disk.
- Disk scheduler chooses optimal read order.
- Disk controller moves head, finds track/sector.
- Data is read into RAM (via cache).
- Application accesses the data.
📌 Summary
- Disk storage is the backbone of long-term data persistence in a computer.
- OS manages disk via scheduling, formatting, caching, and file systems.
- HDDs are mechanical and cheaper; SSDs are faster and more modern.
- Understanding disk structure and performance is key to building efficient systems.