Disk Structure - Physical Organization

Introduction

To understand how operating systems manage storage, you must first understand the physical structure of storage devices. While SSDs are becoming dominant, magnetic hard disk drives (HDDs) remain important — they store the vast majority of the world's data due to their cost advantage for bulk storage. The physical characteristics of disks directly influence how file systems are designed, why disk scheduling algorithms exist, and why fragmentation matters.

Understanding disk geometry also helps you grasp why older systems had limitations (like the 2TB partition limit) and how modern addressing schemes removed those constraints.

Physical Components of a Hard Disk Drive

Platters

Circular aluminum or glass disks coated with a thin layer of magnetic material. Data is stored as magnetic orientation of tiny regions on the surface. Modern drives have 1-5 platters stacked vertically on a central spindle.

Platters spin at constant speed: 5400, 7200, 10000, or 15000 RPM.

Read/Write Heads

One head per platter surface, mounted on a shared actuator arm. All heads move together — you cannot position them independently. The head floats nanometers above the spinning platter on an air cushion.

Head crash: If the head contacts the platter surface (due to shock or manufacturing defect), it physically scratches the magnetic coating, destroying data permanently. This is why you should never move a running HDD.

Tracks and Cylinders

Track: One concentric circle on a single platter surface. A typical drive has thousands of tracks per surface.

Cylinder: The set of tracks at the same radial position across all platter surfaces. Cylinder 0 means track 0 on every surface.

Why cylinders matter: Reading from tracks in the same cylinder requires no head movement (seek time = 0). File systems try to allocate sequential data within the same cylinder for performance.

Sectors

Sector: The smallest addressable unit on a disk — a fixed-length arc of a track. Traditional sector size is 512 bytes; modern drives use 4096 bytes (Advanced Format).

Each sector contains:

• Preamble: Synchronization bytes and sector identification (track number, sector number)
• Data: 512 or 4096 bytes of user data
• ECC: Error Correction Code for detecting and fixing bit errors

Disk Capacity Calculation

Modern drives use Zone Bit Recording (ZBR): outer tracks have more sectors than inner tracks because they have larger circumference. This increases capacity and means the outer tracks have higher data transfer rates.

Disk Addressing

CHS (Cylinder-Head-Sector)

The original addressing scheme. Each sector identified by its physical location:

• Cylinder number (which track, 0-based)
• Head number (which platter surface)
• Sector number (which sector within track, traditionally 1-based)

Limitation: BIOS CHS addressing used 10 bits for cylinder (max 1024), 8 bits for head (max 256), 6 bits for sector (max 63). This created the 8.4 GB limit: 1024 × 256 × 63 × 512 = 8,455,716,864 bytes.

LBA (Logical Block Addressing)

Modern scheme that assigns a sequential number to each sector, starting from 0. The disk controller internally translates LBA to the physical CHS location.

The OS works exclusively with LBA addresses. It says "give me block 1,543,902" and the disk firmware handles physical positioning.

GPT (GUID Partition Table) uses 64-bit LBA, supporting disks up to 9.4 ZB (zettabytes).

Disk Formatting

Low-Level Formatting (Physical)

Performed at the factory. Creates the sector structure on each track:

• Writes preamble, data area, and ECC for each sector
• Defines sector sizes and identifies bad sectors
• Modern drives: done once at manufacture, not user-accessible

Logical Formatting (File System Creation)

Creates the file system structures within a partition:

This process creates:

• Superblock (filesystem metadata)
• Inode table (file metadata storage)
• Data blocks (actual file storage)
• Free space bitmap

Partitioning Schemes

MBR (Master Boot Record)

Limitations: Max 4 primary partitions, max 2TB disk size (32-bit LBA).

GPT (GUID Partition Table)

Advantages: 128 partitions standard, supports disks > 2TB, redundant headers (backup at end), CRC32 checksums for integrity.

Solid State Drive Structure

SSDs have no mechanical components. Their structure differs fundamentally:

• Page: Smallest read/write unit (4-16 KB)
• Block: Smallest erase unit (128-512 pages = 512KB-8MB)
• Write constraint: Can only write to empty pages. To modify data, must erase entire block first.

Key Takeaways

• HDD data is organized in platters → tracks → sectors (physical hierarchy)
• Cylinder = same track position across all platters (fastest access within cylinder)
• LBA replaced CHS addressing, removing the 8.4GB limit and hiding physical geometry
• Sector size is 512B (legacy) or 4KB (modern Advanced Format)
• GPT replaced MBR for disks > 2TB and removes the 4-partition limit
• SSD structure (pages, blocks, channels) requires completely different optimization strategies
• Understanding physical structure explains why sequential I/O is faster than random I/O on HDDs