Data on a hard disk isn’t stored as a linear sequence — it’s physically organized on spinning platters with magnetic surfaces. The OS needs to understand this geometry to efficiently locate and access data.
A hard disk consists of platters, tracks, sectors, cylinders, and read/write heads — the physical organization that determines how data is addressed and accessed.
A hard disk is organized as:
- Platters: Circular magnetic disks stacked on a spindle
- Tracks: Concentric rings on each platter surface
- Sectors: Smallest storage unit on a track (typically 512B or 4KB)
- Cylinder: Set of aligned tracks across all platters
- Read/Write Head: Moves across platters to access data
- Disk Arm: Moves heads to the correct track
- Data addressed by CHS: Cylinder, Head, Sector
- Access time = seek time + rotational delay + transfer time
- Tracks are numbered from outer (0) to inner
- Multiple platters increase capacity without increasing form factor
- Built from: O Devices, Block Driver
- Builds into: Disk Scheduling, Disk Management
- Related: Cylinder, Track, Sector
- Contrasts with: SSD (no moving parts, different structure)
- Sectors aren’t always 512B — modern disks use 4KB physical sectors
- Zone bit recording: outer tracks have more sectors than inner tracks
- Bad sectors must be remapped (handled by disk firmware)