A cylinder on a Hard Disk Drive (HDD) is a fundamental concept related to how data is organized and accessed.
Understanding the HDD Structure
To understand what a cylinder is, it's helpful to know the basic components of an HDD that store data. A hard disk drive is typically structured into three main segments for data storage:
- Sectors: These are the smallest individual units where data is stored. Think of them as tiny segments on a circular track.
- Tracks: A track is a circular path on a single surface of a platter. It is made up of a sequence of many sectors.
- Platters: Hard drives contain one or more rigid, spinning discs called platters, which are coated with a magnetic material to store data. Each platter has two surfaces (top and bottom) that can typically store data.
- Heads: There are read/write heads, usually one for each surface of each platter, that move together across the platters to read or write data.
Defining a Cylinder
Based on this structure, a cylinder is defined as follows:
A cylinder holds a group of tracks that are separated by platters or heads.
In simpler terms, if you imagine stacking all the platters on top of each other, a cylinder is the collection of tracks that are located at the same radial distance from the center spindle on each surface of every platter.
Since the read/write heads for all platters move together as a single unit, they are always positioned over the same cylinder location simultaneously. This organization allows the drive to access data across multiple platter surfaces without having to move the heads radially between accessing tracks within the same cylinder.
How Cylinders Relate to Data Access
When the drive controller needs to access data at a specific location, it uses a Cylinder, Head, Sector (CHS) address (though modern drives often use Logical Block Addressing - LBA).
- The heads move to the correct cylinder (the set of tracks at the target radial position).
- The controller selects the correct head (which corresponds to the correct platter surface).
- The drive waits for the correct sector on that track/surface to rotate under the head.
This method of organization optimizes data access, particularly for sequential reads and writes, as data can be spread across different heads within the same cylinder before requiring the heads to move to a new radial position (cylinder).
In essence, the cylinder is a logical unit that represents a vertical stacking of tracks across all the platters in the drive.
