Physical Structure

A hard disk is made of a stack of circular metal platters, coated with magnetic material. Before a disk can be used it must be formatted. The surface of the disk is divided up into a number of concentric tracks, each of which is subdivided into sectors.

A hard disk is a sealed unit containing a stack of circular platters mounted on a common spindle. Electromagnetic read/write heads are located above and below each platter. The platters rotate at a constant speed, eg: 7200 rpm. While they are spinning the heads can move in towards the centre or out towards the edge. This allows them to reach any location on the platter.

Each platter is divided into thin concentric bands known as tracks. There can be more than a thousand tracks on a 3.5 inch hard disk. The tracks are further subdivided into sectors. These are the smallest physical storage unit on a disk and they are almost always 512 bytes long. A group of tracks which have the same track number, but are on different platters, is sometimes referred to as a cylinder, but this term is no longer widely used.  Tracks are created when the disk is initially formatted. There are normally 1024 tracks on a hard disk, numbered from 0 (at the edge of the disk) to 1023 (near the centre).

One obvious problem with this structure is that the tracks near the centre are shorter than those near the edge of the disk. To compensate for this, they are more densely populated with data, meaning that the same amount of data can be written or read over the same period of time, irrespective of the drive head position.

One side of the first platter has space reserved for hardware-based track-positioning information which is not available to the operating system. This data is written to the disk during assembly and is used by the disk controller to position the drive heads correctly.

We have already noted that a sector is the smallest physical storage unit on the disk and is usually 512 bytes long. Files should ideally be stored in a single contiguous area of disk space. Since most files are longer than 512 bytes, the file system must allocate the number of sectors required to store the file, eg: a 640 byte file would require two sectors. If additional data is appended to the file later, further sectors can be allocated.

Next: Blocks and Clusters