Defining a Disk’s Geometry

Before creating a file system on a disk, you need to understand the basic geometry of a disk drive. Disks come in many shapes and sizes. The number of heads, tracks, and sectors and the disk capacity vary from one model to another.

A hard disk consists of several separate disk platters mounted on a common spindle. Data stored on each platter surface is written and read by disk heads. The circular path that a disk head traces over a spinning disk platter is called a track.

Each track is made up of a number of sectors laid end to end. A sector consists of a header, a trailer, and 512 bytes of data. The header and trailer contain error-checking information to help ensure the accuracy of the data. Taken together, the set of tracks traced across all the individual disk platter surfaces for a single position of the heads is called a cylinder.

Disk Controller

Associated with every disk is a controller, an intelligent device responsible for organizing data on the disk. Some disk controllers are located on a separate circuit board (such as SCSI), and some are integrated with the disk drive (such as IDE/EIDE).

Defect List

Disks might contain areas where data cannot be written and retrieved reliably. These areas are called defects. The controller uses the error-checking information in each disk block’s trailer to determine whether a defect is present in that block. When a block is found to be defective, the controller can be instructed to add it to a defect list and avoid using that block in the future. The last two cylinders of a disk are set aside for diagnostic use and for storing the disk defect list.

Disk Label

A special area of every disk is set aside for storing information about the disk’s controller, geometry, and slices. This information is called the disk’s label or volume table of contents (VTOC).

To label a disk means to write slice information onto the disk. You usually label a disk after defining its slices. If you fail to label a disk after creating slices, the slices will be unavailable because the operating system has no way of knowing about them.

Partition Table

An important part of the disk label is the partition table, which identifies a disk’s slices, the slice boundaries (in cylinders), and the total size of the slices. A disk’s partition table can be displayed by using the format utility.