Disk sector

Figure 1. Disk structures:
(A) Track
(B) Geometrical sector
(C) Track sector
(D) Cluster

In the context of computer disk storage, a sector is a subdivision of a track (Figure 1, item A) on a magnetic disk or optical disc. Each sector stores a fixed amount of data. The typical formatting of these media provides space for 512 bytes (for magnetic disks) or 2048 bytes (for optical discs) of user-accessible data per sector.

Mathematically, the word sector means a portion of a disk between a center, two radii and a corresponding arc (see Figure 1, item B), shaped like a slice of a pie. Thus, the common disk sector (Figure 1, item C) actually refers to the intersection of a track and mathematical sector.

Early on in various computing fields, the term block was used for this small chunk of data, but sector appears to have become more prevalent. One quite probable reason for this is the fact block has often been applied to data chunks of varying sizes for many types of data streams, rather than being limited to the smallest accessible amount of data on a medium. For example, the Linux program dd allows one to set the block size to be used during execution with the parameter bs=bytes. But doing so never changes the actual sector size of a medium, only the size of the blocks that dd will manipulate.

See also

• CD-ROM format
• Cylinder-head-sector
• Block (data storage)
• Disk storage
• Disk formatting
• Hard disk drive partitioning
• File Allocation Table