Block size (data storage and transmission)

In computing (specifically data transmission and data storage), block size indicates a nominal size, usually expressed in bytes or bits, of a block of data. Data thus structured is said to be blocked. The process of putting data into blocks is called blocking. Blocking is used to the facilitate the handling of the data-stream by the computer program receiving the data. Blocked data is normally read a block at a time. Blocking is almost universally employed when storing data to 9-track magnetic tape.

Most filesystems are based on a block device, which is a level of abstraction for the hardware responsible for storing and retrieving specified blocks of data. Under this scheme, individual files will usually require more space to store than the true size of the file, because the block size is more than one byte, and a block is never divided between multiple files. The advantage of a block device is that the physical hardware can almost always be designed to seek more quickly to the start of a block (and then read one or more whole blocks) than it could seek to an arbitrary point on the storage medium (and then read the exact number of bytes required). Thus blocking provides an increase in speed, but at an increased cost in storage space determined by the block size.