11.1 Access Units

Physically, data are written to the disk on both the top ( head 0) and bottom surfaces (head 1) of the disk, which is divided into concentric circles. These concentric circles are called "tracks," and half of a concentric circle is called a "block." The block is the smallest unit of disk access. The size of 1 block varies depending on its location. The more peripheral a block's location on the disk, the larger its size. The blocks in the outermost periphery are largest, at a size of 19,720 bytes. Those in the most central portion of the disk are smallest, at 9,520 bytes. Block size in the intermediate portions does not increase continuously. Rather, each of the surfaces is divided into sections called "zones," and the zones are formatted so that all blocks within the same zone are the same size.