Data Structure

Disk Structure

• Magnetic disk drives are addresed as large 1- dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer. The smallest addressable unit on a disk storage is a block. Logical block size is normally 512 bytes. It also possible that, block size may change with formatting.

• Modern magnetic disk drives are addressed as large one-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer.

• 1-dimensional array of logical blocks is mapped onto the sectors of the disk sequentially. Sector 0 is the first sector of the first track on the outermost cylinder. 

• In theory, this mapping support for conversion of logical block number into an mold-style disk address. That type of address consists of a cylinder number, track number within that cylinder, and a sector number within that track.

• Rotational speed is measured by two ways: CAV and CLV.

• Constant angular velocity (CAV): The rotational speed of the disk is constant. To use the platter in an efficient way, the outer tracks have more sectors than the inner tracks. Used in hard disks.

• Constant Linear velocity (CLV): The density of bits per track is uniform. To get constant data rate the rotation speed is increased as the head moves from the outer to the inner tracks. Used in CD and DVD.

• CD and DVD differ from hard disks in that they use a single track that spirals out from the centre to the periphery.