In computer terminology, a cylinder refers to a series of tracks on a
direct access storage device, or
DASD, that are the same distance from the disk's
axis of rotation.
DASD, or hard disks, are typically composed of several platters, which spin at a high rate. A band of adjacent magnetized spots on the platter is a track. A series of tracks on different platters that occupy the same radius are a cylinder.
Data written to the tracks within a cylinder may be accessed more quickly than data that lies outside the cylinder, because once the disk device's arm is positioned to read one track, every other track within the cylinder can be read without repositioning the arm. Such arm movements, known as seek actions, are among the most costly I/O operations in terms of time.