Cylinder (disk drive)

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A disk drive cylinder is a division of data in a disk drive, as used in the CHS addressing mode of a hard disk (or floppy disk). It is also used in the Cylinder-Head-Record (CCHHRR) addressing mode of CKD disk. The concept is concentric, hollow, cylindrical slices through the physical disks (platters), collecting the respective circular tracks aligned through the stack of platters.

Cylinders are vertically formed by tracks. In other words, the number of cylinders of a disk drive exactly equals the number of tracks on a single surface in the drive.

The term cylinder is sometimes prefaced with the word logical (ie: 'the section of corresponding tracks on all the surfaces form a logical cylinder') to emphasize the fact that it is an abstract concept in Unix, not a cylinder in the physical sense. In ECKD disk architecture, used by z/OS in mainframes, cylinders are part of physical structure. Even when defined as a layer on the top of a Storage Server.

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