- 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 Fixed Block Architecture disk or the cylinder–head–record (CCHHR) addressing mode of a 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. Other forms of Direct Access Storage Device (DASD), such as drum memory devices or the IBM 2321 Data Cell, might give blocks addresses that include a cylinder address, although the cylinder address doesn't select a (geometric) cylindrical slice of the device.
Cylinders are vertically formed by tracks. In other words, track 12 on platter 0 plus track 12 on platter 1 etc. is cylinder 12. 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 (e.g. 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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