Cylinder (disk drive)
A disk drive cylinder is a division of data in a disk drive
Disk storage
Disk storage or disc storage is a general category of storage mechanisms, in which data are digitally recorded by various electronic, magnetic, optical, or mechanical methods on a surface layer deposited of one or more planar, round and rotating disks...

, as used in the CHS
Cylinder-head-sector, also known as CHS, was an early method for giving addresses to each physical block of data on a hard disk drive. In the case of floppy drives, for which the same exact diskette medium can be truly low-level formatted to different capacities, this is still true.Though CHS...

 addressing mode of a Fixed Block Architecture
Fixed Block Architecture
Fixed Block Architecture is a disk layout in which each addressable record on disk is of the same size. The term fell out of use, since nearly all modern disk drives use this principle, termed logical block addressing and usually having a constant addressable block size of 512 bytes.- Count Key...

 disk or the cylinder–head–record (CCHHR) addressing mode of a CKD disk. The concept is concentric, hollow, cylindrical
Cylinder (geometry)
A cylinder is one of the most basic curvilinear geometric shapes, the surface formed by the points at a fixed distance from a given line segment, the axis of the cylinder. The solid enclosed by this surface and by two planes perpendicular to the axis is also called a cylinder...

 slices through the physical disks (platters
Hard disk platter
A hard-disk platter is a component of a hard-disk drive: it is the circular disk on which the magnetic data is stored. The rigid nature of the platters in a hard drive is what gives them their name . Hard drives typically have several platters which are mounted on the same spindle...

), collecting the respective circular tracks aligned through the stack of platters. Other forms of Direct Access Storage Device
Direct access storage device
In mainframe computers and some minicomputers, a direct access storage device, or DASD , is any secondary storage device which has relatively low access time relative to its capacity....

 (DASD), such as drum memory
Drum memory
Drum memory is a magnetic data storage device and was an early form of computer memory widely used in the 1950s and into the 1960s, invented by Gustav Tauschek in 1932 in Austria....

 devices or the IBM 2321 Data Cell
IBM 2321 Data Cell
The IBM 2321 Data Cell announced in April 1964 was a direct access storage device for the IBM System/360. It could hold up to 400 million bytes of data, but its access time was approximately 450 milliseconds.-Characteristics:...

, 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
Track (disk drive)
A disk drive track is a circular path on the surface of a disk or diskette on which information is magnetically recorded and from which recorded information is read....

. 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
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...

, not a cylinder in the physical sense. In ECKD disk architecture, used by z/OS
z/OS is a 64-bit operating system for mainframe computers, produced by IBM. It derives from and is the successor to OS/390, which in turn followed a string of MVS versions.Starting with earliest:*OS/VS2 Release 2 through Release 3.8...

 in mainframe
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...

s, cylinders are part of physical structure, even when defined as a layer on the top of a storage server.
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