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AIX Version 4.3 Kernel Extensions and Device Support Programming Concepts

Chapter 14. Serial Direct Access Storage Device Subsystem

With sequential access to a storage device, such as with tape, a system enters and retrieves data based on the location of the data, and on a reference to information previously accessed. The closer the physical location of information on the storage device, the quicker the information can be processed.

In contrast, with direct access, entering and retrieving information depends only on the location of the data and not on a reference to data previously accessed. Because of this, access time for information on direct access storage devices (DASDs) is effectively independent of the location of the data.

Direct access storage devices (DASDs) include both fixed and removable storage devices. Typically, these devices are hard disks. A fixed storage device is any storage device defined during system configuration to be an integral part of the system DASD. If a fixed storage device is not available at some time during normal operation, the operating system detects an error.

A removable storage device is any storage device you define during system configuration to be an optional part of the system DASD. Removable storage devices can be removed from the system at any time during normal operation. As long as the device is logically unmounted before you remove it, the operating system does not detect an error.

The following types of devices are not considered DASD and are not supported by the logical volume manager (LVM):

DASD Device Block Level Description

The DASD device block (or sector) level is the level at which a processing unit can request low-level operations on a device block address basis. Typical low-level operations for DASD are read-sector, write-sector, read-track, write-track, and format-track.

By using direct access storage, you can quickly retrieve information from random addresses as a stream of one or more blocks. Many DASDs perform best when the blocks to be retrieved are close in physical address to each other.

A DASD consists of a set of flat, circular rotating platters. Each platter has one or two sides on which data is stored. Platters are read by a set of nonrotating, but positionable, read or read/write heads that move together as a unit.

The following terms are used when discussing DASD device block operations:

sector An addressable subdivision of a track used to record one block of a program or data. On a DASD, this is a contiguous, fixed-size block. Every sector of every DASD is exactly 512 bytes.
track A circular path on the surface of a disk on which information is recorded and from which recorded information is read; a contiguous set of sectors. A track corresponds to the surface area of a single platter swept out by a single head while the head remains stationary.

A DASD contains at least 17 sectors per track. Otherwise, the number of sectors per track is not defined architecturally and is device-dependent. A typical DASD track can contain 17, 35, or 75 sectors.

A DASD may contain 1024 tracks. The number of tracks per DASD is not defined architecturally and is device-dependent.

head A head is a positionable entity that can read and write data from a given track located on one side of a platter. Usually a DASD has a small set of heads that move from track to track as a unit.

There must be at least 43 heads on a DASD. Otherwise, the number is not defined architecturally and is device-dependent. A typical DASD has 8 heads.

cylinder The tracks of a DASD that can be accessed without repositioning the heads. If a DASD has n number of vertically aligned heads, a cylinder has n number of vertically aligned tracks.

Related Information

Programming in the Kernel Environment Overview.

Special Files Overview in AIX Version 4.3 Files Reference.

Understanding Physical Volumes and the Logical Volume Device Driver.

Serial DASD Subsystem Device Driver, scdisk SCSI Device Driver in AIX Version 4.3 Technical Reference: Kernel and Subsystems Volume 2.

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