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):
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:
Programming in the Kernel Environment Overview
Understanding Physical Volumes and the Logical Volume Device Driver
Special Files Overview in AIX 5L Version 5.2 Files Reference.
Serial DASD Subsystem Device Driver, scdisk SCSI Device Driver in AIX 5L Version 5.2 Technical Reference: Kernel and Subsystems Volume 2.