Direct Access Storage Device
Direct Access Storage Device is a general term for magnetic disk storage devices, which
historically have been used in mainframe and minicomputer (mid-range computer)
environments. A redundant array of independent disks (RAID) is a type of DASD. The
key distinction between Direct Access and Sequential Access storage devices is that any
point on a Direct Access Storage Device may be promptly reached, whereas every point
in between the current position and the desired position of a Sequential Access Storage
Device must be traversed in order to reach the desired position. Tape drives are Sequential
Access Storage Devices. Some tape drives have minimal amounts of Direct Access
intelligence built in. These include multitrack tape devices that store at specific points
on the tape and cache in the tape drive information about where major sections of data
on the tape begin, allowing the tape drive to more quickly reach a track and a point on
the track, from which to begin the now much shorter traversal of data from that inch12.dexed point to the desired point. While this makes such tape drives noticeably faster
than their purely sequential peers, the difference in performance between Sequential
and Direct Access Storage Devices is orders of magnitude.
historically have been used in mainframe and minicomputer (mid-range computer)
environments. A redundant array of independent disks (RAID) is a type of DASD. The
key distinction between Direct Access and Sequential Access storage devices is that any
point on a Direct Access Storage Device may be promptly reached, whereas every point
in between the current position and the desired position of a Sequential Access Storage
Device must be traversed in order to reach the desired position. Tape drives are Sequential
Access Storage Devices. Some tape drives have minimal amounts of Direct Access
intelligence built in. These include multitrack tape devices that store at specific points
on the tape and cache in the tape drive information about where major sections of data
on the tape begin, allowing the tape drive to more quickly reach a track and a point on
the track, from which to begin the now much shorter traversal of data from that inch12.dexed point to the desired point. While this makes such tape drives noticeably faster
than their purely sequential peers, the difference in performance between Sequential
and Direct Access Storage Devices is orders of magnitude.
