Direct access storage device

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A Direct Access Storage Device ( DASD ) is a device for direct storage access .

With a data storage device of the DASD category, information can be accessed directly without having to work through all memory areas sequentially to the desired element. While a floppy disk drive is a DASD unit, this is not the case with a magnetic tape device , since the data is stored here as linear blocks .

Although a DASD essentially refers to hard disks, the term is mainly used in the mainframe environment (e.g. with operating systems IBM Z / OS , MVS , Z / VM , less often with Siemens BS2000 ) but also in the area of ​​medium-sized data processing by IBM ( OS / 400 , i-Series ) used.

Various DASD disciplines (mainframe computers)

ECKD (Extended Count Key Data)

  • Mainly used DASD discipline
  • Different block sizes in non- Linux systems
  • Two different formats (ldl (Linux disk layout) and cdl (Compatible Disk Layout))

FBA (Fixed Block Architecture)

  • Less common
  • Fixed block size (mostly 512 bytes)

DIAG (DIAG-accessed)

  • Seldom used
  • Kernel 2.4 - CMS-Reserved MiniDisk
  • Kernel 2.6 - any VM-administered disk (alternative method)

In contrast to the usual use of addressing the data areas via (E) CKD in the mainframe environment, when used in medium-sized data processing ( OS / 400 , i-Series , AIX , HP-UX , etc.) and small computers ( Windows , Linux , MAC- OS ) addressing via FBA (512/520 bytes per block) is used.

In addition to floppy disk drives (also "exotic" such as ZIP ) optical data carriers ( CD , DVD , WORM media, etc.) also count among the DASD storage devices.