Direct access storage device

In mainframe computers and some minicomputers, a direct access storage device, or DASD (play /ˈdæzd/), is any secondary storage device which has relatively low access time relative to its capacity.

Historically, IBM introduced the term to cover three different device types:

  1. disk drives
  2. magnetic drums
  3. data cells

The direct access capability, occasionally and incorrectly called random access (although that term survives when referring to memory or RAM), of those devices stood in contrast to sequential access used in tape drives. The latter required a proportionally long time to access a distant point in a medium.

Contents

Architecture

IBM mainframes access I/O devices through 'channels', a type of subordinate mini-processor. Channel programs write to, read from, and control the given device.

CTR (CHR)

Channel programs address data through a scheme called module-bin-cyl-trk-rec or MBBCCHHRR, an eight byte address divided into 16 bit-components representing the module and bin (for data cells), cylinder (for discs), head (or track), and the record number. When the data cell was discontinued in January 1975,[1] the addressing scheme and the device itself was referred to as CHR or CTR for cylinder-track-record, as the bin number was always 0.

IBM referred to the data records programmers worked with as logical records, and how they were stored on disc as blocks or physical records. One block could contain several logical (or user) records or, in some schemes, partial logical records.

Physical records could have any size up to the limit of a cylinder, although in usual practice, blocks or physical records did not exceed the capacity of a single track.

CKD

CHR/CTR acronyms should not be confused with CKD, which refers to Count Key Data, the layout of an addressable data record on a CTR disc.

FBA

In the 1970s, IBM introduced fixed block architecture, or FBA. At the programming level, these devices did not use the traditional CHR addressing, but referenced fixed-length blocks by number, much like sectors in mini-computers. More correctly, the application programmer remained unaware of the underlying storage arrangement, which stored the data in fixed physical block lengths of 512, 1024, 2048, or 4096.

For many applications, FBA not only offered simplicity, but an increase in throughput. GOAL Systems of Columbus, Ohio, discovered that an FBA emulator written for VM by Bill Jurist delivered an unexpected boost of speed.

Access

The programming interface macros and routines were collectively called[by whom?] DAM: direct access methods.

DOS/VSE

  • DAmod/DTFDA – direct access
  • SDmod/DTFSD – sequential disc
  • ISmod/DTFIS - indexed sequential
  • VSAM – Virtual Storage Access Method

MVS, OS/390

  • BSAM - Basic Sequential Access Method
  • QSAM - Queued Sequential Access Method
  • BPAM - Basic Partitioned Access Method
  • BDAM - Basic Direct Access Method
  • VSAM – Virtual Storage Access Method

Present terminology

Both drums and data cells have disappeared as products, so DASD remains as a synonym of a disk device. Modern DASD used in mainframes only very rarely consist of single disk-drives: most commonly "DASD" means large disk arrays utilizing RAID schemes.

See also

  • Hard disk
  • DFSMS - a standard software managing DASD usage
  • ESCON - a protocol for mainframe peripheral communication, used by most DASD devices
  • FICON - new protocol to replace ESCON
  • IBM Enterprise Storage Server - an example of large DASD
  • Global Mirror - DASD remote synchronization product
  • Metro Mirror - DASD remote synchronization product
  • History of IBM magnetic disk drives

References


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