Direct access storage device
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In mainframe computers and some minicomputers, a direct access storage device, or DASD //, is any secondary storage device in which "each physical record has a discrete location and a unique address."
Historically, IBM introduced the term to cover three different device types:
Currently CD-ROM and DVD-ROM devices are also classified as DASD.
The direct access capability, occasionally and incorrectly called random access (although that term survives when referring to memory or RAM), stands in contrast to sequential access used in tape drives. A record on DASD can be accessed without having to read through intervening records from the current location, whereas reading anything other than the "next" record on tape requires skipping over intervening records, and requires a proportionally long time to access a distant point in a medium.
The storage class of DASD includes both fixed and removable media.
The access methods for DASD are sequential, indexed and direct.
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.
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, 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 refers to the data records programmers work with as logical records, and the format on disc as blocks or physical records. One block might contain several logical (or user) records or, in some schemes, partial logical records.
Physical records can have any size up to the limit of a cylinder, although in usual practice, blocks or physical records do not exceed the capacity of a single track.
The access methods are responsible for blocking and deblocking logical records as they are written to or read from external media.
In the 1970s, IBM introduced fixed block architecture (FBA) for mainframes. At the programming level, these devices do not use the traditional CHR addressing, but reference fixed-length blocks by number, much like sectors in mini-computers. More correctly, the application programmer remains unaware of the underlying storage arrangement, which stores the data in fixed physical block lengths of 512, 1024, 2048, or 4096.
For many applications, FBA not only offers simplicity, but an increase in throughput.
The programming interface macros and routines are collectively called[by whom?] DAM: direct access methods.
- DAmod/DTFDA – direct access
- SDmod/DTFSD – sequential disc
- ISmod/DTFIS - indexed sequential
- VSAM – Virtual Storage Access Method
- BSAM - Basic Sequential Access Method
- BISAM - Basic Indexed Sequential Access Method
- QSAM - Queued Sequential Access Method
- QISAM - Queued Indexed Sequential Access Method
- BPAM - Basic Partitioned Access Method
- BDAM - Basic Direct Access Method
- VSAM – Virtual Storage Access Method
Both drums and data cells have disappeared as products, so DASD remains as a synonym of disk and optical devices. Modern DASD used in mainframes only very rarely consist of single disk-drives. Most commonly "DASD" means large disk arrays utilizing RAID schemes.
- 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
- IBM Corporation (1975). Introduction to IBM Direct-Access Storage Devices and and Organization Methods. p. 1-1.
- IBM Corporation. "Serial Direct Access Storage Device Subsystem". IBM AIX 6.1 Information Center. Retrieved December 22, 2013.
- IBM Corporation. "IBM Archives: IBM 2321 data cell drive". Retrieved 8 Nov 2011.