Direct-access storage device: Difference between revisions
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A '''direct access storage device''', or '''DASD''' ([[International Phonetic Alphabet for English|IPA]] {{IPA|[ˈdæzdi]}}) is a form of magnetic disk storage, historically used in the [[mainframe]] and [[minicomputer]] (mid-range) environments. A [[redundant array of independent disks]] (RAID) is a form of DASD. |
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A '''direct-access storage device''' ('''DASD''', pronounced {{IPAc-en|ˈ|d|æ|z|d|iː}}) is any [[secondary storage]] device in which "each physical record has a discrete location and a unique address."<ref name=Intro>{{cite book|last=IBM Corporation|title=Introduction to IBM Direct-Access Storage Devices and Organization Methods|year=1975|page=1-1|url=http://bitsavers.trailing-edge.com/pdf/ibm/dasd/GC20-1649-9_DASDintro_Dec75.pdf}}</ref> [[IBM]] developed DASDs for use with [[mainframe computer]]s and some [[minicomputer]]s. [[Disk drive]]s, [[Drum memory|magnetic drums]], [[IBM 2321 Data Cell|data cells]] and [[optical disc drive]]s are all classified as DASDs.<ref>{{cite web|last=IBM Corporation|title=Serial Direct Access Storage Device Subsystem|url=https://publib.boulder.ibm.com/infocenter/aix/v6r1/index.jsp?topic=%2Fcom.ibm.aix.kernelext%2Fdoc%2Fkernextc%2Fserial_direct_storage.htm|work=IBM AIX 6.1 Information Center|accessdate=December 22, 2013}}</ref> |
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"Direct access" means that all data can be accessed directly — in a form of indexing also known as "random access" — as opposed to storage systems based on seeking sequentially through the data (e.g., tape drives). |
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[[Access methods]] for DASDs include ''sequential'', ''indexed'', and ''direct''. The ''[[direct access (computing)|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 [[magnetic tape data storage|tape]] [[tape drive|drive]]s. A record on a 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. |
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The DASD ''storage class'' includes both fixed and removable media. |
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* [[DFSMS]] |
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* [[ESCON]] |
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* [[FICON]] |
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* [[Global Mirror]] |
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* [[Metro Mirror]] |
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==Architecture== |
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[[Category:Mainframe computers]] |
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IBM mainframes access I/O devices through ''[[Channel I/O|channels]]'', a type of subordinate mini-processor. ''[[Channel program]]s'' write to, read from, and control the given device. |
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===CTR (CHR)=== |
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image:dasd |
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Channel programs address data using an eight-byte absolute module-bin-cylinder-track-record block address, or MBBCCHHR, divided into 16 bit-components representing the ''module'' and ''bin'' (for data cells), ''cylinder'' (for discs), ''head'' (or ''track''), and the ''record'' (block) number. When the data cell was discontinued in January 1975,<ref>{{cite web |
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{{compu-storage-stub}} |
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|url = http://www-03.ibm.com/ibm/history/exhibits/storage/storage_2321.html |
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|title = IBM Archives: IBM 2321 data cell drive |
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|author = IBM Corporation |
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|accessdate = 8 Nov 2011 |
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}}</ref> 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. |
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IBM refers to the data records programmers work with as ''logical records'', and the format on disc as [[Block (data storage)|blocks]] or ''physical records''. One block might contain several logical (or user) records or, in some schemes, partial logical records. |
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Physical records can have any size up to the limit of a [[Cylinder (disk drive)|cylinder]], although in usual practice, blocks or physical records do not exceed the capacity of a single track. |
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The access methods are responsible for ''blocking'' and ''deblocking'' logical records as they are written to or read from external media. |
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====CKD==== |
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CHR/CTR acronyms should not be confused with [[Count Key Data|CKD]], which refers to [[Count Key Data]], the layout of an addressable data record on a CTR. |
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===FBA=== |
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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. |
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For many applications, FBA not only offers simplicity, but an increase in throughput. |
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FBA is supported by [[VM (operating system)|VM/370]] and [[VSE (operating system)|DOS/VSE]], but not [[MVS]] or successor operating systems in the OS/360 line. |
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==Access== |
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The programming interface macros and routines are collectively called{{By whom|date=March 2009}} DAM: <u>d</u>irect <u>a</u>ccess <u>m</u>ethods. |
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===DOS/VSE=== |
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* DAmod/DTFDA – direct access |
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* SDmod/DTFSD – sequential disc |
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* ISmod/DTFIS - indexed sequential |
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* VSAM – Virtual Storage Access Method |
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===MVS, OS/390=== |
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* [[Basic Sequential Access Method|BSAM]] - Basic Sequential Access Method |
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* [[Basic Indexed Sequential Access Method|BISAM]] - Basic Indexed Sequential Access Method |
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* [[Queued Sequential Access Method|QSAM]] - Queued Sequential Access Method |
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* [[Queued Indexed Sequential Access Method|QISAM]] - Queued Indexed Sequential Access Method |
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* [[BPAM]] - Basic Partitioned Access Method |
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* [[BDAM]] - Basic Direct Access Method |
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* [[VSAM]] – Virtual Storage Access Method |
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==Present terminology== |
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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 array]]s utilizing [[redundant array of independent disks|RAID]] schemes. |
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{{Portal|Information technology}} |
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* [[DFSMS]]{{snd}} a standard software managing DASD usage |
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* [[ESCON]]{{snd}} a protocol for mainframe peripheral communication, used by most DASD devices |
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* [[FICON]]{{snd}} new protocol to replace ESCON |
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* [[IBM Enterprise Storage Server]]{{snd}} an example of large DASD |
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* [[Global Mirror]]{{snd}} DASD remote synchronization product |
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* [[Metro Mirror]]{{snd}} DASD remote synchronization product |
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* [[History of IBM magnetic disk drives]] |
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==References== |
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{{Reflist}} |
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[[Category:IBM storage devices]] |
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[[Category:IBM mainframe operating systems]] |
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Revision as of 18:39, 8 April 2015
 | This article needs additional citations for verification. (December 2009) |
A direct-access storage device (DASD, pronounced /ˈdæzdiː/) is any secondary storage device in which "each physical record has a discrete location and a unique address."[1] IBM developed DASDs for use with mainframe computers and some minicomputers. Disk drives, magnetic drums, data cells and optical disc drives are all classified as DASDs.[2]
Access methods for DASDs include sequential, indexed, and direct. 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 a 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 DASD storage class includes both fixed and removable media.
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 using an eight-byte absolute module-bin-cylinder-track-record block address, or MBBCCHHR, divided into 16 bit-components representing the module and bin (for data cells), cylinder (for discs), head (or track), and the record (block) number. When the data cell was discontinued in January 1975,[3] 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.
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.
FBA
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.
FBA is supported by VM/370 and DOS/VSE, but not MVS or successor operating systems in the OS/360 line.
Access
The programming interface macros and routines are 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
- 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
Present terminology
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.
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
- ^ IBM Corporation (1975). Introduction to IBM Direct-Access Storage Devices and Organization Methods (PDF). 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.