Editing Direct-access storage device

{{Short description|Secondary storage device}}
{{About|a class of computer storage devices|other uses of DASD|DASD (disambiguation)}}
{{More citations needed|date=December 2009}}

A '''direct-access storage device''' ('''DASD''') (pronounced {{IPAc-en|ˈ|d|æ|z|d|iː}}) is a [[secondary storage]] device in which "each physical record has a discrete location and a unique address". The term was coined by [[IBM]] to describe devices that allowed [[random access]] to data, the main examples being [[drum memory]] and [[hard disk drive]]s.<ref name=Intro>{{cite book|author=IBM Corporation|title=Introduction to IBM Direct-Access Storage Devices and Organization Methods|date=1975|page=1-1|url=http://bitsavers.trailing-edge.com/pdf/ibm/dasd/GC20-1649-9_DASDintro_Dec75.pdf|access-date=2013-12-23|archive-date=2013-12-24|archive-url=https://web.archive.org/web/20131224113411/http://bitsavers.trailing-edge.com/pdf/ibm/dasd/GC20-1649-9_DASDintro_Dec75.pdf|url-status=dead}}</ref>  Later, [[optical disc drive]]s and  [[flash memory]] units are also classified as DASD.<ref>{{cite web |author=IBM Corporation |title=Serial Direct Access Storage Device Subsystem |url=https://www-01.ibm.com/support/knowledgecenter/ssw_aix_61/com.ibm.aix.kernextc/serial_direct_storage.htm |work=IBM AIX V6.1 Documentation |date=2015 |access-date=December 28, 2015 |archive-url=https://web.archive.org/web/20160304070746/https://www-01.ibm.com/support/knowledgecenter/ssw_aix_61/com.ibm.aix.kernextc/serial_direct_storage.htm |archive-date=2016-03-04}}</ref><ref>{{cite web |last1=DellEMC |title=Powermax 8000 Joins the Mainframe Solutions Portfolio |url=https://www.dellemc.com/en-us/storage/mainframe.htm |website=Dell EMC Products for Mainframe Environments |access-date=August 7, 2018 |archive-url=https://web.archive.org/web/20180808104243/https://www.dellemc.com/en-us/storage/mainframe.htm |archive-date=2018-08-08}}</ref>

The term DASD contrasts with [[sequential access]] storage device such as a [[tape drive|magnetic tape drive]], and [[unit record equipment]] such as a [[punched card]] device. 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 or deck of cards requires skipping over intervening records, and requires a proportionally long time to access a distant point in a medium. [[Access method]]s for DASD include sequential, [[Data set (IBM mainframe)#Partitioned data set|partitioned]], [[ISAM|indexed]], and [[Basic direct access method|direct]]. 

The DASD storage class includes both fixed and removable media.

==Architecture==
IBM mainframes access I/O devices including DASD through ''[[Channel I/O|channels]]'', a type of subordinate mini-processor. ''[[Channel program]]s'' write to, read from, and control the given device.<ref name="360Sum" />

===CTR (CHR)===
The operating system uses a four byte relative track and record (TTR) for some access methods and for others an eight-byte extent-bin-cylinder-track-record block address, or '''MBBCCHHR''',  Channel programs address DASD using a six byte seek address ('''BBCCHH''') and a five byte record  identifier ('''CCHHR'''). {{Citation needed|date=September 2024}}
*'''M''' represents the extent number within the allocation
*'''BB''' representing the '''B'''in (from [[IBM 2321 Data Cell|2321 data cells]]), 
*'''CC''' representing the '''C'''ylinder, 
*'''HH''' representing the '''H'''ead (or track), and 
*'''R''' representing the '''R'''ecord (block) number. 
When the 2321 data cell was discontinued in January 1975,<ref>{{cite web
|url         = http://www-03.ibm.com/ibm/history/exhibits/storage/storage_2321.html
|archive-url = https://web.archive.org/web/20060226172127/http://www-03.ibm.com/ibm/history/exhibits/storage/storage_2321.html
|url-status  = dead
|archive-date = February 26, 2006
|title       = IBM Archives: IBM 2321 data cell drive
|author      = IBM Corporation
|date = 23 January 2003
|access-date  = 8 Nov 2011
}}</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.

IBM refers to the data records programmers work with as ''logical records'', and the format on DASD{{efn|The concept of physical record also applies to other device types, e.g., tape.}} as [[Block (data storage)|blocks]] or ''physical records''. One block might contain several logical (or user) records or, in some schemes, called ''spanned records'', partial logical records.

Physical records can have any size up to the limit of a track, but some devices have a track overflow feature that allows breaking a large block into track-size segments within the same cylinder.

The queued access methods, such as [[QSAM]], are responsible for ''blocking'' and ''deblocking'' logical records as they are written to or read from external media.  The basic access methods, such as [[Basic_sequential_access_method|BSAM]], require the user program to do it. 

====CKD====
{{main|Count key data}}
CKD is an acronym for [[Count Key Data]], the physical layout of a block on a DASD device, and should not be confused with BBCCH and CCHHR, which are the addresses used by the channel program. CTR in this context may refer to either type of address, depending on the channel command. {{Citation needed|date=September 2024}}

===FBA===
In 1979 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, depending on the device type. As part of the FBA interface IBM introduced new channel commands for asynchronous operation that are very similar to those introduced for ECKD.
 
For some applications, FBA not only offers simplicity, but an increase in throughput.

FBA is supported by [[VM (operating system)|VM/370]] and [[VSE (operating system)|DOS/VSE]], but not [[MVS]]{{efn|However, the privileged routine IOSFBA provides limited access to these devices.}} or successor operating systems in the OS/360 line.

===FCP attached SCSI===
Processors with [[FICON]] channels can access SCSI drives using [[Fibre Channel Protocol]] (FCP). While z/VM and z/VSE fully support FCP, z/OS provides only limited support through IOSFBA.

==Access==
{{Cleanup weighted|section|Software|date=May 2021}}
Some programming interface macros and routines are collectively referred to as ''access methods'' with names ending in <u>A</u>ccess <u>M</u>ethod.

===DOS/360 and successors===
DOS/360 through [[z/VSE]] support datasets on DASD with the following access methods: {{Citation needed|date=September 2024}}
* Logical IOCS (LIOCS)
** [[sequential access method]] (SAM)
** direct access method (DAM)
** [[indexed sequential access method]] (ISAM)
** [[virtual storage access method]] (VSAM)
* Physical IOCS (PIOCS)
** [[Execute channel program]] (EXCP)

===OS/360 and successors===
OS/360 through [[z/OS]] support datasets on DASD with the following access methods: {{Citation needed|date=September 2024}}
* [[Basic sequential access method]] (BSAM)
* [[Basic indexed sequential access method]] (BISAM)
* [[Queued sequential access method]] (QSAM)
* [[Queued indexed sequential access method]] (QISAM)
* [[Basic partitioned access method]] (BPAM)
* [[Basic direct access method]] (BDAM)
* [[Virtual storage access method]] (VSAM)
* [[Execute Channel Program]] (EXCP)
* [[Execute Channel Program in Real Storage]] (EXCPVR)

In [[MVS]], starting with [[OS/VS2]] Release 2 and continuing through [[z/OS]], all of the access methods including EXCP[<nowiki/>VR], use the privileged [[Start Input/Output|STARTIO]] macro.

==Terminology==
IBM in its 1964 first version of the "IBM System/360 System Summary" used the term ''File'' to collectively described devices now called DASD.  Files provided "random access storage'"<ref name="360Sum">{{cite book |last1=IBM Corporation |title=IBM System/360 System Summary |date=1964 |page=24 |url=http://bitsavers.org/pdf/ibm/360/systemSummary/A22-6810-0_360sysSummary64.pdf |access-date=Aug 7, 2018}}</ref> At the same time IBM's product reference manual described such devices as "direct access storage devices<ref>{{cite book |title=IBM System/360 Component Descriptions - 2841 Storage Control Unit et. al.
 |url=http://www.bitsavers.org/pdf/ibm/2841/A26-5988-0_2841_2311_2321_7320_Descr.pdf |publisher=IBM |date=1964
 |quote=The IBM 2841 Storage Control Unit provides for the attachment of direct access storage devices to the IBM System/360.}}</ref>" without any acronym.

An early public use of the acronym DASD is in IBM's March 1966 manual, "Data File Handbook.<ref>{{cite book
 |title=Data File Handbook |url=http://www.bitsavers.org/pdf/ibm/generalInfo/C20-1638-1_Data_File_Handbook_Mar66.pdf
 |publisher= IBM}} 85 usages.</ref>" The earliest non-IBM use of the acronym DASD found by the "Google ngram viewer" to refer to storage devices dates from 1968.<ref>{{cite news |url=https://books.google.com/books?id=75NRAAAAYAAJ |pages=40–43 |last=Brown |first=Robert  
|title=Cost and Advantages of On-line DP |work=Datamation |agency=Cahners Publishing Company |date=March 1968}}</ref> From then on use of the term grew exponentially until 1990 after which its usage declined substantially.<ref>{{cite web|title="Ngram_chart: occurrences of DASD"
 |url=https://books.google.com/ngrams/graph?content=DASD&year_start=1960&year_end=2019&corpus=26&smoothing=0}}</ref>

Both drums and data cells have disappeared as products, so DASD remains as a synonym of disk, flash 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. Current devices emulate CKD on FBA hardware.

==See also==

* [[Hard disk drive]]
* [[DFSMS]]{{snd}} a standard software managing DASD usage
* [[ESCON]]{{snd}} a protocol for mainframe peripheral communication, used by most DASD devices
* [[FICON]]{{snd}} new protocol to replace ESCON
* [[IBM Enterprise Storage Server]]{{snd}} an example of large DASD
* [[Global Mirror]]{{snd}} DASD remote synchronization product
* [[Metro Mirror]]{{snd}} DASD remote synchronization product
* [[History of IBM magnetic disk drives]]
* [[History of IBM CKD Controllers]]

==Notes==
{{Notelist}}

==References==
{{Reflist}}

{{DEFAULTSORT:Direct Access Storage Device}}
[[Category:IBM storage devices]]
[[Category:IBM mainframe operating systems]]