View source for Virtual Storage Access Method

'''Virtual Storage Access Method''' ('''VSAM''')<ref>{{cite magazine  |magazine=Datamation
   |url=https://www.datamation.com/netsys/article.php/3677001/New-Life-for-Legacy-Systems-at-LaBarge.htm
   |title=New Life for Legacy Systems at LaBarge
   |date=May 11, 2007}}</ref> is an [[Company:IBM|IBM]] direct-access storage device (DASD) file storage [[Access method|access method]], first used in the OS/VS1, OS/VS2 Release 1 (SVS) and [[Software:MVS|Release 2]] (MVS) operating systems, later used throughout the Multiple Virtual Storage (MVS) architecture and now in z/OS. Originally a [[Record-oriented filesystem|record-oriented filesystem]],{{#tag:ref|With the exception of catalogs, page spaces and swap<ref group=NB>No longer used.</ref> spaces, which unauthorized applications could access only via specialized OS services. Not to mention the fact that it's been in VSE for ever too and is used in z/VSE|group="NB"|name=notfile}} VSAM comprises four<ref group=NB name=notfile/> [[Data set (IBM mainframe)|data set]] ''organizations'': [[Key Sequenced Data Set|key-sequenced]] (KSDS), [[Relative Record Data Set|relative record]] (RRDS), [[Entry Sequenced Data Set|entry-sequenced]] (ESDS) and [[Linear Data Set|linear]] (LDS).<ref name="demystified">{{cite book|url=http://www.redbooks.ibm.com/pubs/pdfs/redbooks/sg246105.pdf |series=Redbooks |title=VSAM Demystified |first1=Mary |last1=Lovelace |first2=Jose |last2=Dovidauskas |first3=Alvaro |last3=Salla |first4=Valeria |last4=Sokal |pages=5 |chapter=1.3.2 Record management |edition=3 |publisher=IBM |date=March 2013}}</ref> The KSDS, RRDS and ESDS organizations contain records, while the LDS organization (added later to VSAM) simply contains a sequence of pages with no intrinsic record structure, for use as a [[Memory-mapped file|memory-mapped file]].

==Overview==
An IBM ''Redbook'' named "VSAM PRIMER" (especially when used with the "Virtual Storage Access Method (VSAM) Options for Advanced Applications" manual) explains the concepts needed to make use of VSAM.<ref>{{cite web
   |url=http://www.computinghistory.org.uk/det/9393/IBM-International-Systems-Centers-VSAM-Primer-for-the-Intergrated-Catalog-Facility-in-an-MVS-Environment
   |title=VSAM Primer}}</ref> IBM uses the term ''data set'' in official documentation as a synonym of ''file'', and ''direct access storage device'' (''DASD'') because it supported other devices similar to ''disk drives''.

VSAM records can be of fixed or variable length. They are organised in fixed-size blocks called Control Intervals (CIs),<ref name="VSAM.CI1">{{cite web |title=VSAM – Components |url=https://www.tutorialZpoint.com/vsam/vsam_components.htm}}</ref><ref name=VSAM.CI2>{{cite web
|url=https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.1.0/com.ibm.zos.v2r1.idad400/d4221.htm
|title=Control Interval Size Limitations|website=[[Company:IBM|IBM]] |date=27 March 2014 }}</ref> and then into larger divisions called Control Areas (CAs). Control Interval sizes are measured in bytes{{snd}} for example 4 [[Kilobyte|kilobyte]]s{{snd}} while Control Area sizes are measured in disk tracks or cylinders. Control Intervals are the units of transfer between disk and computer so a read request will read one complete Control Interval. Control Areas are the units of allocation so, when a VSAM data set is defined, an integral number of Control Areas will be allocated.

The Access Method Services utility program [[Software:IBM mainframe utility programs#IDCAMS|IDCAMS]] is commonly used to manipulate ("delete and define") VSAM data sets. Custom programs can access VSAM datasets through Data Definition (DD) statements in [[Job Control Language]] (JCL), via dynamic allocation or in online regions such as in [[Software:CICS|Customer Information Control System]] (CICS).

Both IMS/DB{{fact|date=April 2021}} and [[Software:IBM Db2|Db2]]<ref name="demystified"/>{{rp|41}}<ref>{{cite web
|url=https://www-01.ibm.com/servers/resourcelink/svc00100.nsf/pages/izodav110sc279034/%24file/azk1b100.pdf
|title=User's Guide}}</ref> are implemented on top of VSAM and use its underlying [[Data structure|data structure]]s.

==VSAM files==
The physical organization of VSAM data sets differs considerably from the organizations used by other access methods, as follows.

A VSAM file is defined as a cluster of VSAM components, e.g., for KSDS a DATA component and an INDEX component.

===Control Intervals and Control Areas===
VSAM components consist of fixed length physical blocks grouped into fixed length control intervals<ref name=VSAM.CI1/><ref name=VSAM.CI2/> (CI) and control areas (CA). The size of the CI and CA is determined by the [[Software:IBM mainframe utility programs#IDCAMS|Access Method Services]] (AMS), and the way in which they are used is normally not visible to the user. There will be a fixed number of control intervals in each control area.

A control interval normally contains multiple records. The records are stored within the control interval starting from the low address upwards. Control information is stored at the other end of the control interval, starting from the high address and moving downwards. The space between the records and the control information is free space. The control information comprises two types of entry: a control interval descriptor field (CIDF) which is always present, and record descriptor fields (RDF) which are present when there are records within the control interval and describe the length of the associated record. Free space within a CI is always contiguous.

When records are inserted into a control interval, they are placed in the correct order relative to other records. This may require records to be moved out of the way inside the control interval. Conversely, when a record is deleted, later records are moved down so that the free space remains contiguous. If there is not enough free space in a control interval for a record to be inserted, the control interval is split. Roughly half the records are stored in the original control interval while the remaining records are moved into a new control interval. The new control interval is taken from a pool of free control intervals within the same control area as the original control interval. If there is no remaining free control interval within that control area, the control area itself is split and the control intervals are distributed equally between the old and the new control areas.

You can use three types of record-orientated file organization with VSAM (the contents of linear data sets have no record structure):

===Sequential VSAM organization===
{{further|Entry Sequenced Data Set}}

An ESDS may have an index defined to it to enable access via keys, by defining an Alternate Index. Records in ESDS are stored in order in which they are written by address access.<ref>{{cite web
   |url=https://walton.uark.edu/enterprise/IBM/systemZ/downloads/module3/VSAM-Introductory.ppt
   |title=VSAM: introductory}}</ref><ref>{{cite web |title=Server Functionality |url=https://supportline.microfocus.com/documentation/books/tps62/mafunc.htm |quote=Sequential (VSAM ESDS – Entry Sequenced Dataset)}}</ref><ref>{{cite web
      |url=http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.469.8853&rep=rep1&type=pdf
      |quote=An ESDS VSAM data set contains records in the order in which they were entered
      |title=ABCs of z/OS System Programming Volume 3|citeseerx=10.1.1.469.8853
 }}</ref> Records are loaded irrespective of their contents and their byte addresses cannot be changed.

===Indexed VSAM organization===
{{further|Key Sequenced Data Set}}
A KSDS has two parts: the index component and the data component. These may be stored on separate disk volumes.

While a basic KSDS only has one key (the primary key), alternate indices may be defined to permit the use of additional fields as secondary keys. An Alternate Index (AIX) is itself a KSDS.

The data structure used by a KSDS is nowadays known as a B+ tree.<ref>{{cite web
   |url=https://patents.justia.com/patent/9043295
   |title=US Patent for Providing record-level alternate-index upgrade locking}}</ref><ref>{{cite web
|url=https://www.wisegeek.com/what-is-vsam.htm
|quote=This index is called a B+ tree.
|title=What is VSAM?}}</ref>

===Relative VSAM organization===
{{further|Relative Record Data Set}}

An RRDS may have an index defined to it to enable access via keys, by defining an Alternate Index.

=== Linear VSAM organization ===
{{further|Linear Data Set}}
An LDS is an unstructured VSAM dataset with a control interval size of a multiple of 4K. It is used by certain system services.

== VSAM Data Access Techniques ==
There are four types of access techniques for VSAM data:

* Local Shared Resources (LSR), is optimised for "random" or direct access. LSR access is easy to achieve from [[Software:CICS|CICS]].<ref name=VSAM.LSRorNSR>{{cite web
   |url=https://www.ibm.com/support/knowledgecenter/SSGMCP_5.6.0/tuning/dfht3c00200.html
   |title=Local shared resources (LSR) or nonshared resources|website=[[Company:IBM|IBM]] }}</ref>
* Global Shared Resources (GSR)<ref name=VSAM.GSR>{{cite web  |website=IBM.com (IBM Knowledge Center)
      |url=https://www.ibm.com/support/knowledgecenter/SSLTBW_2.3.0/com.ibm.zos.v2r3.idad400/share.htm
      |quote=describes considerations for sharing VSAM data sets for NSR or LSR/GSR
      |title=Sharing VSAM Data Sets}}</ref>
* Non-Shared Resources (NSR), which is optimised for sequential access. NSR access has historically been easier to use than LSR for batch programs.<ref name=VSAM.LSRorNSR/>
* Distributed File Management (DFM), an implementation of a [[Distributed Data Management Architecture]] server, enables programs on remote computers to create, manage, and access VSAM files.

==Sharing VSAM data==
Sharing of VSAM data between CICS regions can be done by VSAM Record-Level Sharing (RLS). This adds record caching and, more importantly, record locking. Logging and commit processing remain the responsibility of CICS which means that sharing of VSAM data outside a CICS environment is severely restricted.

Sharing between CICS regions and batch jobs requires Transactional VSAM, DFSMStvs. This is an optional program that builds on VSAM RLS by adding logging and two-phase commit, using underlying z/OS system services. This permits generalised sharing of VSAM data.

==History==
VSAM was introduced as a replacement for older access methods<ref>{{cite manual
 |     title = OS/Virtual Storage 1 Features Supplement 
 |        id = GC20-1752-0
 |      date = August 1972
 |   edition = First
 |       url = http://bitsavers.org/pdf/ibm/370/OS_VS1/GC20-1752-0_OS_VS1_Features_Supplement_Aug72.pdf
 | publisher = IBM
 }}
</ref> and was intended to add function, to be easier to use and to overcome problems of performance and device-dependence. VSAM was introduced in the 1970s when IBM announced [[Virtual memory|virtual storage]] operating systems (DOS/VS, OS/VS1 and OS/VS2) for its new System/370 series, as successors of the DOS/360 and OS/360 operating systems running on its System/360 computer series. While backwards compatibility was maintained, the older access methods suffered from performance problems due to the address translation required for virtual storage.

The KSDS organization was designed to replace [[ISAM]], the Indexed Sequential Access Method. Changes in disk technology had meant that searching for data in ISAM data sets had become very inefficient. It was also difficult to move ISAM data sets as there were embedded pointers to physical disk locations which became invalid if the data set was moved. IBM also provided a compatibility interface to allow programs coded to use ISAM to use a KSDS instead.

The RRDS organization was designed to replace BDAM, the Basic Direct Access Method. In some cases, BDAM data sets contained embedded pointers which prevented them from being moved. However, most BDAM data sets did not and the incentive to move from BDAM to VSAM RRDS was much less compelling than that to move from ISAM to VSAM KSDS.

Linear data sets were added later, followed by VSAM RLS and then Transactional VSAM.

== See also ==
* [[Job Control Language]] (JCL)
* [[Software:IBM mainframe utility programs|IBM mainframe utility programs]]
* [[ISAM]]
* Geneva ERS
* Record Management Services, a similar system developed by [[Company:Digital Equipment Corporation|Digital Equipment Corporation]]
* [[Software:Enscribe|Enscribe]], a similar system developed by [[Company:Tandem Computers|Tandem Computers]]
== Notes ==
{{Reflist|group=NB}}

== References ==
{{reflist}}
* [http://www.redbooks.ibm.com/abstracts/sg246105.html?Open VSAM Demystified]
* [https://www-304.ibm.com/servers/resourcelink/svc00100.nsf/pages/zOSV2R3SC236877/$file/idat100_v2r3.pdf DFSMStvs Overview and Planning Guide]

{{Mainframe I/O access methods}}

[[Category:IBM mainframe operating systems]]
[[Category:IBM file systems]]
[[Category:Computer file formats]]

{{Sourceattribution|Virtual Storage Access Method}}