key-sequenced data set

{{Short description|IBM data record organization type}}

{{no footnotes|date=March 2017}}

{{merge|Linear data set|Entry-sequenced data set|Relative record data set|target=Virtual Storage Access Method|discuss=Talk:Virtual Storage Access Method#Merge suggestion|date=March 2025}}

A key-sequenced data set (KSDS) is a type of data set used by IBM's VSAM computer data storage system.{{cite book|url=https://www.redbooks.ibm.com/redbooks/pdfs/sg246105.pdf |series=Redbooks |title=VSAM Demystified |first1=Mary |last1=Lovelace |first2=Jose |last2=Dovidauskas |first3=Alvaro |last3=Salla |first4=Valeria |last4=Sokal |chapter=Chapter 1. VSAM basics |edition=3 |publisher=IBM |date=August 2022}}{{rp|5}} Each record in a KSDS data file is embedded with a unique key.{{rp|20}} A KSDS consists of two parts, the data component and a separate index file known as the index component which allows the system to physically locate the record in the data file by its key value.{{rp|13}} Together, the data and index components are called a cluster.{{rp|15}}

Records can be accessed randomly or in sequence and can be variable-length.

As a VSAM data set, the KSDS data and index components consist of control intervals{{rp|10}} which are further organized in control areas.{{rp|12}} As records are added at random to a KSDS, control intervals fill and need to be split into two new control intervals, each new control interval receiving roughly half of the records. Similarly, as the control intervals in a control area are used up, a control area will be split into two new control areas, each new control area receiving roughly half the control intervals.{{rp|17}}

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.{{rp|15}} An alternate index is itself a KSDS.{{rp|16}}