Virtual storage access method
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Virtual storage access method (VSAM) is an IBM disk file storage access method, first used in the OS/VS2 operating system, later used throughout the Multiple Virtual Storage (MVS) architecture and now in z/OS. Originally a record-oriented filesystem, VSAM comprises four data set organizations: Key Sequenced Data Set (KSDS), Relative Record Data Set (RRDS), Entry Sequenced Data Set (ESDS) and Linear Data Set (LDS). The KSDS, RRDS and ESDS organizations contain records, while the LDS organization (added later to VSAM) simply contains a sequence of bytes with no intrinsic record structure.
IBM uses the term data set in official documentation as a synonym of file, and DASD instead of disk drive.
VSAM records can be of fixed or variable length. They are organised in fixed-size blocks called Control Intervals (CIs), and then into larger divisions called Control Areas (CAs). Control Interval sizes are measured in bytes — for example 4 kilobytes — 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 IDCAMS is commonly used to manipulate ("delete and define") VSAM data sets. Custom programs can access VSAM datasets through data definitions (DDs) in Job Control Language (JCL) or in online regions such as in Customer Information Control Systems (CICS).
Both IMS/DB and DB2 are implemented on top of VSAM and use its underlying data structures.
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[edit] VSAM files
differs considerably from the organizations used by other access methods. data sets are equivalent to QSAM sequential files. The order of the records is fixed.
[edit] VSAM indexed file organization
(Also referred to as VSAM KSDS (key-sequenced data set) organization.) In a VSAM indexed file (KSDS), the records are ordered according to the collating sequence of an embedded prime key field, which you define. The prime key consists of one or more consecutive characters in the records. The prime key uniquely identifies the record and determines the sequence in which it is accessed with respect to other records. The most important that has to be noted while going for the KSDS is that, the key value has to be in a sorted order in the file.A prime key for a record might be, for example, an employee number or an invoice number.
[edit] VSAM relative file organization
(Also referred to as VSAM fixed-length or variable-length RRDS (relative-record data set) organization.) A VSAM relative-record data set (RRDS) contains records ordered by their relative key. The relative key is the relative record number, which represents the location of the record relative to where the file begins. The relative record number identifies the fixed- or variable-length record.
In a VSAM fixed-length RRDS, records are placed in a series of fixed-length slots in storage. Each slot is associated with a relative record number. For example, in a fixed-length RRDS containing 10 slots, the first slot has a relative record number of 1, and the tenth slot has a relative record number of 10.
In a VSAM variable-length RRDS, the records are ordered according to their relative record number. Records are stored and retrieved according to the relative record number that you set.
Throughout this documentation, the term VSAM relative-record data set (or RRDS) is used to mean both relative-record data sets with fixed-length records and with variable-length records, unless they need to be differentiated.
[edit] VSAM Data Access Techniques
There are three types of access technique for VSAM data:
- Local Shared Resources (LSR)
- Global Shared Resources (GSR)
- Non-Shared Resources (NSR)
Each is optimised for different access patterns. For example, LSR is optimised for "random" or direct access, whereas NSR is optimised for sequential access.
Another difference is that some access techniques are more available than others for specific execution and programming environments. For example, LSR access is easy to achieve from CICS while NSR access has historically been easier to use than LSR for batch programs.
