VSAM: Тікелей қол жеткізу әдісі және құрылымы

Virtual Storage Access Method (VSAM) is an IBM direct access storage device (DASD) file storage access method, first used in the OS/VS1, OS/VS2 Release 1 (SVS) and Release 2 (MVS) operating systems, later used throughout the Multiple Virtual Storage (MVS) architecture and now in z/OS. Originally a record oriented filesystem, VSAM comprises four 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.

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. 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), 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 Definition (DD) statements in Job Control Language (JCL), via dynamic allocation or in online regions such as in Customer Information Control System (CICS). Both IMS/DB and Db2 are implemented on top of VSAM and use its underlying data structures.

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.

VSAM components consist of fixed length physical blocks grouped into fixed length control intervals Records are loaded irrespective of their contents and their byte addresses cannot be changed.

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.

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

An LDS is an unstructured VSAM dataset with a control interval size of a multiple of 4K. It is used by certain system services.