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Virtual Storage Access Method (VSAM) [1] 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.
IDCAMS probably has the most functionality of all the utility programs, performing many functions, for both VSAM and non-VSAM files. The following example illustrates the use of IDCAMS to copy a dataset to disk. The dataset has 80-byte records, and the system will choose the block size for the output:
A relative record data set (RRDS) is a type of data set organization used by IBM's VSAM computer data storage system. [1]: 5 Records are accessed based on their ordinal position in the file (relative record number, RRN). [1]: 29 For example, the desired record to be accessed might be the 42nd record in the file out of 999 total.
An entry-sequenced data set (ESDS) is a type of data set used by IBM's VSAM computer data storage system. [1]: 5 Records are accessed based on their sequential order, that is, the order in which they were written to the file; which means that accessing a particular record involves searching all the records sequentially until it is located, or by using a relative physical address (Relative byte ...
VSAM also included a catalog component used for user catalogs and MVS' master catalog. Partitioned data sets (PDS) are sequential data sets subdivided into "members" that could each be processed as sequential files in their own right (like a folder in a file system). The most important use of PDSes was for program libraries - system ...
A key-sequenced data set (KSDS) is a type of data set used by IBM's VSAM computer data storage system. [ 1 ] : 5 Each record in a KSDS data file is embedded with a unique key. [ 1 ] : 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 ...
For example, the AORs might not perform File I/O. Instead there would be a "File-Owning Region" (FOR) that performed the File I/O on behalf of transactions in the AOR – given that, at the time, a VSAM file could only support recoverable write access from one address space at a time.
The indexes are created with the file and maintained by the system. IBM supports indexed files with the Indexed Sequential Access Method on OS/360 and successors. IBM virtual storage operating systems added VSAM, which supports indexed files as Key Sequenced Data Sets (KSDS), with more options.