The Full Form of VSAM is‍  Virtual Storage Access Method.

Virtual storage access method (VSAM) is a file storage access method used in IBM mainframes. VSAM is an expanded version of indexed sequential access method (ISAM), which was the file access method used previously by IBM.

VSAM enables the organization of records in physical or logical sequences. Files can also be indexed by record number. VSAM helps enterprises by providing quick access to data by the use of the inverted index. The records in VSAM can be either variable or fixed length.

VSAM consists of four major types of data sets (a file is represented by data set by IBM), these include:

• Key-sequenced data set (KSDS)
• Relative record data set (RRDS)
• Entry-sequenced data set (ESDS)
• Linear data set (LDS)

KSDS, RRDS and ESDS contain records while LDS consists of page sequences.