What is VSAM?
VSAM stands for Virtual Storage Access Method. It is a method of managing files that is used mainly on mainframes but also on PCs. Specifically, VSAM can speed up access to file data by using a reverse index of records appended to files. This index is called a B+ tree. VSAM is a kind of record-oriented file system. In this kind of dataset, information is stored as a collection of records. VSAM records can be of any length; they need not be of one set length. They are, however, organized into blocks called Control Intervals, which are measured in bytes. These Control Intervals are further organized into Control Areas, which are measured in much larger units. VSAM can be organized into three types of dataset: Entry Sequenced Data Set (ESDS), Key Sequenced Data Set (KSDS), and Relative Record Data Set (RRDS). ESDS items are listed in the order in which they were input, regardless of any other consideration. Each item in a KSDS is assigned a unique numerical key, according to which the dat
The term Virtual Storage Access Method (VSAM) applies to both a data set type and the access method used to manage various user data types. As an access method, VSAM provides much more complex functions than other disk access methods. VSAM keeps disk records in a unique format that is not understandable by other access methods. VSAM is used primarily for applications. It is not used for source programs, JCL, or executable modules. VSAM files cannot be routinely displayed or edited with ISPF. You can use VSAM to organize records into four types of data sets: key-sequenced, entry-sequenced, linear, or relative record. The primary difference among these types of data sets is the way their records are stored and accessed.
