Data Set (IBM Mainframe)

Data Set (IBM Mainframe)

data set (archaic), dataset (preferred), is a computer file having a record organization. The term pertains to the IBM mainframe operating system line, starting with OS/360, and is still used by its successors, including the current z/OS. Those systems historically preferred this term over a file. A dataset is typically stored on direct access storage device (DASD) or magnetic tape, however unit record devices are also supported.

Datasets are not unstructured streams of bytes, but rather are organized in various logical record and block structures determined by the DSORG (data set organization), RECFM (record format), and other parameters. These parameters are specified at the time of the data set allocation (creation), for example with the Job Control Language DD statements. Inside a job they are stored in the Data Control Block (DCB), which is a data structure used to access datasets, for example using access methods.