Suppose an existing program updates several rate tables that reside on DASD. Updates are random throughout the tables. The tables are too large and too many for your program to keep in contiguous storage in its address space. When the program updates a table, it reads that part of the table into a buffer area in the address space, updates the table, and writes the changes back to DASD. Each time it makes an update, it issues instructions that cause I/O operations.

Assume you want to change this application to improve its performance. If the tables were to reside in data spaces, one table to each data space, the tables would then be accessible to the program through assembler instructions. The program could move the tables to the data spaces (through buffers in the address space) once at the beginning of the update operations and then move them back (through buffers in the address space) at the end of the update operations.

If the tables are VSAM linear data sets, data-in-virtual can map the tables and move the data into the data space where a program can access the data. Data-in-virtual can then move the data from the data space to DASD. With data-in-virtual, the program does not have to use address space buffers as an intermediate buffer area for transferring data to and from DASD.