In synchronous mode, VSAM does not return to your program from a PUT or GET operation until it has completed the operation. In asynchronous mode, VSAM returns control to your program before completing a PUT or a GET. A program in asynchronous mode can perform other useful work while a VSAM PUT or GET is completed.

Asynchronous mode can improve throughput with direct processing because it permits processing to overlap with accesses from and to the direct access device. When reading records directly, each read often involves a seek on the direct access device, a slow operation. In synchronous mode, this seek time does not overlap with other processing.