z/OS® XL C/C++ supports extended format sequential data sets. Extended format data sets must be SMS-managed. Optionally, extended format data sets can be striped. Striping spreads a data set over a specified number of volumes such that I/O parallelism can be exploited. Unlike a multivolume data set in which physical record n follows record n-1, a striped data set has physical records n and n-1 on separate volumes. This enables asynchronous I/O to perform parallel operations, making requests for multiple reads and writes faster.
Striped data sets also facilitate repositioning once the relative block number is known. z/OS XL C/C++ exploits this capability when it uses fseek() to reposition. This can result in substantial savings for applications that use ftell() and fseek() with data sets that have RECFMs of V, U, and FB (not FBS). data sets. When a data set is striped, an fseek() can seek directly to the specified block just as an fsetpos() or rewind() can. For a normal data set with the aforementioned RECFMs, z/OS XL C/C++ has to read forward or rewind the data set to get to the desired position. Depending on how large the data set is, this can be quite inefficient compared to a direct reposition. Note that for such data sets, striping pads blocks to their maximum size. Therefore, you may be wasting space if you have short records.