This represents the movement of data between the processor and disk or tape devices. The effective use of I/O resources is important to a site and I/O activity is often an important component of site accounting methodologies.

Because performing input to and output from disk and tape devices typically takes much longer than manipulating the data in real storage, the amount of I/O performed is a key component of an application's Elapsed time. Therefore, reducing I/O generally improves an application's Elapsed time.