I/O activity

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.

I/O activity is primarily measured in the following ways:

EXCPs: The number of execute channel program (EXCP) commands issued (logical I/Os)
SSCHs: The number of start subchannel (SSCH) commands issued (physical I/Os)
Device Connect Time: The amount of time a particular device is dedicated for the I/O transfer
Channel Usage: The percentage of time a channel is busy initiating, transferring, or completing the movement of data between a device and the CPU

While EXCPs are often used as a measure of I/O activity, their counts can be extremely misleading. For example, one EXCP can be used to transfer a few bytes or dozens of megabytes! SSCHs measure the number of physical I/Os to a data set and thus can be more useful than EXCPs. A complete analysis of total I/O performance should consider device connect time, channel usage and SSCHs.