Hot I/O refers to the repeated I/O interruptions that result from hardware malfunctions. Because hot I/O can cause the system to loop or to fill the system queue area with I/O control blocks, operators need to detect hot I/O quickly and correct the problem.

When the number of repeated interruptions exceeds an installation-defined threshold value, the system assumes there is a hot I/O condition. You can establish hot I/O recovery threshold values. If the threshold is reached, the system issues message IOS109I and attempts to recover from the hot I/O condition. The IECIOSxx parmlib member allows you to change threshold default values. See z/OS MVS Initialization and Tuning Reference for information on setting up hot I/O recovery defaults.