Dynamic I/O configuration lets you change your I/O configuration without causing a system outage. In other words, you can select a new I/O configuration definition without performing a power-on-reset (POR) of the hardware or an initial program load (IPL) of the z/OS system. Using I/O definition files (IODFs) created through hardware configuration definition (HCD), dynamic I/O configuration allows you to add, delete, or modify the definitions of channel paths, control units, and I/O devices to the software and hardware I/O configurations. You can change the I/O configuration definitions to both software and hardware or software only. (See z/OS HCD Planning.)