Advanced Topics › z/OS Hardware Configuration Definition (HCD) ACTIVATE Considerations › Overview of HCD ACTIVATE › How You Change Hardware and Software Definitions

How You Change Hardware and Software Definitions

When an I/O configuration is dynamically changed, the definitions to both hardware and software or to software only, can be changed. A software-only change modifies only software control structures, such as the unit control blocks (UCBs) and the eligible device table (EDT). A hardware and software change modifies both hardware and software control structures. In most cases, simultaneous dynamic configuration changes to both hardware and software configuration definitions are made.

A dynamic configuration change causes z/OS to activate a new EDT definition. Under normal operating circumstances, z/OS uses one eligible device table (EDT) to process allocation requests. As a result of dynamically changing the software I/O configuration definition of a system, z/OS must use two EDTs to process the change: a primary and secondary EDT.

The primary EDT processes all current and new allocation requests. z/OS usually runs with only a primary EDT, until a dynamic configuration change is initiated. z/OS then activates a new primary EDT for the new I/O configuration, making the old primary EDT the secondary EDT.

All new allocation requests, from the ACTIVATION point forward, use the new primary EDT. The secondary EDT receives no new allocation requests. z/OS deletes the secondary EDT when the allocation requests issued before the dynamic configuration change (all requests currently using the old EDT) complete.

Notes:

• Allocation does not stop because an ACTIVATE of a new IODF is in progress. New allocations use the new primary EDT built by the ACTIVATE; allocation requests initiated before the ACTIVATE of the new IODF use the secondary EDT.
• The secondary EDT cannot be deleted until all allocation requests using it have completed. Even though an IOS500I ACTIVATE RESULTS message is issued indicating that the ACTIVATE COMPLETED SUCCESSFULLY, until the IOS501I ACTIVATE CLEANUP COMPLETE message is issued, IOS holds an exclusive enqueue for SYSZIOS/Dynamic.

  A subsequent ACTIVATE, initiated on the same system before the IOS501I ACTIVATE CLEANUP COMPLETE message is issued, may fail because residual allocation requests are still bound to the secondary EDT.