Mainframe, MVS and zOS Discussion

Swapping is the process of transferring all of the pages of an address space between central storage and auxiliary storage.

A swapped-in address space is active, having pages in central storage frames and pages in auxiliary storage slots. A swapped-out address space is inactive; the address space resides on auxiliary storage and cannot execute until it is swapped in.

While only a subset of the address space's pages (known as its working set) would likely be in central storage at any time, swapping effectively moves the entire address space. It is one of several methods that z/OS® uses to balance the system workload and ensure that an adequate supply of available central storage frames is maintained.

Swapping is performed by the System Resource Manager (SRM) component, in response to recommendations from the Workload Manager (WLM) component.

z/OS uses a series of tables to determine whether a page is in real or auxiliary storage, and where. To find a page of a program, z/OS checks the table for the virtual address of the page, rather than searching through all of physical storage for it. z/OS then transfers the page into central storage or out to auxiliary storage as needed. This movement of pages between auxiliary storage slots and central storage frames is called paging. Paging is key to understanding the use of virtual storage in z/OS.