Virtual storage overview

While there is no theoretical limit to 64-bit virtual storage ranges, practical limits exist to the finite real storage frames (formally called central storage) and auxiliary storage pages (slots) that back the virtual storage. Thus, estimating and limiting the virtual storage allocated on a system is important primarily because some percentage, for instance 25 percent, of the virtual storage must be backed by real storage for the system to function and perform well. In cases where there is insufficient real storage to contain all the backed virtual storage of all the active address spaces, the system can, based on work importance, postpone new work, or page out the contents of pageable (not fixed) real storage frames to auxiliary storage, or both. Paging frees up the frames for use by more important work. A real storage memory pool limit can also cause the system to page out real frames of pool members even though there are plenty of available real frames on the system.

For information about estimating the amount of auxiliary storage your system will need, see the discussion of paging data space in Auxiliary storage management initialization.

Each installation can use virtual storage parameters to specify how certain virtual storage areas are to be allocated and limited. These parameters have an impact on real storage use and overall system performance. The following overview describes the function of each virtual storage area. For information about identifying problems with virtual storage requests, see Identifying problems in virtual storage (DIAGxx parmlib member).