Accessing a data space

IMS assigns areas to data spaces using a first fit algorithm.

The entire root addressable portion of an area (including independent overflow) resides in the data space. The sequential dependent portion does not reside in the data space.

The amount of space needed for an area in a data space is (CI size) × (number of CIs per UOW) × ((number of UOWs in root addressable portion) + (number of UOWs in independent overflow portion)) rounded to the next 4 KB.

Expressed in terms of the parameters of the DBDGEN AREA statement, this formula is (SIZE parameter value) × (UOW parameter value) × (ROOT parameter value) rounded to the next 4 KB.

The actual amount of space in a data space available for an area (or areas) is two gigabytes (524,288 blocks, 4 KB each) minus an amount reserved by z/OS® (from 0 to 4 KB) minus an amount used by IMS Fast Path (approximately 100 KB). You can use the /DISPLAY FPVIRTUAL command to determine the actual storage usage of a particular area.

During IMS control region initialization, IMS calls DBRC to request a list of all the areas that are defined as VSO. This list includes the PREOPEN or PRELOAD status of each VSO area. If VSO areas exist, IMS acquires the appropriate data spaces. Then IMS opens all areas defined with PREOPEN and opens and loads areas defined with PRELOAD. During a normal or emergency restart, the opening and loading of areas might occur after control region initialization, if you have changed the specifications of the FPOPN parameter in the IMS procedure.