Although a TCB can own many data spaces, it is important that it reference these data spaces carefully. It is more efficient for the system to reference the same data space ten times, than it is to reference each of ten data spaces one time. For example, an application might have a master application region that has many users, each one having a data space. System performance is best if each program completes its work with one data space before it starts work with another data space.

MVS™ limits the number of access list entries and the number of data spaces available to each TCB. Therefore, given a choice, you must use one large data space rather than a number of small data spaces that add up to the size of the one large data space.