When using transaction isolation there is a cost in terms of real storage. Paging problems can result if insufficient real storage is allocated, which then affects performance. The cost is very much based on the number of subspaces in use in the system, and the size of EDSALIM.
Since the pagesize of the EUDSA is one MB, EDSALIM is likely to be very large for a CICS® system which has transaction isolation active. Since this virtual storage needs to be mapped with page and segment tables using real storage, an increase in the real storage usage can occur. In addition to the real storage used to map the virtual storage for the EDSALIM, subspaces also require real storage. For example:
The figures for the real storage usage is in addition to that required for a CICS system that does not have transaction isolation active.