Why does increasing my XA STORAGE POOL use more 24-bit storage?

Document ID : KB000118220
Last Modified Date : 26/10/2018
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Introduction:
CA IDMS provides for storage pools both above and below the line, but it can be shown that increasing the size of an above-the-line pool causes the CV to use more below-the-line storage.
Question:
Starting with an XA STORAGE POOL set to 10,000 K, CV startup shows this:
DC390005 V65 RHDCPARM FREESTG RELEASED...    768K
DC390006 V65 REGION NEEDED TO START UP...  6,212K
DC390020 V65 TOTAL AMOUNT OF XA STORAGE.. 49,708K
DC390007 V65 SYSTEM CONFIGURATION SIZE... 55,148K
DC390008 V65 STORAGE RETURNED TO OPSYS...  1,444K
If the XA STORAGE POOL is increased to 200,000K, CV startup shows this:
DC390005 V65 RHDCPARM FREESTG RELEASED...     768K
DC390006 V65 REGION NEEDED TO START UP...   6,396K
DC390020 V65 TOTAL AMOUNT OF XA STORAGE.. 239,892K
DC390007 V65 SYSTEM CONFIGURATION SIZE... 245,516K
DC390008 V65 STORAGE RETURNED TO OPSYS...   1,260K
So increasing the XA STORAGE POOL by 190,000K results in the CV using 1,444 - 1,260 = 184 K more below-the-line storage.

Why does this happen?
Environment:
CA-IDMS, all supported releases.
Answer:
The CSVCAREA, which resides in 24-bit storage, contains four CSVs (96 bytes) and a CSV page list whose length is determined by the number of pages for all SCTs plus the number of pages for the 24-bit and 31-bit program pools.
The CSV page list is used for storage protection and swapping from the primary to alternate keys.
Any time any of these pools are increased in size, the size of the CSVCAREA will also increase.
This applies to all storage and program pools, both above and below the line.
Additional Information:
Storage Pools