what makes up the REGION NEEDED TO STARTUP storage

Document ID : KB000105997
Last Modified Date : 17/07/2018
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In startup message DC390006 Vnnn REGION NEEDED TO START UP, exactly what makes up this amount of storage? 
Here is a discussion of the startup messages that deal with the amount of 24-bit Region used by an IDMS CV at startup.

Example from one CV:
DC390005 V1900 RHDCPARM FREESTG RELEASED...     2,048K
DC390006 V1900 REGION NEEDED TO START UP...     4,752K
DC390020 V1900 TOTAL AMOUNT OF XA STORAGE..    71,620K
DC390007 V1900 SYSTEM CONFIGURATION SIZE...    74,320K
DC390008 V1900 STORAGE RETURNED TO OPSYS...     3,048K
  The REGION NEEDED TO START UP is the amount of 24-bit region needed for the portion of the IDMS CV system configuration that must reside in 24-bit address space.
  It is made up of the FREESTG value specified in startup parameter, plus all 24-bit storage and program pools, plus a fixed 24-bit area for certain control block areas and system tables.

  Message DC390008 V1900 STORAGE RETURNED TO OPSYS also refers to 24-bit region and is the amount left from the initial GETMAIN (which requested all the 24-bit Region the opsys could give us) after building the 24-bit system configuration.  
  See RI07097 for details of how the 24-bit region is acquired and configured.
  Note from that discussion of how to avoid abends like S40D, S878, S80A that in this example we could move most of the 3048K RETURNED TO OPSYS into FREESTG.  Those kinds of abends can happen in some CVs depending on the processing and possible third party vendor products that could do opsys GETMAINs that could also use LSQA opsys stg after CV is fully up.
  For the most part all of the GETMAINS IDMS does to build the system are done by the time CV is fully up.  The only exception is a Buffer defined to use opsys stg whose areas come up in retrieval mode. That buffer is not opened at startup, but later when one of the areas is accessed.  Only then does IDMS allocate the stg for the buffer (in 31-bit stg).    
  DCMT D MEM MAP gives a good high level picture of the 24-bit region used. The startup module is loaded at the lowest 24-bit address within the 24-bit region IDMS was given and you can see some of the major control block areas as well as the addresses of the 24-bit storage and program pools.  
      D MEM MAP                                                                
RHDCOS00      7000  OPT          292F0  NLT          2AC20  CSA          2CDB0 
LKM          34040  SCAAREA      34AE0  CCE          37AC0  STGPOOL      61000 
PTT          63108  ECBLIST     1DC4A0  CSVCAREA    1DC8C0  TCA         1E1E00 
DCEAREA     1E1EC0  TCEAREA     1E2F00  PGMPOOL     208000  RENTPOOL    221000 
RHDCD05V    24D200  RHDCD07Q    252200  RHDCD00B    253E00  RHDCD0EV    255A00 
RHDCD03Q    264200  HIADDR      2AA044  SVC172      B573C8  EREAREA   289FE140 
ESE       289FE8E0  DMCL1900  3AF0C490  RUA       3AF26D90  DDT       3AF28C08 
LTT       3AF28D88  TCESTACK  3AF5DF88  ABENDSTG  3B09C5E8  QDT       3B0A1808 
TDT       3B0A3508  PDT       3B0B5F08  SCT       3B163888  MPMODTBL  3B165608 
RCA       3B165E88  RLEAREA   3B165F48  RCEAREA   3B1892C0  DPEAREA   3B206540 
ILEAREA   3B20E9E8  XASTGPL   3ED5D000  XAPGMPL   400E5000  XARENTPL  400FE000 
IDMSDBIO  400FE0B8  IDMSDBMS  401210B8  RHDCD04W  403DF600  RHDCD0ZU  403E0C00 
RHDCD0LV  403E7C38  RHDCD0IP  403EA400  RHDCRUSD  40C11400  RHDCLGSD  40C11E00 
RHDCCFSD  40C25800  IDMSLMSD  40C27C00  IDMSDBSD  40C29800                     

  If you take the HIADDR value which is the highest 24-bit address used and subtract the address of RHDCOS00 and add the FREESTG value you should come up with a number very close to the number you see in REGION NEEDED TO START UP.
  In this case HIADDR minus OS00 address =  2764868   2700K
  The amount of 24-bit Region the opsys gives us varies depending on opsys release and other opsys factors, but is typically between 7 and 8 MB. We've seen some sites get over 9MB but you can see from my example by adding REGION NEEDED TO START UP to STORAGE RETURNED TO OPSYS that on my LPAR I was given a total of 7800K.  
  It is important to monitor these startup numbers to avoid exceeding the use of this amount.
  If you over allocate 24-bit storage or program pools you could cause DC390001 or DC390003 indicating INSUFFICIENT REGION SIZE.
  In my example the 24-bit non-FREESTG system config was only about 2700K.
  You want this amount to be a small as possible while not causing any shortages in the 24-bit pools.  

  DCMT D ALL STORAGE POOL  and DCMT D ALL PROGRAM POOL  should be monitored to see if the HWMs shown indicate over-allocation of the 24-bit pools.  Reducing these pools to only a small amount above what is needed will keep REGION NEEDED TO STARTUP small enough to avoid encountering an  INSUFFICIENT REGION SIZE.
  While you want these pools to be as small as possible, you also want them to be large enough that you encounter no SOS conditions in the 24-bit storage pools and the 24-bit Program and Reentrant pools should indicate all programs loaded to these pools have been loaded only once.

   To see that all programs loaded to these pools have been loaded only once, DCMT D ACT PRO  or DCMT D ACT REENT PRO stats should show 0 in the following two lines:

Loads overlaying program not in use         0   0% of loads 
    Loads overlaying program in use         0   0% of loads 

This means no programs had to be loaded more than once.