31-bit z/OS storage utilization by an IDMS CV

Products

• IDMS
• IDMS - Database

Issue/Introduction

This article describes how to calculate how much 31-bit operating system storage a CV uses.

The DC390020 message does not appear to include everything allocated in 31-bit (XA) storage.

Resolution

This is true because some 31-bit opsys storage allocations are done after the startup messages DC390005 thru DC390008 are issued.

DC390020 Vnnn TOTAL AMOUNT OF XA STORAGE  includes all 31-bit pools plus the fixed control block areas and system tables that reside in 31-bit storage. 

This does not include the 31-bit storage used for database buffers as they are allocated after this point in startup. 

The output of a LOOK DMCL shows how much 31-bit operating system storage the database buffers will use. Example: 

         0 Bytes used for CV buffers in DC storage 
15,867,132 Bytes used for CV buffers in OS storage 
15,867,132 Bytes used for CV DMCL Buffers 

This plus the TOTAL AMOUNT OF XA STORAGE in the DC390020 message is the real total 31-bit storage a CV uses. 

This total should be significantly less than the maximum of 2GB for 31-bit storage in an address space.

One more factor that could add to this is SCRATCH if it is allocated in 31-bit. 

SCRATCH is one of the few things that can be allocated in 64-bit storage so if it is defined to use 64-bit storage and the LPAR has 64-bit storage capabilities, then SCRATCH will not add to the amount of 31-bit stg used by the CV.

In the following example SCRATCH is allocated in 64-bit:

 DCMT D SCRATCH                                                                
Total   number of pages         8075    Location                   ANY Storage 
Page size                       5064    Storage address      00000050 00000000 <== 64 bit address. 
Primary   extent #pages         8075    Primary   extent size            39 MB 
Secondary extent #pages         8075    Secondary extent size            39 MB 
Storage   limit  #pages         8075    Storage   limit  size            39 MB 
PUT    scratch requests         4419    Scratch Areas active                 1 
GET    scratch requests          208    Scratch Areas created               17 
DELETE scratch requests         4139    HWM concurrent Scr. Areas            6 
Pages in use                      98    HWM pages in use                   103 
Pages in use percentage            1%   HWM pages in use  percentage         1%
Buffers                          N/A    HWM pages in use  for 1 S.A.       100 
Pages found in buffer            N/A    HWM pages found   for 1 S.A.       N/A 
Pages written                    N/A    HWM pages written for 1 S.A.       N/A 
Pages read                       N/A    HWM pages read    for 1 S.A.       N/A 


If SCRATCH is allocated in 31-bit storage, then it must be added to the DC390020 and Bytes used for CV buffers in OS storage AND again the total should be significantly less than 2GB to avoid system paging.

The system trace table (SYSTRACE) is also allocated later in the startup process and not included in the startup message, so it should be added to the DC390020 storage amount. See IDMS SYSTRACE table size and startup messages DC390020 DC390007.