IDMS "Real Storage" utilization

Products

• IDMS
• IDMS - Database

Issue/Introduction

How does IDMS use "Real Storage"?

SYSVIEW IDMS Menu #1 IDMSLIST will show active IDMS CVs and the column "RealStg" does not always reflect the storage allocation reported in this IDMS startup message:

DC390007 Vnn SYSTEM CONFIGURATION SIZE

Environment

Release: All supported releases.

Resolution

The real storage usage for IDMS will vary depending on what storage pages are needed at any given time.

IDMS does PAGE FIX a small amount of storage at the beginning of the IDMS region. RHDCOESA, SCAs, TCE/DCEs, etc. This is required as some of the routines and some of the control blocks these routines work with require them to be fixed. They cannot handle a page fault occurring. 

To demonstrate. Looking at the real storage for an IDMS system, especially one that is not very active (although this still works for an active system), issue this IDMS command:

DCMT DME STR C'sdughfsayrbjdjfh' (100) 100 

(the value for the STR inside the quotes is just something that will not be found.) 

Then go back and look at the real storage, notice the number has jumped (increased) significantly. Eventually the operating system will take the page back as needed and the real storage value will shrink. 

This is not IDMS specific. This is standard operating system paging rules.