Pages Requested higher in CV vs Local for same retrieval pgm

Products

• IDMS - Database

Issue/Introduction

The same batch retrieval-only program issuing the exact same DML run under CV shows a higher PAGES REQUESTED value than when run in local mode.

Lines Requested and Recs Made Current are identical.

Why? 

Environment

IDMS 19.0 and all prior releases

Cause

Processing differs in CV due to considerations for other transactions that may be accessing the same pages.

Resolution

- Under CV IDMSDBMS always releases its internal lock on a page at the   
   completion of a command to ensure that other run-units can access     
   the buffer.  As a result a subsequent access to the same page is      
   passed to IDMSDBIO even though it may have been the last buffer (page)
   accessed.  This results in an increased PAGES REQUESTED value.        
                                                                         
- For a local mode job there is no concern for conflicts with other      
  run-units so the internal buffer lock is not released until            
  another page is needed by IDMSDBMS.  As a result an additional call to 
  DBIO is not required if we keep accessing the same database page.  The 
  size of the difference depends on the data path characteristics of the 
  run-unit.                                                        

     
This means that the number of pages requested through CV will            
typically be larger than the number of pages requested for a local job.  
Whether a job uses dataspace or prefetch does not matter since the       
statistic is updated by IDMSDBMS before control is passed to IDMSDBIO.   

Additional Information

How to diagnose database application performance issues?