Use of BLOCK CONTAINS and DISPLACEMENT directives within index-specification clause in AREA definition

[Michael Cairns]

Dear all,

Something perhaps for the Guru's here to chime in on I hope.  We have some AREA's defined with the following characteristics (anonymised for posting):

CREATE PHYSICAL AREA TESTSEG.ARONE                         
     PRIMARY SPACE 18000 PAGES  FROM PAGE 1006001     
     MAXIMUM SPACE 18000 PAGES                        
     PAGE SIZE 27648 CHARACTERS                       
     INCLUDE SUBAREA SA-REC1-INDX1                    
         OFFSET 0 PERCENT FOR 61 PERCENT              
     INCLUDE SUBAREA SA-REC2-INDX1                    
         OFFSET 61 PERCENT FOR 16 PERCENT             
     INCLUDE SUBAREA SA-REC2-INDX2                    
         OFFSET 77 PERCENT FOR 16 PERCENT             
     INCLUDE SUBAREA SA-REC2-INDX3                    
         OFFSET 93 PERCENT FOR 7 PERCENT              
     INCLUDE SYMBOLIC INDEX S-REC1-INDX1              
         BLOCK CONTAINS 59 KEYS DISPLACEMENT 750 PAGES
     INCLUDE SYMBOLIC INDEX S-REC2-INDX1              
         BLOCK CONTAINS 45 KEYS DISPLACEMENT 150 PAGES
     INCLUDE SYMBOLIC INDEX S-REC2-INDX2              
         BLOCK CONTAINS 45 KEYS DISPLACEMENT 120 PAGES
     INCLUDE SYMBOLIC INDEX S-REC2-INDX3              
         BLOCK CONTAINS 45 KEYS DISPLACEMENT 60 PAGES 
     WITHIN FILE FIARONE                              
         FROM 1 FOR ALL BLOCKS                        
     ;                                                

Our question is about the meaning of the 'BLOCK CONTAINS nn KEYS DISPLACEMENT nnn PAGES' clauses.  I've read the Fine Documentation a few times now and see that this particular index-specification clause refers to the "number of entries in an SR8 record and the displacement of bottom-level SR8 records from the remainder of the index", but to be honest I'm struggling to understand the meaning and implications of this sentence. 

Can anyone explain to me what the purpose or intention of using these definitions is please?

Thanks and regards - Mike 

[Denise Basore]

Hi - Imagine a pyramid that is made up of boxes stacked on top of each other. 

The top of the pyramid is one box (SR7 if system-owned) that contains the name of the index and pointers to the boxes on the next level. Every box below the top is an SR8.

The bottom of the pyramid is a whole series of boxes (SR8s). Each box contains a bunch of folders. Each folder identifies a database record and where to find it in the database.

We have a rule for these boxes. Let's say they can each only contain up to 45 folders. That rule is your IBC (Index Block Contains).

When we get to folder 46, we have to build another box (SR8) on the bottom level. We also have to build a box above this level that keeps track of the two below and is chained to the SR7 at the top. We keep building boxes and adding levels until all of our database records are represented.

The displacement is the amount of space that we leave between the top of the pyramid and the bottom. This allows for growth as the pyramid is being built. 

The subareas are complicating things for you though. These cause the overall area to be partitioned so you will need to monitor space in each subarea.

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I hope it helps - Denise
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