How to calculate SYSPLEX structure size for 5000 entries in an ACF2 structure

Products

• ACF2 - z/OS

Issue/Introduction

Based on ACF2 manuals if the LID records are 1K in size and for a structure for LIDs 1000 entries should fit in a 1M structure.  A 5M structure was defined but its not even close to 5000 entries, ACF SHOW SYSPLEX display:

PRIMARY STRUCTURE NAME: ACF2_SECURITY1  
ALTERNATE STRUCTURE NAME: N/A           
CURRENT STRUCTURE SIZE= 5,120K          
MAX STRUCTURE SIZE= 5,120K              
NUMBER OF STRUCTURE ENTRIES= 239        
MAX NUMBER OF STRUCTURE ENTRIES= 329    

 

Environment

Release : 16.0

Component : ACF2 for z/OS

Resolution

The data in the CF has additional overhead for management of the structure.  It is not straight line memory where you can put 5000 1K records in 5meg of storage.  Furthermore, this is only an estimate based on Structure Definition Criteria.  Actual storage utilization may be different.

When using the IBM Calculator using an estimate of 5000 1K records and the requisite other information. Because only LIDRECs are being stored,  the total element count is also 5000.  The element increment is 4 representing 1K elements (256*4). This value was selected to correspond to the minimum size record we will store in the structure. With this setting, memory for structure entries is allocated in 1K increments.  IBM estimated the structure as defined above will require 17 to 21 meg.   Allocating 25 meg should be sufficient.

The Max Elements per Entry is set to 32 resulting in the Max Entry size to 32K required to support the feature allowing a dynamic change to the SYSPLEX GSO Record.  Max Element cannot be changed once the structure is allocated.   

Can the 1K records will be stored in a 32k allocation?  The answer is 'no'; the CF allocates the minimum number of elements required to hold the Entry (record).  This is why there may be over allocation since IBM is expecting some overhead to link elements together for larger entries which we are not using in this case.

It was suggested they allocate high and then see where things fall. They can then get a better feel for what is actually needed.  Client was further warned them that IBM has stated requirements may change when upgrading to a newer CF Level. They should be aware of that with regard to this structure and ensure sufficient space.