Prime Related Overflow

Prime Related Overflow, or PRO, is an alternative structure for the Extended Overflow area in an IAM Enhanced Format file. The PRO format is a block-based overflow structure that is tied into the prime key index. This reduces the size of the overflow index because only one entry in the index per Extended Overflow block that is generated when the Extended Overflow block is first used. Therefore, all of the records within any extended overflow block had to have a relationship with the index structure to a prime block, hence the terminology, Prime Related Overflow.

Related topics

IAM-Data-Set-Structure-Overview

Global-Option-settings

PRO is designed for data sets that have a very high volume of insert activity. It is ideally suited for data sets that have insert activity that is approaching a million records or more. The advantages of PRO format files include:

  1. Reduced virtual storage requirements for the index.
  2. Reduction in frequency of file reorganizations.
  3. Reduced I/O to open files with large overflow areas.
  4. Potential for better sequential read performance.

The nature of being a block-oriented overflow structure will mean that such files will likely require more DASD space than a record-oriented overflow and additional I/O when inserting records. PRO files must be used with IAM/RLS or IAM/PLEX to ensure read integrity when being updated by a Batch job or CICS transaction due to the potential movement of existing records.

For most IAM files, the standard record-based overflow area will still provide the best performance. However, for the few files with an exceptionally large volume of records in overflow, PRO will be the better choice.

PRO insert strategy

When a prime block, or a related extended overflow block, has insufficient space remaining to accommodate either a record being inserted, or an updated record that has become longer, an additional extended overflow block is acquired. The original block will be divided, with some of its existing records moved to the new overflow block if necessary. The point of division will be determined by IAM based on the amount of space needed, the logical location of the new or updated record within the block, and the manner in which inserts or updates are occurring. The records moved (if any) will be those with the highest keys in the original block. The new or updated record will be placed in either the original block, or the new overflow block based on space, logical position, and processing patterns.

An index entry will be created for the new overflow block and inserted into the index for the data set in storage, and on DASD. This is another difference in the PRO structure, which is, there will be an index entry for the overflow blocks written out to the actual data set itself.

A PRO overflow block will be available for reuse by either the same or a different prime block, when all of the records in it have been deleted. The index entry for it will be nullified when the block is emptied out, and then reused if an overflow block is needed again, even for a different prime block. This will avoid having wasted overflow blocks when there is heavy insert and deletion activity against a data set.

Sharing PRO files

The PRO format must be used with IAM data sets defined with Share Option 1. Alternately, the IAM file can also be shared with IAM/RLS or IAM/PLEX using Share Options 2, 3 or 4. The PRO format allows multiple records to be moved with an Update or Insert job, causing read integrity issues if IAM/RLS or IAM/PLEX is not controlling the file.

 

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