VSAM to IAM conversion

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IAM definition:

Innovation access method(IAM). It is an enhanced file format of VSAM through which we can improve the performance of the jobs which reads, updates or loads the file without any impact on the existing outputs of the jobs.

Purpose:

VSAM file has three components - CLUSTER , DATA , INDEX
IAM file has only one component - CLUSTER

As IAM format doesn’t have DATA and INDEX components, the I/Os used to access those components in VSAM format could be avoided. This reduces 2/3^rd of EXCP count and results in a considerable amount of elapsed time and CPU time savings. The savings in IAM format is proportional to the number of records in the files and the number of attempts to access the file in a job. This could be strongly realized in load jobs of the files.

Changes required:

The keyword OWNER($IAM) should be included in the cluster definition of VSAM to convert it to IAM.

VSAMCNTL cards are used to define a VSAM file in delete/define jobs. The appropriate card should be selected to include the keyword- OWNER($IAM).
Referring job log details in SAR is a better way of getting the correct del/define card of a file, as JCL overrides could be easily identified in SAR display.

The files which don’t have any scheduled reorg jobs, FRMDSN should be executed to convert the file to IAM.

Testing:

1. Del/define and load steps should be executed for both VSAM and IAM with the same cluster definition except OWNER($IAM) included for IAM. There shouldn’t be any errors in creating and loading the IAM file.
2. Any program which reads the file should be executed with the VSAM and IAM formats of the file. There shouldn’t be any records mismatch in the compare results of the outputs generated with VSAM and IAM.
   

Savings collection:

1. Use SARBCH program to obtain the CPU time and elapsed time of the jobs for its execution with VSAM as well as with IAM.
2. Reformat the output obtained in the previous step by executing the program SARPGM2.
3. Convert the savings details collected into scorecard format.
   

Cases where VSAM can’t be converted to IAM:

The VSAM which are all being used in RLS(record level sharing) mode can not be converted to IAM. The file should have STORAGE CLASS- SCRLS and DATA CLASS–DCRLS for being used in RLS mode. IAM file can not be defined with the above mentioned attributes.

Things to be kept in mind while converting a VSAM file to IAM:

1. If the file selected for IAM conversion is referred as MODEL in definition of another VSAM file which need not to be converted to IAM, then we need to include OWNER(****) to retain the file as VSAM.
2. While considering CPU savings, having data and index buffers for VSAM is more efficient than converting a VSAM to IAM. Because data and index buffers wouldn’t be effective in IAM format, as IAM don’t have data and index components.
   
3. Some times converting a VSAM which has defined with buffers to IAM may lead to negative CPU savings. Eg: IAM conversion of IRAMASTR file resulted in negative CPU savings as it is used with buffers. But this file has been left out as IAM itself by considering large elapsed time savings in critical path.
4. UARTOTAL may be calculated from the DATA component of VSAM file. As the DATA component will not exist after the file has been converted to IAM following changes should be done-
   (1) New DDNAME - SYSPRINT of the JCL step should be replaced with DDNAME – IAMPRINT.
   (2) FRMUAR should be created to get the info required for UARTOTAL from the CLUSTER of the file itself. This FRMUAR was added to refer the word “TOTAL RECORDS” from DDNAME – IAMPRINT instead of referring the word “REC-TOTAL” from DDNAME – SYSPRINT.
   
5. All the jobs which del/defines the file should be run with IAM changes, otherwise IAM files may be converted back to VSAM

Link for better understanding about IAM files -
http://www.ibmmainframes.com/about5359.html