BAJJI
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Joined: 15 Jul 2005
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just want to share with u all some info on IAM files, since nothing on this has been posted previously in our forum...
IAM vs. VSAM:
A Comparison of Data Access Methods
By Harry L. Kirkpatrick
Innovation Data Processing's Innovation Access Method (IAM) is a high-performance, indexed access method for OS/390 and MVS/ESA operating systems, which offers advantages over the IBM?-provided Virtual Storage Access Method (VSAM). Existing as non-VSAM data sets provides IAM with capabilities to eliminate the 4.3-gigabyte file-size restriction in VSAM prior to DFSMS V1.3, and to choose a block size that optimizes space utilization on DASD.
This article touches on the strengths and weaknesses of both products. It should be noted that BMC Software's RECOVERY UTILITY (RU? ) for VSAM supports both IAM and VSAM files.
What is IAM?
IAM, as noted previously, offers numerous advantages over the IBM-provided VSAM access method. IAM files exist on DASD as non-VSAM data sets, with IAM providing a VSAM-compatible Application Programming Interface (API) for key-sequenced data sets (KSDS) and entry-sequenced data sets (ESDS) file types and any associated alternate indexes.
Existing as non-VSAM data sets provides IAM with capabilities to eliminate the 4.3-gigabyte file-size restriction in VSAM prior to DFSMS V1.3, and to choose a block size that optimizes space utilization on each of the different type of DASD devices and architectures available. Along with IAM's unique file structure and IAM's Data Compression feature, user data stored in an IAM file typically requires substantially less DASD space than when stored in a VSAM cluster.
The maximum size of an IAM file is determined by a set of limitations imposed by DFSMS, MVS and the architecture of the DASD devices. IAM's file-size limitation, based on the IBM 3390 DASD architecture, is approximately 201 gigabytes of compressed user data.
IAM has been in the MVS marketplace for more than 20 years, providing an outstanding level of performance compared to VSAM. IAM offers CPU time savings, along with reductions in EXCPs that result in reduced elapsed times for batch jobs and improved response times for online systems. In the past few years, the most important features of IAM, for many customers, have been IAM's ability to support VSAM data sets that have exceeded 4.3 gigabytes in size and the DASD space savings of IAM's data compression.
What is VSAM?
VSAM is a high-performance access method used in MVS, OS/390 and VSE/ESA operating systems. Although more than 30 years old, IBM's VSAM remains popular because of its simplicity and high performance. Installations have been using VSAM data sets to hold more and more of their data to the point where many have reached the 4-gigabyte architectural limit for the size of VSAM data sets. This has been particularly true for users of KSDSs, as determined by several surveys conducted worldwide. IBM has recently enhanced VSAM to support record-level sharing and file sizes larger than 4 gigabytes.
The new architectural limit varies according to the size of the data control interval (CI). For a 512-byte CI size, the maximum size is 2 terabytes; for a KSDS with a data CI size of 32 kilobytes, the maximum size is 128 terabytes.
for more info, refer
http://www.bmc.com/technews/003/IAMvsVSAM.html |
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