BAJJI
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hi,
i am copy - pasting this stuff... u can get an over view what IAM actually is..
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.
Types of VSAM Data Sets
ESDS
KSDS
RRDS (fixed)
VRRDS
LDS
Records are in order as they are entered
Records are in collating sequence by key field
Records are in relative record number order
Records are in relative record number order
No processing at record level
Direct access by RBA
Direct access by key or RBA
Direct access by relative record number
Direct access by relative record number
Access with Data-In-Virtual (DIV)
Alternate indexes allowed
Alternate indexes allowed
No alternate indexes allowed
No alternate indexes allowed
No alternate indexes allowed
A record's RBA cannot change
A record's RBA can change
A record's relative record number cannot change
A record's relative record number cannot change
No processing at record level
Space at the end of the data set is used for adding records
Free space is used for inserting and lengthening records
Empty slots in the data set are used for adding records
Free space is used for inserting and lengthening records
No processing at record level
A record cannot be deleted, but its space can be reused for a record of the same length
Space given up by a deleted or shortened record becomes free space
A slot given up by a deleted record can be used
Space given up by a deleted or shortened record becomes free space
No processing at record level
Spanned records allowed
Spanned records allowed
No spanned records allowed
No spanned records allowed
No spanned records allowed
Extended format allowed
Extended format or compression allowed
Extended format allowed
Extended format allowed
Extended format allowed
VSAM Characteristics
VSAM is not simply a set of access-method routines. It is a powerful facility for handling data within a user installation. Among its major features are:
A format for storing data independently of the type of direct access storage device on which it is stored
Routines for sequential or direct access and for access by key, relative address or relative record number
Options for optimizing performance
A multifunction service program (Access Method Services - IDCAMS) for setting up catalog records and maintaining data sets
IAM Characteristics
The basic principle of IAM is to intelligently utilize virtual storage to reduce the need to perform physical I/O to look up and retrieve data. This is accomplished by using the sophisticated IAM Real Time Tuning concept to manage buffers and by keeping the index for files in virtual storage while they are being processed. IAM requires, at most, one I/O to retrieve any explicitly identified record within the IAM data set. All of the index I/Os and index buffers used by VSAM are eliminated.
IAM's VSAM Transparency
IAM's system-level VSAM Interface (VIF) provides transparency. VIF allows an unaltered application program executing under MVS/ESA or OS/390 to access IAM files in place of VSAM KSDS or VSAM ESDS files. IAM /AIX V7.0 now allows VSAM files with Alternate Indexes to be converted to IAM.
VIF can be used in conjunction with the common programming languages COBOL II, COBOL, Assembler, PL/1, RPG, etc. and any higher-level language products that support keyed access to VSAM files.
IAM supports programs executing in AMODE (31) and VSAM control blocks (example: ACB, RPL) residing above the 16-megabyte line.
VIF supports:
The full range of VSAM file-access commands, such as GET, PUT, INSERT, GETPREV, ERASE, POINT
The file-status commands SHOWCB, TESTCB, GENCB
The VSAM catalog lookup macro SHOWCAT
IAM supports the following functions of IDCAMS, as they relate to VSAM KSDS or ESDS file processing:
BLDINDEX
DEFINE
DELETE
LISTCAT
PRINT
REPRO
VERIFY
DELETE, PRINT, REPRO and VERIFY provide the same services for IAM files as they would for VSAM clusters.
IDCAMS DEFINE creates an IAM file whenever the OWNER($IAM) parameter is specified, $IAM is placed somewhere in the data set name, or $IAM is part of the Data Class or Storage Class name. LISTCAT ALL displays IAM files as non-VSAM in its standard SYSPRINT report. LISTCAT ALL also displays the file's IAM characteristics in an IAMPRINT DD report, which will be dynamically allocated if necessary.
IAM Features
Mass Sequential Deletes
For Enhanced Format files, IAM has incorporated an ability to temporarily delete a record logically from a data block. Then, when that data block is about to be written out to DASD, the records are physically deleted from the data block. This eliminates the overhead of constantly moving records within a data block, as prior records are deleted, only to end up being deleted as well. In the case where every single record is deleted from a data block, this enhancement eliminates the data movement entirely from that process.
Dynamic Region Size Adjustment
IAM V7.0 has the ability to dynamically increase the extended private-region limit. When IAM is opening an Enhanced Format file, it estimates the amount of storage required to open the file. Included in that estimate is storage for the prime and overflow indexes, control information and buffers. IAM then checks to see if that amount of virtual storage is available. If not, IAM attempts to increase the extended private storage limit by the quantity required to open the file, rounded to 4 megabytes. By default, IAM will not increase the extended private region to greater than 512 megabytes. This maximum value can be either increased or decreased through the IAM Override facility or by changing the IAM default in the IAM Global Options Table.
Multiple ACB Support
For Enhanced Format files, IAM recognizes that an ACB has already been opened for a file within the same task, and shares the index structure and buffers. For applications, or CICS regions, where a file was opened under one DDNAME or ACB for read only, and the other for UPDATE, they occasionally experienced an inability to retrieve an updated data record. This will no longer be the case. This support also reduces the storage requirements for having multiple ACBs opened to the same file.
ESDS Extended Addressability
IAM has added support for 8-byte RBA values in the IAM ESDS type of files, which IBM made in DFSMS V1.5. This is referred to by IBM as Extended Addressability, which allows the VSAM ESDS file to exceed 4 gigabytes. An IAM 8-byte RBA file is created by either specifying the XESDS keyword on the IAM CREATE override, or through specification in the DFSMS Data Class. Such IAM ESDS files do not have to reside on DFSMS-managed volumes, as do VSAM ESDS files. Likewise, IAM can support these 8-byte RBA files from DFSMS V1.3 and above; however, application programs may not be able to take advantage of that until the languages provide support for the 8-byte RBA values.
IAM Journaling
IAM provides an optional automatic journaling capability on file updates for Enhanced Format IAM KSDS and ESDS types of files. This capability is provided to assist in improving data availability, particularly for the very large multi-volume data sets. The improved availability of the data is accomplished by reducing the frequency of data set backups and providing a speedier recovery facility for failing batch jobs.
VSAM Enhancements
When initially released by IBM in 1973, VSAM supported only two data organizations: key-sequenced (KSDS) and entry-sequenced (ESDS) data sets. Since that time, IBM has continually introduced major VSAM enhancements, as follows.
Extended Format Data Sets
Beginning with DFSMS V1.3, you can create and use VSAM KSDSs that can be much larger than the 4-gigabyte limit imposed on any VSAM data set defined before this release. DFSMS V1.5 allows non-KSDS file types (ESDS, RRDS, VRRDS and LDS) to exceed 4 gigabytes.
Record Level Sharing
DFSMS V1.3 introduced VSAM Record Level Sharing (RLS), which uses the coupling facility hardware to allow VSAM data to be accessed for update from multiple systems. The coupling facility is used as a store-through cache where records accessed for update can be stored and where locks can be maintained to ensure integrity.
VSAM Striping
This enhancement, introduced in OS/390 V2.10, provides the capacity to define any VSAM data set in extended format (EF) as a striped data set. Any VSAM data set in extended format may be defined in a manner that allows CIs to be interspersed among a group of DASD volumes to enable the CIs to be retrieved simultaneously from all volumes (or stripes).
Transactional VSAM
Transactional VSAM is an enhancement to VSAM Record Level Sharing (RLS). VSAM RLS provided record-level shared access to VSAM data sets via CF-based locking and caching. Batch jobs accessing recoverable VSAM data sets could read them, but not update while CICS had them open in RLS mode. By adding logging and two-phase commit and backout protocols at the file system level, Transactional VSAM allows batch jobs to share recoverable VSAM data sets for both read and update access while CICS is still using them.
Which Access Method will you choose?
Which access method to use is a great question, which has many possible answers. But the decision usually comes down to the best match for the business requirements of an enterprise. The quest for higher and higher availability has to be tempered by what the business actually needs and what is possible with the available technology.
Despite the success of DB2? and IMS, over 50% of the world's operational data still exist in VSAM data sets on the mainframe. DB2, IMS and VSAM applications include both online transactional and batch updates.
IAM is a high-performance indexed access method, providing random and sequential access to user data with minimal computer resources. IAM provides an application program interface that is fully compatible to the OS/390 VSAM access method, supporting the most commonly used features and capabilities.
IAM can be used in place of VSAM KSDS files, which are processed sequentially or randomly by key, and VSAM ESDS files, which are accessed sequentially or randomly by relative byte address, or by control interval. IAM files can be used in place of VSAM KSDS or ESDS files that utilize the IAM supported functions without modification. IAM, with the optional alternate index feature, can be used for any alternate indexes related to VSAM KSDS or ESDS types of file that have been converted to IAM.
While IAM does not use the VSAM LSR buffer pool, IAM can be used by applications that indicate usage of the VSAM LSR buffering, including Batch LSR and CICS. IAM files can be processed by system-utility programs, including IDCAMS and any of the many SORT software products. IAM provides support for the VSAM exit routines, as specified in the ACB EXLST, including the SYNAD, LERAD, EODAD, JRNAD and UPAD exit types.
VSAM data sets often have to be shared among several different applications in an OS/390 image or among applications on several different OS/390 system images. For example, transactions running in different CICS regions may have to access the same VSAM data set at the same time, or CICS transactions may have to access a VSAM data set at the same time that a batch job is using the data set. The requirements for sharing can vary. Sometimes applications only have to read the data set. Sometimes an application has to update the data set while other applications are reading it. The most complex case is when all applications have to update the data set, and all require complete data integrity.
Transactional VSAM Services allows VSAM data set sharing in batch/online and batch/batch environments. It will be available through an extended Early Support program at the time OS/390 V2.10 is available. Transactional VSAM supports recoverable VSAM data sets to allow batch programs and CICS online applications to concurrently share data for read and write processing. This capability will allow CICS applications to stay online along with many batch update applications to help meet the 24x365 data availability requirement. Transactional VSAM requires the use of the Automatic Restart Manager (ARM). Transactional VSAM services will be restarted on another system by ARM in the case of a system failure.
Summary
IAM and VSAM are both great products, each with its share of strengths and weaknesses. The selection of one over the other will always come down to what is the best solution to achieve a business objective.
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About the Author
Harry L. Kirkpatrick is a lead quality assurance representative for BMC Software, Inc. He has more than 20 years experience in Information Technology with expertise in VSAM, software development and MVS system management. He is a frequent contributor to BMC Software's tech NEWS online technical journal.
BMC Software Products Cited in this Article:
RECOVERY UTILITY (RU? )
References
For more information on these products, visit the following:
1. IBM DFSMS: Use [URL] BBCode for External Links
2. Innovation Data Processing: Use [URL] BBCode for External Links
thanks,
Kiran. |
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