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Q I am new to the IBM mainframe environment and one of my first assignments is to improve our IO and DASD utilization. What is VSAM and what are the fundamental architectural concepts that I have to be concerned with?
A VSAM stands for Virtual Sequential Access Method. VSAM can be a complex topic and there isn’t an easy reference to a comparable built-in PC or client/server utilization of the software.

There are five types of entry sequenced datasets:

  • ESDS
  • KSDS
  • RRDS
  • LDS
  • VRRDS
Entry sequenced datasets Key sequenced datasets Relative record datasets Linear datasets  Variable length RRDS

Each of the VSAM datasets contains a cluster. A cluster is a combination of the index, sequence set and data portions of the dataset. The operating system gives program access to the cluster, ie. to all parts of the dataset simultaneously.

VSAM dataset is organized by a Control Interval. Each CI: Control Interval consists of a Which consists of Record Descriptor Field: RDF and Control Interval Descriptor Field.
 
Fixed Size Records Each CI contains two RDF's and each RDF is 3 bytes in length.
Variable Size Records There is a separate RDF for each record in the CI.
CIDF Every CI contains one CIDF which is a field consisting of the last 4 bytes of a CI. It contains information about the offset and the length of free space in the CI.
CI Size Data Portion The size of the data portion of the CI should be multiples of 512 or 2,048 bytes. Max size of the CI is 32K.
CI Size Index Portion The size of the index portion of CI can be one of the following: 512, 1024, 2048, 4096.
 

We cover these concepts, inclusive of hands-on machine workshops, in both our VSAM for Programmers and VSAM: Structures and Strategies courses.
Q Why is it important for me to acquire expertise with VSAM? Isn’t learning COBOL and CICS, or COBOL and IMS or DB2 sufficient for me to perform first rate work on my job?
A VSAM is one of several access methods in z/OS. It only applies to data stored in DASD devices. An access method is re-entrant code contained in DFSMSdfp, a component of the DFSMS z/OS product. This access method makes it more efficient for an application to execute an I/O operation.

During the application development process, the application programmer needs to make the following important decisions:

  • Selecting a VSAM data set type.
  • Specifying the data organization.
  • Choosing the type of access.

It is true that not every application programmer requires a comprehensive knowledge of VSAM. However, many online systems use VSAM. In addition, VSAM is heavily used in DB2 tablespaces. Knowledge of VSAM will improve your expertise with DB2.
Q I am a mainframe programmer with 25 years experience in coding COBOL programs for the big three in IBM systems software: CICS, IMS, and DB2. What improvements have been made to VSAM recently?
A The most significant improvements which have been made are:
  • Extended format and extended addressability which allows a data set to go beyond the 4 GB limitation.
  • SMB: System managed buffering for improved performance, data compression, data striping, RLS: record level sharing, and transactional VSAM.
Q What is extended format?
A Extended format is a technique that affects the way CKD: Count Key Data is stored in a 3390/3380 logical track. Extended format was introduced to implement data striping. It increases the performance and the reliability of an I/O operation.
Q What does the term “split” refer to when working with VSAM?
A

When there isn't enough space in the control interval VSAM performs a control interval split by moving some records to the free control intervals. If there isn't a free control interval, VSAM performs a control area split by allocating a new control area and moving half of the control intervals to it.

There are two types of splits used extensively in VSAM.
CI-SPLIT Movement of some records from an existing CI to another free CI in the same CA because a record add or update cannot be accommodated in the existing one.

A CI split requires a number of I/O operations and this degrades the performance of the VSAM file. The correct amount of the space will need to be allocated for the CI (internal percentage of free space allocation). This will result in two half-empty CI's instead of one full and one empty CI.
CA-SPLIT  Movement of the half of the records in an existing CA to a new CA because a record add or update cannot be accommodated in the existing CA. This results in two approximately half-full CA's instead of one full and one empty CA. This is inefficient because it involves a high level of I/O operations.
Q I attended your VSAM for Programmers course in 2001 and I have a question relating to tuning. How can you determine what is the amount of unused space for a specific VSAM cluster?
A This is done by comparing the higher allocated RBA with the higher used RBA in the LISTCAT function listing.
Q How is a VSAM file renamed?
A The procedure for renaming a VSAM file in z/OS is:
    // EXEC PGM=IDCAMS
    //SYSPRINT DD SYSOUT=*
    //SYSIN DD *
         ALTER -
         SYSED.MYNAME.DATA.CLUSTER -
         NEWNAME(SYSED.YOURNAME.DATA) -
         CATALOG(CATALOG.zOS.MASTER)

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