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Efficient Data Set Usage

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Efficient data set usage is crucial for optimizing the performance of JCL jobs. Proper management of data sets can lead to significant improvements in job execution times and resource utilization. This section will cover best practices and techniques for using data sets efficiently in JCL.

Key Concepts

  1. Data Set Types:

    • Sequential Data Sets: Used for storing data in a linear format.
    • Partitioned Data Sets (PDS): Used for storing multiple members, each of which can be a separate file.
    • VSAM Data Sets: Used for high-performance access to large volumes of data.

  2. Data Set Allocation:

    • Primary and Secondary Space Allocation: Define the initial and additional space for data sets.
    • Block Size and Record Length: Optimize these parameters to improve I/O performance.

  3. Data Set Disposition:

    • DISP Parameter: Controls the status of the data set before and after the job execution.

  4. Data Set Reuse:

    • Temporary Data Sets: Use temporary data sets for intermediate storage to avoid unnecessary I/O operations.
    • Referencing Existing Data Sets: Efficiently reference existing data sets to minimize duplication.

Best Practices

  1. Optimize Space Allocation

Proper space allocation can prevent job failures due to insufficient space and reduce the need for frequent I/O operations.

//MYDATA  DD  DSN=MY.DATA.SET, 
//            DISP=(NEW,CATLG,DELETE), 
//            SPACE=(CYL,(10,5),RLSE), 
//            DCB=(RECFM=FB,LRECL=80,BLKSIZE=800)
  • Primary Space (10 cylinders): Initial space allocated.
  • Secondary Space (5 cylinders): Additional space allocated if needed.
  • RLSE: Release unused space after the job completes.

  1. Use Appropriate Block Size and Record Length

Choosing the right block size and record length can significantly improve data transfer rates.

//MYDATA  DD  DSN=MY.DATA.SET, 
//            DISP=(NEW,CATLG,DELETE), 
//            SPACE=(CYL,(10,5),RLSE), 
//            DCB=(RECFM=FB,LRECL=80,BLKSIZE=800)
  • RECFM=FB: Fixed block format.
  • LRECL=80: Logical record length.
  • BLKSIZE=800: Block size.

  1. Efficient Use of Temporary Data Sets

Temporary data sets are useful for intermediate processing and can be deleted automatically after job completion.

//TEMPDS  DD  DSN=&&TEMP, 
//            DISP=(NEW,PASS), 
//            SPACE=(TRK,(5,5)), 
//            DCB=(RECFM=FB,LRECL=80,BLKSIZE=800)
  • DSN=&&TEMP: Temporary data set.
  • DISP=(NEW,PASS): Pass the data set to the next step and delete it after job completion.

  1. Referencing Existing Data Sets

Referencing existing data sets can save time and resources by avoiding duplication.

//EXISTDS  DD  DSN=EXISTING.DATA.SET, 
//              DISP=SHR
  • DISP=SHR: Share the data set with other jobs.

Practical Example

Let's create a JCL job that demonstrates efficient data set usage by allocating space, using appropriate block sizes, and referencing existing data sets.

//JOBNAME  JOB  (ACCT),'EXAMPLE JOB',CLASS=A,MSGCLASS=X
//STEP1    EXEC PGM=IEFBR14
//NEWDS    DD  DSN=MY.NEW.DATA.SET, 
//              DISP=(NEW,CATLG,DELETE), 
//              SPACE=(CYL,(10,5),RLSE), 
//              DCB=(RECFM=FB,LRECL=80,BLKSIZE=800)
//TEMPDS   DD  DSN=&&TEMP, 
//              DISP=(NEW,PASS), 
//              SPACE=(TRK,(5,5)), 
//              DCB=(RECFM=FB,LRECL=80,BLKSIZE=800)
//EXISTDS  DD  DSN=EXISTING.DATA.SET, 
//              DISP=SHR

Explanation

  • STEP1: Executes the IEFBR14 program (a utility program that does nothing but is used to allocate data sets).
  • NEWDS: Allocates a new data set with optimized space and block size.
  • TEMPDS: Creates a temporary data set for intermediate processing.
  • EXISTDS: References an existing data set to avoid duplication.

Exercises

Exercise 1: Allocate a New Data Set

Write a JCL job to allocate a new data set with the following specifications:

  • Data set name: USER.DATA.SET
  • Primary space: 5 cylinders
  • Secondary space: 2 cylinders
  • Record format: Fixed block (FB)
  • Logical record length: 100
  • Block size: 1000

Solution

//JOBNAME  JOB  (ACCT),'ALLOCATE DATA SET',CLASS=A,MSGCLASS=X
//STEP1    EXEC PGM=IEFBR14
//NEWDS    DD  DSN=USER.DATA.SET, 
//              DISP=(NEW,CATLG,DELETE), 
//              SPACE=(CYL,(5,2),RLSE), 
//              DCB=(RECFM=FB,LRECL=100,BLKSIZE=1000)

Exercise 2: Create a Temporary Data Set

Write a JCL job to create a temporary data set with the following specifications:

  • Data set name: &&TEMPDS
  • Primary space: 3 tracks
  • Secondary space: 1 track
  • Record format: Variable block (VB)
  • Logical record length: 200
  • Block size: 2000

Solution

//JOBNAME  JOB  (ACCT),'TEMP DATA SET',CLASS=A,MSGCLASS=X
//STEP1    EXEC PGM=IEFBR14
//TEMPDS   DD  DSN=&&TEMPDS, 
//              DISP=(NEW,PASS), 
//              SPACE=(TRK,(3,1)), 
//              DCB=(RECFM=VB,LRECL=200,BLKSIZE=2000)

Conclusion

Efficient data set usage in JCL involves optimizing space allocation, choosing appropriate block sizes and record lengths, and effectively using temporary and existing data sets. By following these best practices, you can improve the performance and resource utilization of your JCL jobs. In the next section, we will explore parallel processing techniques to further enhance job performance.

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