Introduction:

In the world of IBM z/OS mainframe systems, data is organized and managed through various types of datasets. Each dataset type has specific characteristics and intended usage. Understanding the different dataset types is crucial for effective mainframe programming and system administration. In this blog post, we will explore the various z/OS dataset types, their purposes, and provide examples of how to use them in COBOL and JCL.

1. Sequential Datasets (SEQ):

Sequential datasets are the most basic and widely used type in z/OS. They consist of a linear sequence of records with fixed or variable lengths. Sequential datasets are accessed sequentially, from the first record to the last. They are commonly used for storing and processing large amounts of data.

To define a sequential dataset in COBOL, you can use the SELECT statement:

SELECT MYSEQFILE
   ASSIGN TO MY.SEQUENTIAL.FILE
   ORGANIZATION IS SEQUENTIAL

In JCL, you can allocate a sequential dataset using a DD statement:

//MYSEQ DD DSN=MY.SEQUENTIAL.FILE,DISP=NEW,
//       UNIT=SYSDA,SPACE=(TRK,(10,10)),DCB=(RECFM=FB,LRECL=80)

2. Partitioned Datasets (PDS/PDSE):

Partitioned datasets are used to store collections of related members, similar to directories in a file system. They allow for the logical grouping of related data and programs. Partitioned datasets come in two varieties: PDS (Physical Sequential) and PDSE (Physical Sequential Extended).

In COBOL, you can specify a partitioned dataset member:

SELECT MYPDSFILE ASSIGN TO MY.PARTITIONED.FILE(MEMBER1)

In JCL, you can allocate a partitioned dataset using a DD statement:

//MYPDS DD DSN=MY.PARTITIONED.FILE,DISP=SHR

To specify a member within the PDS, use the member name after the dataset name.

3. VSAM Datasets:

VSAM (Virtual Storage Access Method) datasets are designed for high-performance access and provide various record organization methods, such as Key-Sequenced, Entry-Sequenced, and Relative Record. VSAM datasets are often used for indexed or direct access to data.

In COBOL, you can define a VSAM dataset using the SELECT statement:

SELECT MYVSAMFILE
   ASSIGN TO MY.VSAM.FILE
   ORGANIZATION IS INDEXED
   ACCESS MODE IS DYNAMIC

In JCL, you can allocate a VSAM dataset using a DEFINE statement within an IDCAMS step:

//DEFVSAM EXEC PGM=IDCAMS
//SYSPRINT DD SYSOUT=*
//SYSIN    DD *
  DEFINE CLUSTER -
    (NAME(MY.VSAM.FILE) -
    INDEXED -
    KEYS(8 0) -
    RECORDSIZE(80 80) -
    TRACKS(10 10)) -
    DATA(NAME(MY.VSAM.FILE.DATA))

4. Temporary Datasets (TEMP):

Temporary datasets are used for temporary storage during program execution. They are automatically created and deleted by the system when no longer needed. Temporary datasets are useful for intermediate results, temporary work files, or holding transient data.

In COBOL, you can specify a temporary dataset using the SELECT statement:

SELECT MYTEMPFILE
   ASSIGN TO SYSOUT=(A,INTRDR)

In JCL, you can allocate a temporary dataset using a DD statement with DISP=(MOD,DELETE,DELETE):

//MYTEMP DD SYSOUT=(A,INTRDR),DISP=(MOD,DELETE,DELETE)

5. Generation Data Groups (GDG):

GDGs are used for version control and historical data retention. A GDG consists of a base name and a generation number. Each time a new version of the dataset is created, it increments the generation number. Older generations can be retained for auditing or recovery purposes.

In COBOL, you can specify a GDG dataset using the SELECT statement:

SELECT MYGDGFILE
   ASSIGN TO MY.GDG.BASE(+1)

In JCL, you can allocate a GDG dataset using a DD statement:

//MYGDG DD DSN=MY.GDG.BASE(+1),DISP=(NEW,CATLG,DELETE),
//         UNIT=SYSDA,SPACE=(TRK,(10,10)),DCB=(RECFM=FB,LRECL=80)

Conclusion:

z/OS provides various dataset types to cater to different data storage and access requirements. Sequential datasets offer straightforward linear access, while partitioned datasets allow logical grouping of related members. VSAM datasets provide indexed or direct access capabilities, while temporary datasets offer temporary storage during program execution. GDGs provide version control and historical data retention capabilities.

Understanding the characteristics and usage of each dataset type is essential for effective mainframe programming and system administration. Whether you’re working with sequential data, organizing related members, accessing data directly, or managing temporary or historical data, utilizing the appropriate dataset type ensures efficient and reliable data management in the z/OS mainframe environment.