Introduction
The DD (Data Definition) statement is a crucial component of JCL (Job Control Language). It is used to describe the data sets that a job will use, including their attributes and how they should be handled. Understanding the DD statement is essential for managing data within JCL jobs.
Key Concepts
- Data Set: A collection of data records that are stored together.
- DD Statement: Defines the data set to be used in a job step.
Basic Structure of a DD Statement
A DD statement typically includes the following components:
- DDNAME: The name of the DD statement.
- Parameters: Various parameters that define the attributes and handling of the data set.
Syntax
Example
Explanation
//MYDATA: The DDNAME, which is a label for the DD statement.DD: Indicates that this is a Data Definition statement.DSN=MY.DATA.SET: Specifies the data set name.DISP=SHR: Specifies the disposition of the data set (in this case, shared).
Common Parameters
Here are some commonly used parameters in DD statements:
| Parameter | Description |
|---|---|
DSN |
Data Set Name. Specifies the name of the data set. |
DISP |
Disposition. Specifies the status of the data set (e.g., NEW, OLD, SHR). |
UNIT |
Unit. Specifies the device type or group where the data set resides. |
SPACE |
Space. Defines the space allocation for new data sets. |
DCB |
Data Control Block. Specifies the attributes of the data set (e.g., record format, block size). |
VOL |
Volume. Specifies the volume serial number where the data set is located. |
Detailed Examples
Example 1: Creating a New Data Set
//NEWDS DD DSN=NEW.DATA.SET, DISP=(NEW,CATLG,DELETE), // UNIT=SYSDA, SPACE=(CYL,(5,1)), // DCB=(RECFM=FB,LRECL=80,BLKSIZE=800)
DSN=NEW.DATA.SET: Name of the new data set.DISP=(NEW,CATLG,DELETE): Disposition parameters (NEW for new data set, CATLG to catalog it, DELETE if the job fails).UNIT=SYSDA: Specifies the device type.SPACE=(CYL,(5,1)): Allocates space in cylinders (5 primary, 1 secondary).DCB=(RECFM=FB,LRECL=80,BLKSIZE=800): Data control block parameters (fixed block format, logical record length of 80, block size of 800).
Example 2: Referencing an Existing Data Set
DSN=EXISTING.DATA.SET: Name of the existing data set.DISP=SHR: Disposition parameter (shared access).
Practical Exercise
Exercise 1: Define a New Data Set
Write a DD statement to define a new data set named USER.DATA.NEW with the following attributes:
- Disposition: New, cataloged if the job completes successfully, deleted if the job fails.
- Unit: SYSDA
- Space: 10 primary cylinders, 2 secondary cylinders.
- Data Control Block: Fixed block format, logical record length of 100, block size of 1000.
Solution
//NEWUSER DD DSN=USER.DATA.NEW, DISP=(NEW,CATLG,DELETE), // UNIT=SYSDA, SPACE=(CYL,(10,2)), // DCB=(RECFM=FB,LRECL=100,BLKSIZE=1000)
Exercise 2: Reference an Existing Data Set
Write a DD statement to reference an existing data set named PROD.DATA.OLD with shared access.
Solution
Common Mistakes and Tips
- Incorrect Disposition: Ensure the disposition parameters are correctly specified to avoid data loss.
- Space Allocation: Properly allocate space to avoid job failures due to insufficient space.
- DCB Parameters: Ensure the DCB parameters match the data set's requirements to avoid data corruption.
Conclusion
The DD statement is a fundamental part of JCL, allowing you to define and manage data sets effectively. By understanding its structure and parameters, you can control how data is accessed and manipulated within your JCL jobs. Practice writing DD statements with different parameters to become proficient in managing data sets in JCL.
JCL (Job Control Language) Course
Module 1: Introduction to JCL
Module 2: JCL Statements and Syntax
Module 3: Data Definition (DD) Statements
Module 4: Procedures and Symbolic Parameters
Module 5: Advanced JCL Concepts
- Conditional Processing
- JCLLIB and INCLUDE Statements
- Generation Data Groups (GDGs)
- Restart and Checkpoint
Module 6: Error Handling and Debugging
- Common JCL Errors
- Interpreting JCL Error Messages
- Debugging Techniques
- Using JES2/JES3 for Troubleshooting