Data Set Attributes

Key Characteristics

• • Record Format (RECFM): Specifies how records are structured within the data set, such as FB (Fixed Blocked), VB (Variable Blocked), U (Undefined), or F (Fixed). This attribute dictates how the system reads and writes individual records.
  • Logical Record Length (LRECL): Defines the maximum length of a logical record in bytes. For fixed-format data sets, this is the exact record length; for variable-format data sets, it's the maximum allowed length, including control bytes.
  • Block Size (BLKSIZE): Determines the physical block size in bytes that is written to or read from the storage device. An optimal BLKSIZE can significantly impact I/O performance, typically a multiple of LRECL for blocked records, up to 32760 bytes for DASD.
  • Data Set Organization (DSORG): Identifies the fundamental structure of the data set, such as PS (Physical Sequential), PO (Partitioned Organization - PDS/PDSE), VSAM (Virtual Storage Access Method), or DA (Direct Access). This dictates the access methods used.
  • Space Allocation (SPACE): Specifies the primary and secondary space requirements on a direct access storage device (DASD), typically in tracks, cylinders, or blocks, along with an optional directory block allocation for PDS/PDSE.
  • Expiration Date (EXPDT) / Retention Period (RETPD): Defines the date until which a data set should be retained or the number of days it should be kept. The system prevents accidental deletion of data sets before this period expires.
  • Storage Management Subsystem (SMS) Classes: For SMS-managed data sets, attributes like DATACLAS (data class), STORCLAS (storage class), and MGMTCLAS (management class) define data set characteristics, storage device selection, and retention/backup policies, respectively.

Use Cases

• • JCL Data Set Definition: Attributes are explicitly defined in JCL DD statements when allocating new data sets or overriding existing attributes for specific job steps.

  jcl
  //MYDD DD DSN=MY.PDS.LIB,DISP=(NEW,CATLG,DELETE),
  //          UNIT=SYSDA,VOL=SER=WORK01,SPACE=(TRK,(10,2,5)),
  //          DCB=(RECFM=FB,LRECL=80,BLKSIZE=27920,DSORG=PO)

  • VSAM Cluster Definition: When defining VSAM clusters using IDCAMS DEFINE CLUSTER, attributes like RECORDSIZE, KEYLEN, KEYS, and FREESPACE are specified to control the cluster's structure and performance.
  • ISPF Data Set Information: Users can browse and view the attributes of existing data sets through ISPF option 3.4 (Data Set List Utility) to understand their configuration and properties.
  • System Administration and Performance Tuning: System programmers and administrators analyze data set attributes to optimize storage utilization, I/O performance, and ensure data integrity and availability.
  • Backup and Recovery: Data set attributes are critical for backup and recovery processes, ensuring that restored data sets retain their original characteristics and can be properly accessed by applications.

Related Concepts