Dump

Enhanced Definition

A dump, in the mainframe context, is a snapshot of the contents of main storage (memory) and/or auxiliary storage at a specific point in time, written to an external medium. Its primary purpose is to provide diagnostic information for problem determination, debugging, and post-mortem analysis of program or system failures (ABENDs).

Key Characteristics

- Types: Can be categorized as system dumps (e.g., SVC Dumps, Standalone Dumps for OS failures), job step dumps (e.g., SYSUDUMP, SYSABEND, SYSMDUMP for batch job failures), or transaction dumps (e.g., CICS transaction dumps).
- Triggering: Dumps can be automatically generated by the system upon an ABEND, explicitly requested by a program using macros like SDUMP or SNAP, or initiated by an operator command.
- Content: Typically includes the contents of CPU registers, program storage, application data areas, system control blocks (e.g., PSA, CSA, SQA), and other relevant system areas.
- Output Format: Raw dumps are usually in hexadecimal format, often accompanied by character representations, and are written to specific dump datasets or DD statements.
- Analysis Tool: The primary tool for analyzing z/OS dumps is the Interactive Problem Control System (IPCS), which allows users to format, browse, and interpret the dump content.
- Resource Impact: Generating large dumps can consume significant disk space and I/O resources, potentially impacting system performance.

Use Cases

- ABEND Diagnosis: The most common use is to analyze why a batch job, online transaction, or system component terminated abnormally (ABENDed), pinpointing the exact instruction and data state at the time of failure.
- Debugging Complex Logic: Developers use dumps to debug intricate program logic errors, storage overlays, or data corruption that are difficult to reproduce or trace in real-time execution.
- System Problem Determination: System programmers utilize system dumps to investigate operating system failures, hangs, loops, or performance degradation issues within the z/OS environment.
- CICS Transaction Analysis: CICS transaction dumps provide specific CICS-related control blocks and application storage for debugging issues within a CICS region, such as transaction ABENDs.
- Post-mortem Analysis: Dumps enable offline analysis of a problem, allowing support personnel to diagnose issues without requiring the failing application or system to be actively running.

Related Concepts

Dumps are intrinsically linked to ABENDs (Abnormal Ends), as they are the primary diagnostic output generated when an ABEND occurs. They are often specified in JCL (Job Control Language) via DD statements like SYSUDUMP, SYSABEND, or SYSMDUMP to control their generation for batch jobs. The raw dump data is then processed and interpreted using IPCS (Interactive Problem Control System), which understands the structure of control blocks and program areas within z/OS to present meaningful information to the analyst.

Best Practices:

Selective Dumping: Utilize options to limit the scope of a dump (e.g., SDUMP parameters, CHNGDUMP command) to only relevant storage areas, reducing dump size and analysis time.
Proper DD Statement Configuration: Ensure SYSUDUMP, SYSABEND, or SYSMDUMP DD statements in JCL are correctly defined with sufficient space and appropriate disposition (DISP=SHR for shared dump datasets, DISP=MOD for appending).
IPCS Proficiency: Develop strong skills in using IPCS commands and facilities to efficiently navigate, format, and interpret dump content, including understanding common control blocks.
Dump Dataset Management: Implement robust procedures for managing system dump datasets (SYS1.DUMPxx), including monitoring free space, offloading, and deleting old or unnecessary dumps to prevent system outages.
Security Considerations: Restrict access to dump datasets as they can contain sensitive information, including passwords, encryption keys, and confidential business data.