Image

Enhanced Definition

In the mainframe context, an **Image Copy** refers to a complete, point-in-time backup of a data set, a database object (such as a DB2 table space or IMS database), or an entire DASD volume. It captures the data exactly as it exists at a specific moment, primarily serving as a critical component for data recovery and disaster preparedness.

Key Characteristics

- Point-in-Time Snapshot: Represents the data's state at the exact moment the copy was taken, ensuring consistency for recovery operations.
- Full or Incremental: Can be a full copy of all data or an incremental copy capturing only changes since the last full or incremental copy, optimizing storage and time.
- Online or Offline: Can be taken while the data is offline (not accessible to applications) for maximum consistency, or online (while applications are active) using specialized utilities that manage data integrity.
- Recovery Baseline: Serves as the primary restore point for data recovery, often combined with log records (e.g., DB2 logs, IMS logs) for forward recovery.
- Storage Medium: Typically stored on magnetic tape for cost-effectiveness and archival purposes, or on other DASD volumes for faster access.

Use Cases

- Database Recovery: Restoring a DB2 table space, IMS database, or VSAM KSDS to a previous consistent state after data corruption, accidental deletion, or system failure.
- Disaster Recovery: Providing a baseline copy of critical data sets or entire volumes to be stored off-site, enabling system and data restoration in the event of a major data center outage.
- Data Migration and Replication: Creating a consistent copy of data for moving it between systems, environments, or for populating test and development environments.
- Auditing and Compliance: Archiving historical data states for regulatory compliance, audit trails, or long-term data retention requirements.

Related Concepts

Image copies are fundamental to Data Recovery and Business Continuity strategies on z/OS. They work in conjunction with Logging/Journaling mechanisms (e.g., DB2 logs, IMS logs, VSAM journals) to enable Forward Recovery (applying changes from logs to an image copy to bring it to a more current state) and Backward Recovery (undoing changes). Utilities like DB2 Utilities (COPY), IMS Utilities, or DFSMSdss are used to create and manage them. They are a core component of Backup and Recovery procedures, ensuring data integrity and availability.

Best Practices:

Regular Scheduling: Implement a robust schedule for taking full and incremental image copies, aligning with defined Recovery Point Objectives (RPOs) and Recovery Time Objectives (RTOs).
Off-site Storage: Store critical image copies off-site to protect against site-wide disasters and ensure data survivability.
Verification and Testing: Periodically verify the integrity of image copies and conduct recovery drills to ensure they are readable and usable for actual recovery scenarios.
Log Integration: Ensure that image copies are taken in a manner that allows seamless integration with corresponding log records for complete and consistent recovery.
Performance Considerations: Schedule image copy jobs during off-peak hours or leverage online copy utilities with minimal impact to production workloads.