Image Copy

Enhanced Definition

An image copy, in the context of mainframe databases like DB2 for z/OS and IMS, is a consistent, point-in-time backup of a database object (e.g., a table space, index space, or database area). Its primary purpose is to serve as a baseline for database recovery operations, allowing data to be restored to a previous state.

Key Characteristics

- Types: Can be a full image copy (a complete copy of the entire object) or an incremental image copy (a copy of only the pages or blocks that have changed since the last full or incremental image copy).
- Consistency: Ensures data integrity by capturing the database object's state at a specific moment, crucial for reliable recovery.
- Utility-Driven: Created using specialized database utilities, such as the COPY utility for DB2 for z/OS or various utilities managed by DBRC (Database Recovery Control) for IMS.
- Storage: Typically stored on sequential access datasets (e.g., tape volumes or DASD datasets) or in dedicated recovery utility files.
- Recovery Baseline: Serves as the starting point for database recovery, often combined with database logs (LOGREC for DB2, IMS logs) for forward recovery to a desired point in time.
- Online/Offline: Can be taken while the database is online and accessible to applications (concurrent copy) or offline, requiring exclusive access.

Use Cases

- Disaster Recovery: Essential for restoring entire databases or specific objects after catastrophic hardware failures, data corruption, or logical errors.
- Point-in-Time Recovery: Used to revert a database object to a specific historical state, for example, to undo the effects of an erroneous application update or accidental data deletion.
- Data Migration and Cloning: Creating copies of production databases for use in development, testing, quality assurance, or for migrating data to new environments.
- Database Reorganization: Often taken before and after a database reorganization (REORG) to provide a recovery point and to back up the newly reorganized data.
- Auditing and Compliance: Retaining historical image copies to meet regulatory requirements for data retention and audit trails.

Related Concepts

Image copies are fundamental to the overall database recovery strategy, working in conjunction with database logs (DB2 LOGREC, IMS logs) which record all changes made to the database. While image copies provide the baseline, logs enable forward recovery (rolling forward changes from the image copy) and backward recovery (undoing changes). They are managed by recovery utilities (e.g., DB2 RECOVER, IMS DBRC) and are integral to the robust data management capabilities of DB2 for z/OS and IMS DB.

Best Practices:

Regular Schedule: Implement a consistent schedule for both full and incremental image copies, balancing recovery point objective (RPO) with performance impact.
Retention Policy: Define and enforce clear retention policies for image copies based on recovery time objective (RTO), RPO, and regulatory compliance requirements.
Verification: Periodically verify the integrity and recoverability of image copies to ensure they are usable when needed, using utilities like DSN1COPY CHECK for DB2.
Offsite Storage: Store critical image copies offsite to protect against site-wide disasters and ensure business continuity.
Concurrent Copy Techniques: Utilize advanced concurrent copy features (e.g., IBM FlashCopy) to minimize application downtime during image copy operations.
Automation: Automate image copy processes using JCL, scheduling tools, and database management system utilities to ensure consistency and reduce manual errors.