Archive Log
An `archive log` is a historical, sequential record of all committed changes made to a database (typically DB2 or IMS) on a z/OS system, generated when an `active log` becomes full. It captures a complete history of transactions and is crucial for database recovery, enabling the restoration of data to a consistent state after an outage, data corruption, or for point-in-time recovery.
Key Characteristics
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- Persistent Storage: Archive logs are typically written to sequential datasets on DASD (Direct Access Storage Device) or, more commonly, to magnetic tape for long-term, cost-effective, and offline storage.
- Sequential Nature: They contain a chronological sequence of database changes, ensuring that transactions are applied in the correct order during recovery operations.
- Input for Recovery: They serve as the primary input for forward recovery, applying changes from a database backup up to a specific point in time or the end of the log.
- Managed by Subsystem: For DB2, the
Data Sharing Group(DSG) or single DB2 subsystem automatically manages the archiving process. For IMS,DBRC(Database Recovery Control) manages theOLR(Online Log Data Sets) andSLDS(System Log Data Sets), which are the IMS archive logs. - Critical for Data Integrity: A complete chain of archive logs is indispensable for full database recovery to the most recent consistent state after a backup.
- Offline Availability: Unlike active logs which are online, archive logs can reside on offline media (e.g., tape cartridges) and are mounted as needed for recovery processes.
Use Cases
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- Forward Recovery: Applying committed transactions from a database backup up to the point of failure to restore the database to its most current consistent state.
- Point-in-Time Recovery: Restoring a database to a specific, earlier point in time (e.g., just before an erroneous update or data corruption) by applying archive log records up to that precise timestamp.
- Disaster Recovery: Rebuilding an entire database environment at a remote disaster recovery site using a combination of full database backups and subsequent archive logs to bring the data current.
- Data Replication/Synchronization: In advanced scenarios, archive logs can be utilized by specialized tools to replicate database changes to another system or for maintaining synchronized copies of data.
- Auditing and Compliance: While not their primary function, archive logs contain a detailed historical record of data modifications, which can be leveraged for auditing specific changes, especially in conjunction with specialized log analysis tools.
Related Concepts
Archive logs are intrinsically linked to Active Logs, serving as the overflow and long-term storage for transaction data initially written to active logs. They are managed by the respective database subsystem (e.g., DB2 or IMS) and their associated recovery components like DBRC (Database Recovery Control for IMS) or DB2's recovery utilities. They work in conjunction with database backups (image copies) to provide a complete recovery strategy: a backup establishes a baseline, and archive logs bring the database forward from that baseline. In DB2 Data Sharing environments, archive logs are crucial for maintaining data consistency