Database Mirroring - Real-time copy

Enhanced Definition

Database mirroring, in the z/OS environment, refers to the continuous, real-time replication of database changes from a primary system or storage volume to a secondary, identical copy. This process ensures an up-to-date duplicate database is available, primarily for high availability, disaster recovery, or workload distribution purposes, significantly minimizing data loss and downtime. In the mainframe context, "Database Mirroring - Real-time copy" refers to the process of maintaining a continuously updated, consistent duplicate of a production database to ensure high availability, facilitate disaster recovery, or enable workload offloading. This is achieved through various z/OS technologies that replicate data changes as they occur, minimizing data loss and downtime.

Key Characteristics

- Synchronous or Asynchronous Replication: Can operate in synchronous mode (writes committed on both primary and secondary before acknowledging to application, ensuring zero data loss) or asynchronous mode (writes acknowledged after primary commit, with secondary catching up, potentially minimal data loss but better performance over distance).
- Storage-level or Log-level Implementation: Often implemented at the storage subsystem level (e.g., PPRC, Metro Mirror) for entire volumes, or at the database log level (e.g., InfoSphere Data Replication for DB2/IMS) for granular data changes.
- Automated Failover/Failback: Typically integrated with GDPS (Geographically Dispersed Parallel Sysplex) or similar automation to manage automatic or semi-automatic switchover to the mirrored copy in case of a primary system failure.
- Data Consistency: Ensures data integrity and transactional consistency on the secondary copy, often using consistency groups for storage replication or transactional apply for log-based replication.
- Reduced Recovery Time Objective (RTO) and Recovery Point Objective (RPO): Significantly minimizes the time to recover from an outage and the amount of data lost, often achieving near-zero RTO and RPO.
- Transparent to Applications: Once configured, applications typically interact only with the primary database, largely unaware of the underlying replication mechanism.

Use Cases

- Disaster Recovery (DR): Maintaining a geographically separate, up-to-date copy of critical databases to ensure business continuity in the event of a regional disaster affecting the primary data center.
- High Availability (HA): Providing immediate failover capability within a data center or metropolitan area to minimize downtime from localized failures (e.g., hardware failure, software error).
- Data Migration/Upgrades: Facilitating near-zero-downtime database migrations or upgrades by replicating to a new environment and then switching over production workload.
- Reporting/Query Offloading: In some advanced configurations (especially log-based replication), the mirrored copy can be used for read-only reporting or batch processing, offloading workload from the primary transactional system.