Disk Mirroring
Disk mirroring, also known as RAID-1, is a data redundancy technique in mainframe environments where data is simultaneously written to two or more separate physical storage devices. Its primary purpose is to provide high availability and fault tolerance, ensuring continuous data access even if one of the mirrored disks fails. Disk mirroring is a data redundancy technique in which data is simultaneously written to two separate physical disk volumes or storage devices, creating an identical copy. In the mainframe z/OS environment, its primary purpose is to ensure high availability and data protection by providing an immediate, transparent failover mechanism in case of a primary disk failure.
Key Characteristics
-
- Synchronous Data Replication: Data writes are committed to both primary and mirrored disks concurrently, ensuring that both copies are always identical and up-to-date.
- Storage Subsystem Implementation: Typically managed by the intelligent storage controller (e.g., IBM DS8000, Hitachi VSP, Dell EMC PowerMax) rather than directly by the z/OS operating system.
- Immediate Data Availability: In the event of a failure on one disk, the system automatically switches to the mirrored copy, providing uninterrupted access to data with no data loss.
- Performance Impact: While local mirroring (within the same storage subsystem) has minimal performance overhead, remote synchronous mirroring across distances can introduce latency due to the need to confirm writes at both sites.
- Capacity Requirement: Requires double the storage capacity for the mirrored data, as an exact copy is maintained.
Use Cases
-
- High Availability for Critical Datasets: Protecting essential z/OS system datasets (
SYSRES,PAGE,SPOOL), DB2 logs and critical tablespaces, CICS journals, and IMS databases to ensure continuous operation. - Disaster Recovery (Remote Mirroring): Implementing
Global MirrororMetro Mirror(IBM DS8000 terminology) to replicate data to a geographically separate recovery site for business continuity in the event of a regional disaster. - Data Protection Against Disk Failure: Safeguarding against physical disk failures, controller failures (if mirrored across different controllers), or even entire storage subsystem failures (with remote mirroring).
- Ensuring Data Integrity: Providing an immediate, consistent backup copy of data, which can be crucial for applications requiring extremely high levels of data integrity and uptime.
- High Availability for Critical Datasets: Protecting essential z/OS system datasets (
Related Concepts
Disk mirroring is a foundational component of RAID (Redundant Array of Independent Disks) strategies, specifically RAID-1. It is often implemented within advanced Storage Subsystems like the IBM DS8000 series, which provide additional features such as FlashCopy (point-in-time copies) and various remote replication technologies (Metro Mirror, Global Mirror). It is a critical element in overall Data Availability and Disaster Recovery (DR) planning, working in conjunction with Parallel Sysplex to provide highly resilient and continuously available mainframe environments. While z/OS doesn't directly manage the mirroring, it relies on the underlying storage infrastructure to present highly available logical volumes.
- Monitor Mirror Status Continuously: Implement robust monitoring tools to track the health, synchronization status, and performance of mirrored pairs, alerting administrators to any degradation or failure.
- Test Disaster Recovery Procedures: Regularly conduct planned failover and failback tests for remote mirrored environments to validate recovery procedures and ensure business continuity plans are effective.
- Optimize I/O Paths: Ensure sufficient bandwidth and low latency, especially for synchronous remote mirroring, to prevent performance bottlenecks that could impact application response times.
- Capacity Planning: Accurately plan for the increased storage capacity required by mirroring, ensuring adequate resources are available for both primary and mirrored copies.
- Combine with Other Data Protection: While powerful, disk mirroring should be part of a layered data protection strategy, complementing traditional backups, snapshots, and archival solutions.