D2D - Disk to Disk

Enhanced Definition

In the context of IBM mainframe systems and z/OS, D2D (Disk to Disk) refers to the process of transferring data directly from one disk storage device to another. This method is predominantly used for high-speed backups, data replication, and migration, leveraging the capabilities of modern Direct Access Storage Devices (DASD) and storage subsystems.

Key Characteristics

- High Performance: D2D transfers are significantly faster than traditional disk-to-tape (D2T) operations due to the higher I/O throughput of disk systems and the elimination of sequential access limitations.
- Random Access Capability: Data stored on disk allows for immediate, random access, which is crucial for rapid recovery of specific datasets or files, unlike the sequential nature of tape.
- Automated Management: D2D processes are often managed and automated by storage management software like DFSMS (Data Facility Storage Management Subsystem) and advanced storage subsystem features.
- Data Reduction Technologies: Modern D2D solutions frequently incorporate deduplication and compression to optimize storage capacity and reduce the amount of data transferred.
- Replication Foundation: D2D is fundamental to synchronous and asynchronous disk replication technologies (e.g., Metro Mirror, Global Mirror, FlashCopy) used for disaster recovery and business continuity.
- Virtual Tape Library (VTL) Integration: While VTLs emulate tape, their internal operations involve D2D movement of data between different disk tiers or for virtual tape copying.

Use Cases

- High-Speed Backups: Performing daily or incremental backups of critical z/OS datasets (e.g., VSAM, sequential files, PDS/PDSE) to another disk volume or storage pool for quick recovery.
- Disaster Recovery (DR) Replication: Replicating entire storage volumes or key datasets from a primary data center to a secondary DR site using disk replication for rapid failover.
- Data Migration: Moving large volumes of data between different storage tiers, from older DASD arrays to newer ones, or between different LPARs/systems without involving physical tape.
- Test/Development Environment Refresh: Quickly refreshing test or development environments with copies of production data by copying relevant datasets or volumes from production disk.
- Virtual Tape Copying: Within a VTL, copying virtual tapes from one virtual tape library or pool to another, which is an internal D2D operation.

Related Concepts

D2D is a core component of DFSMS (Data Facility Storage Management Subsystem), which automates data placement and movement based on policies across DASD and other storage types. It is critical for Business Continuity and Disaster Recovery strategies, often leveraging advanced IBM DS8000 storage subsystems and Storage Area Networks (SANs). D2D also underpins the functionality of Virtual Tape Libraries (VTLs) by providing the underlying disk infrastructure for virtual tape emulation and internal data movement.

Best Practices:

Implement DFSMS Policies: Utilize DFSMS to define and automate D2D operations, ensuring data is placed on appropriate storage tiers and managed according to defined service levels and retention policies.
Leverage Data Reduction: Employ deduplication and compression features available in modern disk subsystems or VTLs to optimize storage consumption and improve the efficiency of D2D transfers.
Monitor Performance and Capacity: Continuously monitor the I/O performance of D2D operations and the capacity of target disk pools to ensure transfers complete within SLAs and prevent storage exhaustion.
Regularly Test Recovery: Periodically test data recovery procedures from D2D backups and replicated data to validate data integrity and the effectiveness of the recovery process.
Secure Data: Ensure that data transferred D2D is encrypted, both in transit (e.g., over SAN links) and at rest on the target disk, to comply with security and regulatory requirements.