Disk Array

Enhanced Definition

A **Disk Array** in the mainframe context refers to a high-performance, fault -tolerant storage subsystem composed of multiple physical disk drives (often `DASD` or `FlashSystem` technology) managed as a single logical unit. It provides robust data storage, enhanced performance, and data redundancy for critical z/OS workloads and applications. A disk array, in the context of IBM mainframes and z/OS, refers to a collection of physical disk drives (often `DASD` - Direct Access Storage Devices) that are managed as a single, integrated storage unit by a specialized controller. Its primary purpose is to enhance data capacity, improve I/O performance, and provide data redundancy and availability for critical mainframe applications and data.

Key Characteristics

- High Availability and Redundancy: Employs various RAID (Redundant Array of Independent Disks) levels (e.g., RAID-5, RAID-6, RAID-10) to protect against individual drive failures and ensure continuous data access.
- Scalability and Performance: Designed to scale in capacity and I/O performance, handling millions of I/O operations per second (IOPS) required by large-scale enterprise applications.
- Advanced Features: Modern mainframe disk arrays (like IBM DS8000 series) include features such as thin provisioning, data compression, encryption, and FlashCopy (point-in-time copies).
- Host Connectivity: Connects to z/OS mainframes via high-speed fibre channel protocols like FICON (Fibre Connection) or older ESCON (Enterprise Systems Connection) for efficient data transfer.
- Logical Unit Numbers (LUNs) / Volumes: Presents storage to the mainframe as logical volumes (often referred to as DASD volumes or LUNs), which z/OS formats and manages as VTOC (Volume Table of Contents) controlled devices.
- Microcode/Firmware: Managed by sophisticated internal microcode that handles data placement, error recovery, caching, and other advanced storage functions.

Use Cases

- Primary z/OS Data Storage: Hosting critical system datasets, user data, VSAM files, PDS/PDSE libraries, and sequential files for applications running on z/OS.
- Database Management Systems: Providing high-performance and reliable storage for DB2 for z/OS databases, IMS databases, and their associated logs and recovery areas.
- Transaction Processing Systems: Storing data and logs for high-volume transaction processing environments like CICS and IMS TM, ensuring data integrity and rapid access.
- Disaster Recovery and Business Continuity: Utilizing built-in replication capabilities (e.g., Metro Mirror, Global Mirror) to create remote copies of data for disaster recovery and high availability solutions.
- Backup and Recovery: Serving as the target for disk-to-disk backups, often using FlashCopy for rapid point-in-time snapshots that can be used for recovery or application testing.

Related Concepts

A Disk Array is the physical realization of DASD (Direct Access Storage Device) in modern mainframe environments, providing the underlying storage infrastructure for z/OS. It connects to the mainframe via FICON or ESCON