Storage Subsystems

This direct attach storage (DAS) paradigm dates back to the early days of mainframe computing, when disk drives were located close to the CPU and cabled directly to the computer system via some control circuits. Computing evolved from being computation-centric to storage-centric. The decentralization of departmental computing into networked individual workstations requires efficient sharing of data. Storage subsystems, basically a collection of disk drives, and perhaps some other backup storage devices such as tape and optical disks that can be managed together, evolved out of necessity. There are two orthogonal aspects of storage subsystems that are discussed. One aspect has to do with how multiple drives within a subsystem can be organized together, cooperatively, for better reliability and performance. A second aspect deals with how a storage subsystem is connected to its clients and accessed. Some form of networking is usually involved. The storage subsystem software understands what organization is being used and knows how to map the logical addresses of the virtual disk to the addresses of the underlying physical devices. This concept is described as virtualization in the storage community. The technique of using data striping as an organization is purely for improving performance.