Direct Attach Storage was the original storage architecture. The term DAS came later. One of the first disk systems was called Direct Access Storage Devices, or DASD, and was popular in the IBM mainframe environment. The choices of storage architecture changed in the 1990s with the introduction of Network Attached Storage (NAS) and Storage Area Networks (SAN). Hard drives are the primary online storage media, with high speed and high capacity. Tape, CD-ROM/RW, DVD-ROM/RW, and magneto-optical systems are used mainly for backup and archive, as well as software distribution. Aggregation into large systems provides benefits in speed, logical capacity, and data protection. Removable media libraries and jukeboxes reduce the chance of error, increase availability, and allow multiple computers to access different media simultaneously. When data is accessed directly as blocks, it is called block I/O. If it is accessed through a file system, it is referred to as file I/O. RAID is a way of increasing performance and data protection by writing and reading data to multiple disks at the same time. Major RAID functions including striping, the writing of different data to many disks simultaneously, and mirroring, which is the writing of the same data to several disks. SCSI is a high-performance standard for transferring data to and from devices. It is used extensively for mass storage, and it encompasses both hardware specifications and a software protocol. ATA is the most popular storage technology today. It is used in most desktop and laptop computers. It is an in-the-box technology, almost never used to attach storage externally. There is a new serial implementation called SATA or Serial ATA. Network Attached Storage (NAS) devices are highly optimized file servers. They use standard protocols to communicate with a large number of clients. They provide high performance, can be quite scalable, and are easy to install and inexpensive to maintain. NAS uses a file head, sometimes called the NAS head, to provide a file system, management, and an interface to the network. A SAN is a storage architecture that performs block I/O over a network. SANs have advantages over DAS in terms of distance capabilities, address space, the ability to support many-to-many device configurations, better cable plan management, greater scalability, and higher availability. Fibre Channel (FC) is a high-speed, low-latency technology that marries networks with I/O channels. It is often used for SANs. Fibre Channel supports three topologies: fabric, arbitrated loop, and point-to-point. Fabric is the most common and allows for full-bandwidth connections between all nodes in the network. It also implements naming, discovery, and time protocols as part of the fabric.
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