RAID

For general consumer use, SCSI has not achieved the same mass appeal as IDE. The expectation regarding SCSI was that the ability to add a large number of devices would outweigh the complexity of the interface. But that was before alternative technologies like Universal Serial Bus (USB) and FireWire (IEEE 1394) came into play. In fact, the only mainstream desktop computer standardized on SCSI was the Apple Macintosh, and that was because of a design mistake. The original Mac was a closed system, which means that there were no expansion slots or other means to easily add extra components. As the Mac grew in popularity, users began to clamor for some way to upgrade their system. Apple decided to add a built-in SCSI controller with an external SCSI port as a way to enable expansion of the system. Until recently, virtually every Mac has contained onboard SCSI. But with the rise of USB and Firewire, Apple has finally removed SCSI as a standard feature on most of its systems. Where you commonly see SCSI is on servers and workstation computers. The main reason for this is RAID. Redundant array of independent disks (RAID) uses a series of hard drives to increase performance, provide fault tolerance or both. The hard drives are connected together and treated as a single logical entity. Basically, this means that the computer sees the series of drives as one big drive, which can be formatted and partitioned just like a normal drive. Performance is enhanced because of striping, which means that more than one hard drive can be writing or reading information at the same time. The SCSI RAID controller determines which drive gets which chunk of data and sends the appropriate data to the appropriate drive. While that drive is writing the data, the controller sends another chunk of data to the next drive or reads a chunk of data from another drive. Simultaneous data transfers allow for faster performance. Fault tolerance, the ability to maintain data integrity in the event of a crash or failure, is achieved in a couple of ways. The first is called mirroring. Basically, mirroring makes an exact duplicate of the data stored on one hard drive to a second hard drive. A RAID controller can be set to automatically send two hard drives the exact same data. To avoid potential complications, both drives should be exactly the same size. Mirroring can be an expensive type of fault tolerance since it requires that you have twice as much storage space as you have data. The more popular method of fault tolerance is parity. Parity requires a minimum of three hard drives, but will work with several more. What happens is that data is written sequentially to each drive in the series, except the last one. The last drive stores a number that represents the sum of the data on the other drives. For more information on RAID and fault tolerance, check out this page.
Digital video is another prime example of the right time to use SCSI. Because of the demanding storage and speed requirements of full-motion, uncompressed video, most video workstations use a SCSI RAID with extremely fast SCSI hard drives.