SCSI is a high-speed standard for connecting peripherals and computers, defining hardware connections and data exchange methods. It supports device-specific commands and protocols and is commonly used on servers and high-performance computers. SCSI evolved from the 1980s to the early 2000s, and its command protocol defines several dozen operations. SCSI has several advantages over competing technologies, including long data cables, simultaneous use of multiple high-performance devices, and organization into logical units, targets, and initiators. SCSI’s parallel nature caused termination and cabling problems at high speeds, leading to redesign as Serial Attached SCSI (SAS), which offers several advantages over parallel SCSI. SAS is compatible with SATA storage devices and improves fault isolation.
Small Computer System Interface is a high-speed standard for connecting peripherals and computers. Also known by the acronym SCSI, it defines both hardware connections and data exchange methods. For each type of device it supports, SCSI defines device-specific commands and protocols. SCSI is commonly used on servers and high-performance computers such as those for audio and video production. It is also often used with RAID (Redundant Array of Independent Disks) and network storage technology.
SCSI was created in the late 1970s and was originally named Shugart Associates System Interface after its corporate inventor. Small Computer System Interface has several advantages over competing technologies. Its data cables can be quite long, making it easy to connect many external devices to a computer. More than one high-performance device on the cable can be active simultaneously, simplifying storage-intensive applications. For example, editing software can simultaneously read data from two hard drives and burn a digital video disc (DVD).
From the 1980s through the early 2000s, system interface specifications for small computers evolved dramatically. The parallel SCSI bus went from eight bits to 16 bits, and device bandwidths often doubled from one version to another. Many motherboards included SCSI controllers or supported SCSI host bus adapters (HBAs). Drive manufacturers usually introduced their high-performance drives with SCSI support before other technologies. However, low-cost Integrated Drive Electronics (IDE) disks remained a common choice for personal computers.
Small Computer System Interface organizes connected devices into logical units, targets and initiators. A device is an initiator if it can initiate SCSI commands, such as a SCSI controller. Targets, such as disk drives, DVD drives, and similar devices, respond to requests from initiators. Each target device can have more than one logical drive and many logical blocks of data. In particular, large capacity storage devices are usually accessed as multiple virtual drives.
The Small Computer System Interface command protocol defines several dozen operations. This includes commands for device management, status collection, and data transfer. There are four variants of reading data from a device as well as four different write commands. The 32-bit Cyclic Redundancy Check (CRC32) method has been used for data transfers since 1996, when the SCSI-3 specification was released.
By the early 2000s, the SCSI bus clock rate had increased to 160 Megahertz (MHz) under the Ultra 640 specification. The parallel nature of SCSI began causing termination and cabling problems at very high speeds. These problems were solved by redesigning SCSI to carry data in series rather than in parallel. The changes were implemented in the late 2000s as Serial Attached SCSI (SAS). An important variation, FC-AL (Fibre Channel Arbitrated Loop) uses a very fast clock—4 Gigahertz (GHz)—with fiber optic cables.
SAS offers several advantages over parallel SCSI in addition to increased device throughput. Device connections are hot-swappable, which means they can be disconnected and reconnected as needed without powering down the servers. SAS is compatible with Serial Advanced Technology Attachment (SATA) storage devices. This allows you to use the cheapest and most popular SATA drives, the successor of IDE, with the advanced technology based on SCSI. SAS also improves fault isolation over the original Small Computer System Interface.
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