SCSI

Parallel interface

SCSI is derived from the Shugart Associates System Interface (SASI),^[4] developed beginning 1979^[5] and publicly disclosed in 1981.^[1] Larry Boucher is considered to be the "father" of SASI and ultimately SCSI due to his pioneering work first at Shugart Associates and then at Adaptec, which he founded in 1981.^[6]

A SASI controller provided a bridge between a hard disk drive's low-level interface and a host computer, which needed to read blocks of data. SASI controller boards were typically the size of a hard disk drive and were usually physically mounted to the drive's chassis. SASI, which was used in mini- and early microcomputers, defined the interface as using a 50-pin flat ribbon connector, which was adopted as the SCSI-1 connector. SASI is a fully compliant subset of SCSI-1, so that many, if not all, of the then-existing SASI controllers were SCSI-1 compatible.^[7]

In around 1980, NCR Corporation had been developing a competing interface standard by the name of BYSE. In the summer of 1981, NCR abandoned its in-house efforts in favor of pursuing SASI and improving on its design for their own computer systems. Fearing that their extension of the SASI standard would induce market confusion, however, NCR briefly cancelled their contract with Shugart. NCR's proposed improvements to the design of SCSI piqued the interest of Optimem, a subsidiary of Shugart, which requested that NCR and Shugart collaborate on a unified standard. In October 1981, the two companies agreed to co-develop SASI and present their standard jointly with ANSI.^[8][9]

Until at least February 1982, ANSI developed the specification as "SASI" and "Shugart Associates System Interface".^[10] However, the committee documenting the standard would not allow it to be named after a company. Almost a full day was devoted to agreeing to name the standard "Small Computer System Interface", which Boucher intended to be pronounced "sexy", but ENDL's^[11] Dal Allan pronounced the new acronym as "scuzzy" and that stuck.^[12]

The NCR facility in Wichita, Kansas developed the industry's first SCSI controller chip, the NCR 5385, released in 1983. According to its developers, the chip worked the first time it was tested.^[13][14] A number of companies, such as Adaptec and Optimem, were early supporters of SCSI.^[10] By late 1990 at least 45 manufactures offered 251 models of parallel SCSI host adapters^[15] Today, such host adapters have largely been displaced by the faster serial SCSI (SAS) host adapters.^[16]

The "small" reference in "small computer system interface" is historical; since the mid-1990s, SCSI has been available on even the largest of computer systems.

Since its standardization in 1986, SCSI has been commonly used in the Amiga, Atari, Apple Macintosh and Sun Microsystems computer lines and PC server systems. Apple started using the less-expensive parallel ATA (PATA, also known as IDE) for its low-end machines with the Macintosh Quadra 630 in 1994, and added it to its high-end desktops starting with the Power Macintosh G3 in 1997. Apple dropped on-board SCSI completely in favor of IDE and FireWire with the (Blue & White) Power Mac G3 in 1999, while still offering a PCI SCSI host adapter as an option on up to the Power Macintosh G4 (AGP Graphics) models.^[17] Sun switched its lower-end range to Parallel ATA (PATA) with introduction of their Ultra 5 and 10 low end workstations using CMD640 IDE controller and continued this trend with the later Blade 100 and 150 entry level systems and did not switch to contemporary SATA interface even with the introduction of the Blade 1500 in 2003 while the higher end Blade 2500 released at the same time used Ultra320 Parallel SCSI-3. Sun moved to SATA and SAS interfaces with their last UltraSPARC-III based workstations in 2006 with the entry-level Ultra 25 and mid-range Ultra 45. Commodore included SCSI on the Amiga 3000/3000T systems and it was an add-on to previous Amiga 500/2000 models. Starting with the Amiga 600/1200/4000 systems, Commodore switched to the IDE interface. Atari included SCSI as standard in its Atari MEGA STE, Atari TT and Atari Falcon computer models. SCSI has never been popular in the low-priced IBM PC world, owing to the lower cost and adequate performance of the ATA hard disk standard. However, SCSI drives and even SCSI RAIDs became common in PC workstations for video or audio production.

Modern SCSI

Recent physical versions of SCSI‍—‌Serial Attached SCSI (SAS), SCSI-over-Fibre Channel Protocol (FCP), and USB Attached SCSI (UAS)‍—‌break from the traditional parallel SCSI bus and perform data transfer via serial communications using point-to-point links. Although much of the SCSI documentation talks about the parallel interface, all modern development efforts use serial interfaces. Serial interfaces have a number of advantages over parallel SCSI, including higher data rates, simplified cabling, longer reach, improved fault isolation and full-duplex capability. The primary reason for the shift to serial interfaces is the clock skew issue of high-speed parallel interfaces, which makes the faster variants of parallel SCSI susceptible to problems caused by cabling and termination.^[18]

The non-physical iSCSI preserves the basic SCSI paradigm, especially the command set, almost unchanged, through embedding of SCSI-3 over TCP/IP. Therefore, iSCSI uses logical connections instead of physical links and can run on top of any network supporting IP. The actual physical links are realized on lower network layers, independently of iSCSI. Predominantly, Ethernet is used, which is also of a serial nature.

SCSI is popular on high-performance workstations, servers, and storage appliances. Almost all RAID subsystems on servers have used some kind of SCSI hard disk drives for decades (initially Parallel SCSI, interim Fibre Channel, recently SAS), though a number of manufacturers offer SATA-based RAID subsystems as a cheaper option. Moreover, SAS offers compatibility with SATA devices, creating a much broader range of options for RAID subsystems together with the existence of nearline SAS (NL-SAS) drives. Instead of SCSI, modern desktop computers and notebooks typically use SATA interfaces for internal hard disk drives, with NVMe over PCIe gaining popularity as SATA can bottleneck modern solid-state drives.

Parallel SCSI

Initially, the SCSI Parallel Interface (SPI) was the only interface using the SCSI protocol. Its standardization started as a single-ended 8-bit bus in 1986, transferring up to 5 MB/s, and evolved into a low-voltage differential 16-bit bus capable of up to 320 MB/s. The last SPI-5 standard from 2003 also defined a 640 MB/s speed, which failed to be realized.

Parallel SCSI specifications include several synchronous transfer modes for the parallel cable and an asynchronous mode. The asynchronous mode is a classic request/acknowledge protocol, which allows systems with a slow bus or simple systems to also use SCSI devices. Faster synchronous modes are used more frequently.

SCSI interfaces

SCSI Parallel Interface

Internal parallel SCSI cables are usually ribbons, with two or more 50–, 68–, or 80–pin connectors attached. External cables are typically shielded (but may not be), with 50– or 68–pin connectors at each end, depending upon the specific SCSI bus width supported. The 80–pin Single Connector Attachment (SCA) is typically used for hot-pluggable devices

Fibre Channel

Fibre Channel can be used to transport SCSI information units, as defined by the Fibre Channel Protocol for SCSI (FCP). These connections are hot-pluggable and are usually implemented with optical fiber.

Serial attached SCSI

Serial attached SCSI (SAS) uses a modified Serial ATA data and power cable.

iSCSI

iSCSI (Internet Small Computer System Interface) usually uses Ethernet connectors and cables as its physical transport, but can run over any physical transport capable of transporting IP.

SRP

The SCSI RDMA Protocol (SRP) is a protocol that specifies how to transport SCSI commands over a reliable RDMA connection. This protocol can run over any RDMA-capable physical transport, e.g. InfiniBand or Ethernet when using RoCE or iWARP.

USB Attached SCSI

USB Attached SCSI allows SCSI devices to use the Universal Serial Bus.

Automation/Drive Interface

The Automation/Drive Interface − Transport Protocol (ADT) is used to connect removable media devices, such as tape drives, with the controllers of the libraries (automation devices) in which they are installed. The ADI standard specifies the use of RS-422 for the physical connections. The second-generation ADT-2 standard defines iADT, use of the ADT protocol over IP (Internet Protocol) connections, such as over Ethernet. The Automation/Drive Interface − Commands standards (ADC, ADC-2, and ADC-3) define SCSI commands for these installations.

Parallel interface

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On a parallel SCSI bus, a device (e.g. host adapter, disk drive) is identified by a "SCSI ID", which is a number in the range 0–7 on a narrow bus and in the range 0–15 on a wide bus. On earlier models a physical jumper or switch controls the SCSI ID of the initiator (host adapter). On modern host adapters (since about 1997), doing I/O to the adapter sets the SCSI ID; for example, the adapter often contains an Option ROM (SCSI BIOS) program that runs when the computer boots up and that program has menus that let the operator choose the SCSI ID of the host adapter. Alternatively, the host adapter may come with software that must be installed on the host computer to configure the SCSI ID. The traditional SCSI ID for a host adapter is 7, as that ID has the highest priority during bus arbitration (even on a 16-bit bus).

The SCSI ID of a device in a drive enclosure that has a back plane is set either by jumpers or by the slot in the enclosure the device is installed into, depending on the model of the enclosure. In the latter case, each slot on the enclosure's back plane delivers control signals to the drive to select a unique SCSI ID. A SCSI enclosure without a back plane often has a switch for each drive to choose the drive's SCSI ID. The enclosure is packaged with connectors that must be plugged into the drive where the jumpers are typically located; the switch emulates the necessary jumpers. While there is no standard that makes this work, drive designers typically set up their jumper headers in a consistent format that matches the way that these switches are implemented.

Setting the bootable (or first) hard disk to SCSI ID 0 is an accepted IT community recommendation. SCSI ID 2 is usually set aside for the floppy disk drive, while SCSI ID 3 is typically for a CD-ROM drive.^[20]

General

Note that a SCSI target device (which can be called a physical unit) is sometimes divided into smaller logical units. For example, a high-end disk subsystem may be a single SCSI device but contain dozens of individual disk drives, each of which is a logical unit. Further, a RAID array may be a single SCSI device, but may contain many logical units, each of which is a "virtual" disk—a stripe set or mirror set constructed from portions of real disk drives. The SCSI ID, WWN, etc. in this case identifies the whole subsystem, and a second number, the LUN, identifies a disk device (real or virtual) within the subsystem.

It is quite common, though incorrect, to refer to the logical unit itself as a "LUN".^[21] Sometimes, redundantly, the actual LUN may be called a "LUN number" or "LUN id".^[22]

In modern SCSI transport protocols, there is an automated process for the "discovery" of the IDs. The SSA initiator (normally the host computer through the host adaptor) "walk the loop" to determine what devices are connected and then assigns each one a 7-bit "hop-count" value. Fibre Channel – Arbitrated Loop (FC-AL) initiators use the LIP (Loop Initialization Protocol) to interrogate each device port for its WWN (World Wide Name). For iSCSI, because of the unlimited scope of the (IP) network, the process is quite complicated. These discovery processes occur at power-on/initialization time and also if the bus topology changes later, for example, if an extra device is added.

SCSI has the CTL (Channel, Target or Physical Unit Number, Logical Unit Number) identification mechanism per host bus adapter, or the HCTL (HBA, Channel, PUN, LUN) identification mechanism; one host adapter may have more than one channel.^[23]