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What is SCSI?

...from the original standard of the 1980s
...upto today's SCSI Express (SCSI for PCIe SSDs)

article by Performance Technologies and the editor of StorageSearch.com


What is SCSI?

The "Small Computer System Interface", or "SCSI", began as a standard parallel interface between computers and peripheral devices. Although it has been an official ANSI standard since 1986, its concept dates back to mainframe computers of the 1960's. The following is a brief description of SCSI and its capabilities.

The primary objective of the SCSI interface is to provide host computers with independence from proprietary devices. With that as the case, different speed and vendor disk drives, tape drives, printers, communications devices, optical media drives, and other devices can be added to the host computers without requiring modifications to generic system hardware or software.

This is achieved by having a SCSI compliant controller within the host computer, and a SCSI compliant peripheral attached to it. Unique is the fact that each of those pieces of hardware has the ability to behave with intelligence as either/both a target, or initiator of information. The behavior is based on the SCSI standard and therefore consistent from machine to machine, and device to device.

Its name, however, is misleading. The term "Small" creates a misnomer in that SCSI peripherals can operate in both high end SPARC and other workstations running Solaris, Sun OS, or Windows NT as easily as in low end personal computers, or even mainframes.

With SCSI, the computer uses a standard set of commands to move data back and forth between host and peripheral. For the peripheral vendors, this means writing one driver for each operating system environment, as opposed to one driver for each computer vendor.

More importantly, what does this mean to the network administrator, system integrator, or end user? Workstations or personal computers can be configured, and reconfigured to have a certain set of SCSI peripherals working with any given machine, regardless of peripheral vendor. Any additional hardware device added to the machine does not require a laborious development cycle to provide software support - most software drivers are made available from the hardware vendors and can be obtained with the product.

As with any evolved standard, the newer capabilities and inclusions are performance based. Therefore, you can understand the evolution of SCSI through its three versions, (SCSI-I, SCSI-II, and SCSI-III progressively), with several categories within those versions - each category representing a different level of throughput performance, as well as different mechanisms of multi-vendor hardware peripheral connections, (i.e. serial, fibre, etc.).

Perhaps the most dramatic distinction in performance is the distinction between the Narrow (an 8 bit bus) versus Wide (a 16 bit bus) bus widths. Wide was incorporated in the SCSI-II specification and doubles the amount of data transferred in the same period of time as the Narrow.

The following briefly outlines some of the categories you will be exposed to when evaluating and attempting to choose SCSI devices:


FAST SCSI FAST refers to transfer rates of 10 MB/Sec with a narrow device, and 20 MB/Sec transfer rate for a Wide/FAST device.

FAST-20 (Ultra) SCSI FAST-20 refers to transfer rates of 20 MB/Sec with a narrow device, and 40 MB/Sec transfer rate for a ULTRA/Wide/FAST device.

FAST-40 SCSI Fast-40 refers to transfer rates of 40 MB/Sec with a narrow device, and 80 MB/Sec transfer rate for a wide device.

FAST-80 SCSI Fast-80 refers to transfer rates of 80 MB/Sec with a narrow device, and 160 MB/Sec transfer rate for a wide device.

Ultra320 Ultra320 provides burst R/W rates upto 320 MB/sec - and marks the top end of the parallel SCSI standard. The next part of the SCSI roadmap in performance terms is Serial Attached SCSI.

iSCSI internet SCSI was proposed as a concept in 2001 to provide SCSI command functionality over the internet.

Most iSCSI systems in the market today operate at 10Gbs - but the technology will scale with faster ethernet speeds when they become common.

SAS Serial Attached SCSI was first proposed as a concept in 2001 to provide SCSI command functionality over a SATA physical interface.

The latest generations of SAS SSDs operate at 12Gbps and can support throughput rates of 1GB/s in each direction concurrently.

SCSI Express SCSI Express was first proposed as a standard in 2012 to bring SCSI concepts to PCIe.

See also:- 2.5" removable PCIe SSDs

Having defined the above categories, it is clear to see that another advantage of SCSI is its speed. Depending on the SCSI controller and peripheral device, it potentially can provide extremely fast I/O between the host system and peripheral device, depending on the category mode of the SCSI devices, (FAST vs. FAST-20).

SCSI also allows multi-threaded I/O to occur at one time, enabling the handling of multiple outstanding I/O. This form of multitasking is referred to as Command Tagged Queuing. With the use of FAST Wide SCSI host adapters, multi-disk and RAID (redundant array of inexpensive disks) configurations can be arranged while maintaining high system performance and saving precious system bus slots. Disks can be connected to a single port using a daisy chain configuration, (controller connected to the first disk, first disk connected to the second, and so on). In some cases, depending on system configuration, a SCSI controller can support up to 15 devices. Systems can also be configured to boot from one of the SCSI disks if the controller is equipped with a bootable device PROM.

Expandability, flexibility, performance, ease of use, and maturity are several great reasons why the SCSI definition has longevity in the rapidly evolving computer industry. But for the end-user, it provides the ability to get the highest performance and compatibility out of existing and additional computer systems.


Parts of this article were first published in the 1997 edition of the SPARC Product Directory.

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SCSI - Small Computer System Interface:- is a high performance parallel interface for connected mass storage devices such as disk drives to a computer.
SCSI was the first open systems method of virtualizing access to fast enterprise storage drives.

Its ease of use, fast performance and support for large storage capacities made it the popular choice in the 1980s for manufacturers who wanted to offer high performance in their embedded systems, servers and workstations.

Originally specified in the early 1980s using logic level (TTL) signals over ribbon cable to daisy chain upto 8 devices, this standard has been enhanced over 30 years to include newer voltage levels, higher speeds and more devices.

The standard enables large amounts of data to be requested using a small number of intelligent commands.

Due to the popularity of this standard some manufacturers have developed extenders which enable connection over a long distance, converters such as IDE to SCSI, and routers which translate the SCSI interface and commands to fibre channel compatible signals.

Newer variations such as Serial Attached SCSI (SAS), Internet SCSI (iSCSI) and SCSI Express (SCSI for PCIe SSDs) preserve compatibility at the SCSI driver and command set level, but offer different connection and routing methods. ...from Megabyte's Storage Dictionary
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