What is SCSI?

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:

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.