Why SCSI

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Most servers spend a considerable amount of their time reading and writing to storage devices. Currently the SCSI interface provides the fastest method to perform this operation.

Most workstations use the IDE or EIDE interface. Probably the reason is that IDE drives are cheaper than SCSI type devices. For most workstation usage, the IDE interface is more than adequate. But if high speed access is crucial, the choice should be SCSI. Other articles in our newsletter have referred to the fact that you can’t measure a system’s performance based on the CPU speed alone. This is especially true in the case of a server when tasks are not normally CPU bound, but are file I/O bound.

Are all SCSI interfaces created equal? Of course not, this is computer technology. As time marches on, improve- ments have been made to the original SCSI design. The terminology that has been used to define the improvements does not exactly represent the changes. There are terms like Fast SCSI, Fast Wide SCSI, Ultra SCSI, Wide Ultra SCSI, Ultra2 SCSI and Wide Ultra2 SCSI. From these titles can you tell what is the best? My guess is probably not. Should you know the difference? My guess is yes, if you are planning to acquire new equipment. So we have finally arrived at the purpose of this article. That is to define the differences and to provide you with the knowledge that will help you what the %&*$ computer sales person is telling you.

The definitions below are listed in order from the slowest to the fastest type. In simple terms there are two measurements to consider: Bus Width and Bus Speed. One other consideration is the number of SCSI devices that can be configured in one machine.

Bus Width

Bus width is measured in bits and is either 8 or 16 bits. Easist way to understand this is to relate to lanes on a highway. A 16 lane highway will allow for more traffic than an 8 lane highway. Of course the 16 lane highway will cost more.

Bus Speed

Bus speed is measured in megabytes (MB). The rating represents how many megabytes can be accessed in a second. (MB/s) A higher rating represents that faster access.

SCSI Types

SCSI or SCSI I

This is the original interface. It only provides an 8 bit bus width and a speed of 5 MB/s. Maximum devices is 8.

Fast SCSI

Fast SCSI basically uses a clock doubling technique. It is still limited to an 8 bit bus width and the speed is improved up to 10 MB/s. Maximum devices is 8.

Ultra SCSI

Ultra SCSI was developed using a technique to once again double the clock speed. Still limited to an 8 bit bus but the speed is up to 20 MB/s. Maximum devices is 8.

Fast Wide SCSI

This design provides increased speed by increasing the bus width to 16 bits. It provides the same speed as Ultra SCSI, up to 20 MB/s. The number of maximum devices jumps to 16.

Ultra2 SCSI

This design increases the speed to 40 MB/s using only an 8 bit bus. Maximum devices is 8.

Wide Ultra SCSI

This design is the first in the Ultra category to use a 16 bit bus. The speed is no faster than Ultra2 with the 8 bit bus, 40 MB/s. The number of maximum devices jumps to 16.

Wide Ultra2 SCSI

This design is the fastest. It uses a 16 bit bus and provides a speed of 80 MB/s. Maximum devices is 16.

Are your eyes glazing over? One hint to help remember the differences is that whenever the "wide" term is used there is a 16 bit bus involved vs. the 8 bit bus.

Yes, the 16 bit bus versions are fastest but can also cost more. The other hint is that the "Ultra" technology is basically replacing the "Fast" technology.

If your budget can afford it, the Wide Ultra2 SCSI is the fastest. Because it is also the most recent, you may not find it available or as an option on all equipment.

Other SCSI Benefits

Besides the speed issues, another important feature of SCSI interfaces is that backwards compatibility has been incorporated in all new improvements. A rare situation for computer technology. Thus, if you have older SCSI devices, they will still work as you upgrade to newer technology. Different SCSI technologies can be mixed on the same machine. Even though a new SCSI method may be used, older SCSI devices will not run any faster.

SCSI devices provide the capability of interfacing with the widest variety of devices. Supported peripherals are tape drives, optical drives, hard disk drives, scanners, printers, disk array subsytems (RAID) and CD-ROM drives.

Last Update: September 07, 1998.
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by Advanced Horizons, Inc. -
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