Performance with SCSI Drives

SCSI stands for Small Computer System Interface and is pronounced scuzzy. SCSI supports more than just drives. SCSI devices include hard disks, CD-ROM players, tape backup devices, and other hardware peripherals. Any SCSI device must communicate with the computer through a connection to a SCSI adapter. Because SCSI hard drives must be attached to an intelligent SCSI adapter, this is a more expensive disk storage solution when compared to IDE.

Small Computer System Interface (SCSI)

An interface that connects SCSI devices to the computer. This interface uses high-speed parallel technology to connect devices that include hard disks, CD-ROM players, tape backup devices, and other hardware peripherals.

array

A set of objects, all of which are the same size and type.

These are some benefits of SCSI:

• With a SCSI adapter, you can easily add or remove SCSI devices to or from your computer.

• SCSI is a high-performance storage solution with faster transfer rates and high-speed drive mechanisms.

• You can take advantage of new SCSI standards and adapters while maintaining compatibility with older SCSI devices.

• Multiple SCSI drives connected to a special adapter offer options that can greatly improve performance by enabling all the SCSI drives to work together as one; this approach is known as an array. Arrays are important technologies for digital video editing systems.

• SCSI is a widely available, widely used, mature technology.

• SCSI drives have a long and successful history as high-performance storage solutions for high-end workstations and servers.

SCSI Standards

SCSI has been in development for nearly two decades. Since the first standard was released, SCSI has become the premier storage system for servers. Although other drive systems such as IDE have improved dramatically in performance, SCSI is still the first choice of network administrators despite its higher cost. SCSI decisions come down to the SCSI standard that will be supported on your workstations and servers. The bus speed can significantly affect performance, so select wisely.

The table below outlines the SCSI standards.

SCSI Standard	Bus Width	MBps Bus Speed	Maximum Number of Supported Devices
SCSI-1	8-bit	5	8
Fast SCSI (SCSI-2)	8-bit	8	8
Fast and Wide SCSI (SCSI-2)	16-bit	20	16
Ultra SCSI	8-bit	20	8-16
Ultra Wide SCSI (SCSI-3)	16-bit	40	8-16
Ultra2 SCSI	8-bit	40	8
Ultra2 Wide SCSI	16-bit	80	16
Ultra3 SCSI, or Ultra160	16-bit	160	16
Ultra320	16-bit	320	16

SCSI-1 and SCSI-2

The original SCSI-1 standard was introduced in 1986. SCSI-1 had an 8-bit bus with a 5MBps transfer speed. It wasn't until 1994 that SCSI-2 was ratified. SCSI-2 is essentially SCSI-1 with the addition of some options. The most significant options were Fast SCSI and Wide SCSI. These two options could be used with compatible devices to boost transfer speeds to 20MBps. A SCSI device connected to a SCSI-2 adapter will still function but at the lower speed of 8MBps.

SCSI-3

SCSI-3 was developed to overcome several limitations of SCSI-2. SCSI-3 increased the total number of SCSI devices connected together from 8 in SCSI-2 to 16. The next problem was much more difficult to solve. SCSI-2 had a cable distance limitation of 3 meters. The limitation was a problem especially as the data transfer speeds increased because there was no way for SCSI devices to calculate transmission delays. Keeping the signals in order was difficult. SCSI-3 improved on this design by adding timing information to the data being sent. The additional timing information would keep the order of the data intact.

daisy chain

To connect a series of devices, one after the other. When signals are transmitted on a daisy chain, they go to the first device, then to the second, and so on, until termination is reached.

termination

The use of a terminator at both ends of a SCSI daisy chain to keep data signals from bouncing back on the SCSI bus after they reach the end. The terminator is a small plastic connector that has a resistor (ceramic-based material that absorbs electricity) inside it.

Ultra3 SCSI and Ultra320

Ultra3 SCSI takes SCSI-3 technology one giant leap forward by doubling the transfer rate to 160MBps. Like SCSI-3, Ultra3 SCSI uses timing information in the signal, but it adds a second piece of timing information to push double the amount of data.

Ultra320 is a similar specification to Ultra3 SCSI with the exception that it has a 16-bit wide bus that can deliver a whopping 320MBps transfer rate.

Note 

Be sure to check which SCSI technology your computer will support. Some older implementations are not compatible with the newer Ultra Wide SCSI or Ultra3 SCSI. Also, keep in mind that if you use an older SCSI drive on a much faster Ultra3 SCSI bus, the SCSI bus will be only as fast as the older SCSI drive. So, if your drive is Ultra SCSI but your bus supports Ultra3, the SCSI bus will operate at a transfer rate of 20MBps rather than the 160MBps supported by Ultra3.

SCSI Termination

Traditionally, SCSI adapters enable you to daisy chain up to seven devices off each controller. The controller contains termination. You must also terminate the last device in the SCSI chain. The termination at the beginning and the end forms the SCSI chain. Some devices use active termination, which means you don't have to do anything. Other devices require you to manually remove the termination from devices in the middle of the chain and ensure that the last device is terminated.

The next graphic illustrates the daisy-chain technique used by SCSI and the locations of the termination. The ID numbers listed next to the drives are an important part of SCSI devices. Each SCSI device must have a unique address. The ID is used to address data to the correct device in the chain. Depending on the type of SCSI technology you are using, the ID can range from 0 to 15.

physical hard drive

The physical (or real, as opposed to conceptual) drive; for example, drive 0 or drive 1 in a two-drive configuration.

volume

A part of a physical disk that is identified by a single drive label.

Note 

On internal SCSI cables, the terminators are built into the ends of the cables.