Chapter 9 Introduction to the Server
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Small Computer
Systems Interface
(SCSI)
a computer bus tech-
nology that allows
you to connect
multiple devices to a
single controller.
Small Computer Systems Interface (SCSI)
Small Computer Systems Interface (SCSI), pronounced skuzzy, is a
computer bus technology that allows you to connect multiple devices to a
single controller. The benefi ts of SCSI technology can be readily observed in
the chart in Figure 9-7. SCSI technology not only allows multiple devices to
connect to a single controller, it also supports high-data transfer rates. This is
quite a performance improvement when compared to the traditional IDE or ATA
attachment. SCSI is ideal for servers that must hold large amounts of data that is
accessed by numerous clients.
Most SCSI drives are hot-swap devices. Often, SCSI drives are arranged in
the server with hot-swap bays open to the outside of the case to provide easy
access. This allows you to connect or disconnect a drive without opening the
server case. See Figure 9-8.
SCSI technology has evolved over the years. With this evolution, new names
have emerged to describe the improved technology. The term Wide is used to
indicate 16-bit data transfers in place of 8-bit data transfers. To refl ect the increase
in frequency, the term Fast was used and then the term Ultra. Combination of
the words, such as Ultra/Wide, are also used to express the newer technologies.
Ultra/Wide means the SCSI device is faster and supports 16-bit transfers. A close
Figure 9-7.
SCSI technology
specifications.
Common Name Class Devices Bus Width Speed MBps
in bits
SCSI-1 SCSI-1 8 8 5 MHz 4-5 MBps
Wide SCSI SCSI-2 16 8 5 MHz 10 MBps
Fast SCSI SCSI-2 8 8 10 MHz 10 MBps
Fast/Wide SCSI SCSI-2 16 16 10 MHz 20 MBps
Ultra SCSI SCSI-3 8 8 20 MHz 20 MBps
Ultra/Wide SCSI-3 8 16 20 MHz 20 MBps
Ultra2 SCSI-3 8 8 40 MHz 40 MBps
Ultra2/Wide SCSI SCSI-3 16 16 40 MHz 80 MBps
Ultra3 SCSI SCSI-3 16 16 40 MHz 160 MBps
Ultra Wide 320 SCSI-3 16 16 80 MHz 320 MBps
Ultra Wide 640 SCSI-3 16 16 160 MHz 640 MBps
Figure 9-8.
Hot-swap drives are
typically accessible
from the front of
the server for easy
access. (Hewlett-
Packard Company)
Hot-swap
SCSI drive