Chapter 9 Introduction to the Server
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logical unit number
(LUN)
a numbering scheme
to identify SCSI
devices attached to
an extender card.
Serial attached SCSI
(SAS)
SCSI technology
that transfers data in
serial fashion.
electrical connection is made. The pairs of pins represent the binary number
system. The jumpers are applied in a binary pattern that represents the SCSI ID
number. Figure 9-10 illustrates each of the binary patterns and the related SCSI
ID number.
Logical unit number (LUN)
SCSI devices are not limited to internal devices. The SCSI chain can extend
outside the case by adding an extender card to the SCSI chain. The extender card
allows more devices to be attached to existing SCSI systems. The extender card
is an integrated circuit card connected to the SCSI chain as a SCSI device. See
Figure 9-11.
When additional devices are attached, they are identifi ed separately from the
original chain of devices. Each additional device connected to the SCSI extender
is identifi ed with a logical unit number (LUN). SCSI bus extenders are also
referred to as SCSI expanders, repeaters, and regenerators.
External SCSI device chains are commonly used in RAID systems. RAID
systems are covered later in this chapter.
Serial Attached SCSI
Serial attached SCSI (SAS) is the next generation of SCSI. It uses a serial
port similar to the SATA serial drive port, Figure 9-12. This eliminates the need
of the wide SCSI cable and produces faster data transfer rates. First generation
SAS devices can achieve data rates as high as 3 Gbps. Second generation SAS
devices can achieve rates of 6 Gbps. Note that these speeds are expressed in bits
per second (bps), not bytes per second (Bps). This is because SAS transfers data
in a serial fashion. The SAS design allows 128 devices to be attached directly.
Figure 9-10.
SCSI binary patterns.
=0
=1
=2
=3
=4
=5
=6
=7
Connection
pins
Jumper