<s>
Small	B-Architecture
Computer	I-Architecture
System	I-Architecture
Interface	I-Architecture
(	O
SCSI	B-Architecture
,	O
)	O
is	O
a	O
set	O
of	O
standards	O
for	O
physically	O
connecting	O
and	O
transferring	O
data	O
between	O
computers	O
and	O
peripheral	O
devices	O
.	O
</s>
<s>
The	O
SCSI	B-Architecture
standards	O
define	O
commands	O
,	O
protocols	O
,	O
electrical	O
,	O
optical	O
and	O
logical	O
interfaces	B-Application
.	O
</s>
<s>
The	O
SCSI	B-Architecture
standard	O
defines	O
command	O
sets	O
for	O
specific	O
peripheral	O
device	O
types	O
;	O
the	O
presence	O
of	O
"	O
unknown	O
"	O
as	O
one	O
of	O
these	O
types	O
means	O
that	O
in	O
theory	O
it	O
can	O
be	O
used	O
as	O
an	O
interface	O
to	O
almost	O
any	O
device	O
,	O
but	O
the	O
standard	O
is	O
highly	O
pragmatic	O
and	O
addressed	O
toward	O
commercial	O
requirements	O
.	O
</s>
<s>
The	O
initial	O
Parallel	O
SCSI	B-Architecture
was	O
most	O
commonly	O
used	O
for	O
hard	B-Device
disk	I-Device
drives	I-Device
and	O
tape	O
drives	O
,	O
but	O
it	O
can	O
connect	O
a	O
wide	O
range	O
of	O
other	O
devices	O
,	O
including	O
scanners	O
and	O
CD	B-Device
drives	O
,	O
although	O
not	O
all	O
controllers	O
can	O
handle	O
all	O
devices	O
.	O
</s>
<s>
The	O
ancestral	O
SCSI	B-Architecture
standard	O
,	O
X3.131-1986	O
,	O
generally	O
referred	O
to	O
as	O
SCSI-1	O
,	O
was	O
published	O
by	O
the	O
X3T9	O
technical	O
committee	O
of	O
the	O
American	O
National	O
Standards	O
Institute	O
(	O
ANSI	O
)	O
in	O
1986	O
.	O
</s>
<s>
SCSI-2	O
was	O
published	O
in	O
August	O
1990	O
as	O
X3.T9.2/86	O
-109	O
,	O
with	O
further	O
revisions	O
in	O
1994	O
and	O
subsequent	O
adoption	O
of	O
a	O
multitude	O
of	O
interfaces	B-Application
.	O
</s>
<s>
SCSI	B-Architecture
is	O
derived	O
from	O
"	O
SASI	O
"	O
,	O
the	O
"	O
Shugart	O
Associates	O
System	O
Interface	O
"	O
,	O
developed	O
beginning	O
1979	O
and	O
publicly	O
disclosed	O
in	O
1981	O
.	O
</s>
<s>
Larry	O
Boucher	O
is	O
considered	O
to	O
be	O
the	O
"	O
father	O
"	O
of	O
SASI	O
and	O
ultimately	O
SCSI	B-Architecture
due	O
to	O
his	O
pioneering	O
work	O
first	O
at	O
Shugart	O
Associates	O
and	O
then	O
at	O
Adaptec	O
.	O
</s>
<s>
A	O
SASI	O
controller	O
provided	O
a	O
bridge	O
between	O
a	O
hard	B-Device
disk	I-Device
drive	I-Device
's	O
low-level	O
interface	O
and	O
a	O
host	O
computer	O
,	O
which	O
needed	O
to	O
read	O
blocks	O
of	O
data	O
.	O
</s>
<s>
SASI	O
controller	O
boards	O
were	O
typically	O
the	O
size	O
of	O
a	O
hard	B-Device
disk	I-Device
drive	I-Device
and	O
were	O
usually	O
physically	O
mounted	O
to	O
the	O
drive	O
's	O
chassis	O
.	O
</s>
<s>
SASI	O
,	O
which	O
was	O
used	O
in	O
mini	O
-	O
and	O
early	O
microcomputers	O
,	O
defined	O
the	O
interface	O
as	O
using	O
a	O
50-pin	O
flat	O
ribbon	O
connector	O
which	O
was	O
adopted	O
as	O
the	O
SCSI-1	O
connector	O
.	O
</s>
<s>
SASI	O
is	O
a	O
fully	O
compliant	O
subset	O
of	O
SCSI-1	O
so	O
that	O
many	O
,	O
if	O
not	O
all	O
,	O
of	O
the	O
then-existing	O
SASI	O
controllers	O
were	O
SCSI-1	O
compatible	O
.	O
</s>
<s>
Almost	O
a	O
full	O
day	O
was	O
devoted	O
to	O
agreeing	O
to	O
name	O
the	O
standard	O
"	O
Small	B-Architecture
Computer	I-Architecture
System	I-Architecture
Interface	I-Architecture
"	O
,	O
which	O
Boucher	O
intended	O
to	O
be	O
pronounced	O
"	O
sexy	O
"	O
,	O
but	O
ENDL	O
's	O
Dal	O
Allan	O
pronounced	O
the	O
new	O
acronym	O
as	O
"	O
scuzzy	B-Architecture
"	O
and	O
that	O
stuck	O
.	O
</s>
<s>
A	O
number	O
of	O
companies	O
such	O
as	O
NCR	O
Corporation	O
,	O
Adaptec	O
and	O
Optimem	O
were	O
early	O
supporters	O
of	O
SCSI	B-Architecture
.	O
</s>
<s>
The	O
NCR	O
facility	O
in	O
Wichita	O
,	O
Kansas	O
is	O
widely	O
thought	O
to	O
have	O
developed	O
the	O
industry	O
's	O
first	O
SCSI	B-Architecture
controller	O
chip	O
;	O
it	O
worked	O
the	O
first	O
time	O
.	O
</s>
<s>
The	O
"	O
small	O
"	O
reference	O
in	O
"	O
small	B-Architecture
computer	I-Architecture
system	I-Architecture
interface	I-Architecture
"	O
is	O
historical	O
;	O
since	O
the	O
mid-1990s	O
,	O
SCSI	B-Architecture
has	O
been	O
available	O
on	O
even	O
the	O
largest	O
of	O
computer	O
systems	O
.	O
</s>
<s>
Since	O
its	O
standardization	O
in	O
1986	O
,	O
SCSI	B-Architecture
has	O
been	O
commonly	O
used	O
in	O
the	O
Amiga	B-Device
,	O
Atari	O
,	O
Apple	B-Device
Macintosh	I-Device
and	O
Sun	O
Microsystems	O
computer	O
lines	O
and	O
PC	O
server	O
systems	O
.	O
</s>
<s>
Apple	O
started	O
using	O
the	O
less-expensive	O
parallel	B-Protocol
ATA	I-Protocol
(	O
PATA	B-Protocol
,	O
also	O
known	O
as	O
IDE	O
)	O
for	O
its	O
low-end	O
machines	O
with	O
the	O
Macintosh	B-Device
Quadra	I-Device
630	O
in	O
1994	O
,	O
and	O
added	O
it	O
to	O
its	O
high-end	O
desktops	O
starting	O
with	O
the	O
Power	O
Macintosh	B-Device
G3	O
in	O
1997	O
.	O
</s>
<s>
Apple	O
dropped	O
on-board	O
SCSI	B-Architecture
completely	O
in	O
favor	O
of	O
IDE	O
and	O
FireWire	B-Protocol
with	O
the	O
(	O
Blue	O
&	O
White	O
)	O
Power	O
Mac	O
G3	O
in	O
1999	O
,	O
while	O
still	O
offering	O
a	O
PCI	B-Protocol
SCSI	B-Architecture
host	B-Architecture
adapter	I-Architecture
as	O
an	O
option	O
on	O
up	O
to	O
the	O
Power	O
Macintosh	B-Device
G4	O
(	O
AGP	O
Graphics	O
)	O
models	O
.	O
</s>
<s>
Sun	O
switched	O
its	O
lower-end	O
range	O
to	O
Serial	B-Protocol
ATA	O
(	O
SATA	O
)	O
.	O
</s>
<s>
Commodore	O
included	O
SCSI	B-Architecture
on	O
the	O
Amiga	B-Device
3000/3000T	O
systems	O
and	O
it	O
was	O
an	O
add-on	O
to	O
previous	O
Amiga	B-Device
500/2000	O
models	O
.	O
</s>
<s>
Starting	O
with	O
the	O
Amiga	B-Device
600/1200/4000	O
systems	O
Commodore	O
switched	O
to	O
the	O
IDE	O
interface	O
.	O
</s>
<s>
Atari	O
included	O
SCSI	B-Architecture
as	O
standard	O
in	O
its	O
Atari	B-Device
MEGA	I-Device
STE	I-Device
,	O
Atari	B-Device
TT	I-Device
and	O
Atari	B-Device
Falcon	I-Device
computer	O
models	O
.	O
</s>
<s>
SCSI	B-Architecture
has	O
never	O
been	O
popular	O
in	O
the	O
low-priced	O
IBM	O
PC	O
world	O
,	O
owing	O
to	O
the	O
lower	O
cost	O
and	O
adequate	O
performance	O
of	O
ATA	O
hard	B-Device
disk	I-Device
standard	O
.	O
</s>
<s>
However	O
,	O
SCSI	B-Architecture
drives	O
and	O
even	O
SCSI	B-Architecture
RAIDs	B-Architecture
became	O
common	O
in	O
PC	O
workstations	O
for	O
video	O
or	O
audio	O
production	O
.	O
</s>
<s>
Recent	O
physical	O
versions	O
of	O
SCSISerial	O
Attached	O
SCSI	B-Architecture
(	O
SAS	O
)	O
,	O
SCSI-over-Fibre	O
Channel	O
Protocol	O
(	O
FCP	O
)	O
,	O
and	O
USB	B-Protocol
Attached	O
SCSI	B-Architecture
(	O
UAS	O
)	O
break	O
from	O
the	O
traditional	O
parallel	O
SCSI	B-Architecture
bus	B-Architecture
and	O
perform	O
data	O
transfer	O
via	O
serial	B-Protocol
communications	I-Protocol
using	O
point-to-point	B-Architecture
links	O
.	O
</s>
<s>
Although	O
much	O
of	O
the	O
SCSI	B-Architecture
documentation	O
talks	O
about	O
the	O
parallel	O
interface	O
,	O
all	O
modern	O
development	O
efforts	O
use	O
serial	B-Protocol
interfaces	B-Application
.	O
</s>
<s>
Serial	B-Protocol
interfaces	B-Application
have	O
a	O
number	O
of	O
advantages	O
over	O
parallel	O
SCSI	B-Architecture
,	O
including	O
higher	O
data	O
rates	O
,	O
simplified	O
cabling	O
,	O
longer	O
reach	O
,	O
improved	O
fault	O
isolation	O
and	O
full-duplex	O
capability	O
.	O
</s>
<s>
The	O
primary	O
reason	O
for	O
the	O
shift	O
to	O
serial	B-Protocol
interfaces	B-Application
is	O
the	O
clock	O
skew	O
issue	O
of	O
high	O
speed	O
parallel	O
interfaces	B-Application
,	O
which	O
makes	O
the	O
faster	O
variants	O
of	O
parallel	O
SCSI	B-Architecture
susceptible	O
to	O
problems	O
caused	O
by	O
cabling	O
and	O
termination	O
.	O
</s>
<s>
The	O
non-physical	O
iSCSI	B-Protocol
preserves	O
the	O
basic	O
SCSI	B-Architecture
paradigm	O
,	O
especially	O
the	O
command	O
set	O
,	O
almost	O
unchanged	O
,	O
through	O
embedding	O
of	O
SCSI-3	O
over	O
TCP/IP	B-Protocol
.	O
</s>
<s>
Therefore	O
,	O
iSCSI	B-Protocol
uses	O
logical	O
connections	O
instead	O
of	O
physical	O
links	O
and	O
can	O
run	O
on	O
top	O
of	O
any	O
network	B-Architecture
supporting	O
IP	B-Protocol
.	O
</s>
<s>
The	O
actual	O
physical	O
links	O
are	O
realized	O
on	O
lower	O
network	B-Architecture
layers	O
,	O
independently	O
from	O
iSCSI	B-Protocol
.	O
</s>
<s>
Predominantly	O
,	O
Ethernet	O
is	O
used	O
which	O
is	O
also	O
of	O
serial	B-Protocol
nature	O
.	O
</s>
<s>
SCSI	B-Architecture
is	O
popular	O
on	O
high-performance	O
workstations	O
,	O
servers	O
,	O
and	O
storage	O
appliances	O
.	O
</s>
<s>
Almost	O
all	O
RAID	B-Architecture
subsystems	O
on	O
servers	O
have	O
used	O
some	O
kind	O
of	O
SCSI	B-Architecture
hard	B-Device
disk	I-Device
drives	I-Device
for	O
decades	O
(	O
initially	O
Parallel	O
SCSI	B-Architecture
,	O
interim	O
Fibre	B-Architecture
Channel	I-Architecture
,	O
recently	O
SAS	O
)	O
,	O
though	O
a	O
number	O
of	O
manufacturers	O
offer	O
SATA-based	O
RAID	B-Architecture
subsystems	O
as	O
a	O
cheaper	O
option	O
.	O
</s>
<s>
Moreover	O
,	O
SAS	O
offers	O
compatibility	O
with	O
SATA	O
devices	O
,	O
creating	O
a	O
much	O
broader	O
range	O
of	O
options	O
for	O
RAID	B-Architecture
subsystems	O
together	O
with	O
the	O
existence	O
of	O
nearline	O
SAS	O
(	O
NL-SAS	O
)	O
drives	O
.	O
</s>
<s>
Instead	O
of	O
SCSI	B-Architecture
,	O
modern	O
desktop	O
computers	O
and	O
notebooks	O
typically	O
use	O
SATA	O
interfaces	B-Application
for	O
internal	O
hard	B-Device
disk	I-Device
drives	I-Device
,	O
with	O
NVMe	B-Application
over	O
PCIe	O
gaining	O
popularity	O
as	O
SATA	O
can	O
bottleneck	O
modern	O
solid-state	B-Device
drives	I-Device
.	O
</s>
<s>
SCSI	B-Architecture
is	O
available	O
in	O
a	O
variety	O
of	O
interfaces	B-Application
.	O
</s>
<s>
The	O
first	O
was	O
parallel	O
SCSI	B-Architecture
(	O
also	O
called	O
SCSI	B-Architecture
Parallel	O
Interface	O
or	O
SPI	O
)	O
,	O
which	O
uses	O
a	O
parallel	O
bus	B-Architecture
design	O
.	O
</s>
<s>
Since	O
2005	O
,	O
SPI	O
was	O
gradually	O
replaced	O
by	O
Serial	B-Protocol
Attached	O
SCSI	B-Architecture
(	O
SAS	O
)	O
,	O
which	O
uses	O
a	O
serial	B-Protocol
design	O
but	O
retains	O
other	O
aspects	O
of	O
the	O
technology	O
.	O
</s>
<s>
Many	O
other	O
interfaces	B-Application
which	O
do	O
not	O
rely	O
on	O
complete	O
SCSI	B-Architecture
standards	O
still	O
implement	O
the	O
SCSI	B-Architecture
command	O
protocol	O
;	O
others	O
drop	O
physical	O
implementation	O
entirely	O
while	O
retaining	O
the	O
SCSI	B-Architecture
architectural	O
model	O
.	O
</s>
<s>
iSCSI	B-Protocol
,	O
for	O
example	O
,	O
uses	O
TCP/IP	B-Protocol
as	O
a	O
transport	O
mechanism	O
,	O
which	O
is	O
most	O
often	O
transported	O
over	O
Gigabit	O
Ethernet	O
or	O
faster	O
network	B-Architecture
links	O
.	O
</s>
<s>
SCSI	B-Architecture
interfaces	B-Application
have	O
often	O
been	O
included	O
on	O
computers	O
from	O
various	O
manufacturers	O
for	O
use	O
under	O
Microsoft	B-Application
Windows	I-Application
,	O
classic	B-Application
Mac	I-Application
OS	I-Application
,	O
Unix	B-Application
,	O
Amiga	B-Device
and	O
Linux	B-Application
operating	I-Application
systems	I-Application
,	O
either	O
implemented	O
on	O
the	O
motherboard	B-Device
or	O
by	O
the	O
means	O
of	O
plug-in	O
adaptors	O
.	O
</s>
<s>
With	O
the	O
advent	O
of	O
SAS	O
and	O
SATA	O
drives	O
,	O
provision	O
for	O
parallel	O
SCSI	B-Architecture
on	O
motherboards	B-Device
was	O
discontinued	O
.	O
</s>
<s>
Initially	O
,	O
the	O
SCSI	B-Architecture
Parallel	O
Interface	O
(	O
SPI	O
)	O
was	O
the	O
only	O
interface	O
using	O
the	O
SCSI	B-Architecture
protocol	O
.	O
</s>
<s>
Its	O
standardization	O
started	O
as	O
a	O
single-ended	O
8-bit	O
bus	B-Architecture
in	O
1986	O
,	O
transferring	O
up	O
to	O
5MB/s	O
,	O
and	O
evolved	O
into	O
a	O
low-voltage	O
differential	O
16-bit	O
bus	B-Architecture
capable	O
of	O
up	O
to	O
320MB/s	O
.	O
</s>
<s>
Parallel	O
SCSI	B-Architecture
specifications	O
include	O
several	O
synchronous	O
transfer	O
modes	O
for	O
the	O
parallel	O
cable	O
,	O
and	O
an	O
asynchronous	O
mode	O
.	O
</s>
<s>
The	O
asynchronous	O
mode	O
is	O
a	O
classic	O
request/acknowledge	O
protocol	O
,	O
which	O
allows	O
systems	O
with	O
a	O
slow	O
bus	B-Architecture
or	O
simple	O
systems	O
to	O
also	O
use	O
SCSI	B-Architecture
devices	O
.	O
</s>
<s>
Interface	O
Alternativenames	O
Specificationbody	O
/	O
document	O
Width(bits )	O
Clock	O
Line	B-Protocol
code	I-Protocol
Maximum	O
Throughput	O
Length	O
Devices	O
Ultra-320	O
SCSI	B-Architecture
Ultra-4	O
;	O
Fast-160	O
SPI-5	O
(	O
INCITS	O
367-2003	O
)	O
16	O
80	O
MHz	O
DDR	O
none	O
320	O
MB/s	O
(	O
2560	O
Mbit/s	O
)	O
12	O
m	O
16	O
SSA	B-Architecture
Serial	B-Architecture
Storage	I-Architecture
Architecture	I-Architecture
T10	O
/	O
INCITS	O
309-1997	O
serial	B-Protocol
200	O
Mbit/s	O
8b10b	B-Protocol
20	O
MB/s	O
(	O
160	O
Mbit/s	O
)	O
25	O
m	O
96	O
SSA	B-Architecture
40	O
T10	O
/	O
INCITS	O
309-1997	O
serial	B-Protocol
400	O
Mbit/s	O
40	O
MB/s	O
(	O
320	O
Mbit/s	O
)	O
25	O
m	O
96	O
Fibre	B-Architecture
Channel	I-Architecture
1Gbit	O
1GFC	B-Architecture
T11	O
/	O
X3T11/94	O
-175v0	O
FC-PH	B-Architecture
Draft	O
,	O
Revision	O
4.3	O
serial	B-Protocol
1.0625	O
Gbit/s	O
8b10b	B-Protocol
98.4	O
MB/s	O
(	O
850	O
Mbit/s	O
)	O
500	O
m	O
/	O
10	O
km	O
127	O
(	O
FC-AL	B-Architecture
)	O
224	O
(	O
FC-SW	B-Architecture
)	O
Fibre	B-Architecture
Channel	I-Architecture
2Gbit	O
2GFC	B-Architecture
T11	O
/	O
X3T11/96	O
-402v0	O
FC-PH-2	O
,	O
Rev	O
7.4	O
serial	B-Protocol
2.125	O
Gbit/s	O
197	O
MB/s	O
(	O
1,700	O
Mbit/s	O
)	O
500	O
m	O
/	O
10	O
km	O
127/224	O
Fibre	B-Architecture
Channel	I-Architecture
4Gbit	O
4GFC	B-Architecture
T11	O
/	O
INCITS	O
Project	O
2118-D	O
/	O
Rev	O
6.10	O
serial	B-Protocol
4.25	O
Gbit/s	O
394	O
MB/s	O
(	O
3,400	O
Mbit/s	O
)	O
500	O
m	O
/	O
10	O
km	O
127/224	O
Fibre	B-Architecture
Channel	I-Architecture
8Gbit	O
8GFC	B-Architecture
T11	O
/	O
INCITS	O
Project	O
2118-D	O
/	O
Rev	O
6.10	O
serial	B-Protocol
8.5	O
Gbit/s	O
788	O
MB/s	O
(	O
6,800	O
Mbit/s	O
)	O
500	O
m	O
/	O
10	O
km	O
127/224	O
Fibre	B-Architecture
Channel	I-Architecture
16Gbit	O
16GFC	B-Architecture
T11	O
/	O
INCITS	O
Project	O
2118-D	O
/	O
Rev	O
6.10	O
serial	B-Protocol
14.025	O
Gbit/s	O
64b66b	O
1,575	O
MB/s	O
(	O
13,600	O
Mbit/s	O
)	O
500	O
m	O
/	O
10	O
km	O
127/224	O
SAS	O
1.1	O
Serial	B-Protocol
attached	O
SCSI	B-Architecture
T10	O
/	O
INCITS	O
417-2006	O
serial	B-Protocol
3	O
Gbit/s	O
8b10b	B-Protocol
300	O
MB/s	O
(	O
2,400	O
Mbit/s	O
)	O
6	O
m	O
16,256	O
SAS	O
2.1	O
T10	O
/	O
INCITS	O
478-2011	O
serial	B-Protocol
6	O
Gbit/s	O
600	O
MB/s	O
(	O
4,800	O
Mbit/s	O
)	O
6	O
m	O
16,256	O
SAS	O
3.0	O
T10	O
/	O
INCITS	O
519	O
serial	B-Protocol
12	O
Gbit/s	O
1,200	O
MB/s	O
(	O
9,600	O
Mbit/s	O
)	O
6	O
m	O
16,256	O
SAS	O
4.0	O
T10	O
/	O
INCITS	O
534	O
(	O
draft	O
)	O
serial	B-Protocol
22.5	O
Gbit/s	O
128b150b	O
2,400	O
MB/s	O
(	O
19,200	O
Mbit/s	O
)	O
tbd	O
16,256	O
IEEE	O
1394-2008	O
Firewire	B-Protocol
S3200	O
,	O
i.Link	B-Protocol
,	O
Serial	B-Protocol
Bus	B-Architecture
Protocol	O
(	O
SBP	O
)	O
IEEE	O
Std	O
.	O
</s>
<s>
Internal	O
parallel	O
SCSI	B-Architecture
cables	O
are	O
usually	O
ribbons	O
,	O
with	O
two	O
or	O
more	O
50	O
–	O
,	O
68	O
–	O
,	O
or	O
80	O
–	O
pin	O
connectors	O
attached	O
.	O
</s>
<s>
External	O
cables	O
are	O
typically	O
shielded	O
(	O
but	O
may	O
not	O
be	O
)	O
,	O
with	O
50	O
–	O
or	O
68	O
–	O
pin	O
connectors	O
at	O
each	O
end	O
,	O
depending	O
upon	O
the	O
specific	O
SCSI	B-Architecture
bus	B-Architecture
width	O
supported	O
.	O
</s>
<s>
Fibre	B-Architecture
Channel	I-Architecture
can	O
be	O
used	O
to	O
transport	O
SCSI	B-Architecture
information	O
units	O
,	O
as	O
defined	O
by	O
the	O
Fibre	B-Architecture
Channel	I-Architecture
Protocol	O
for	O
SCSI	B-Architecture
(	O
FCP	O
)	O
.	O
</s>
<s>
Serial	B-Protocol
attached	O
SCSI	B-Architecture
(	O
SAS	O
)	O
uses	O
a	O
modified	O
Serial	B-Protocol
ATA	O
data	O
and	O
power	O
cable	O
.	O
</s>
<s>
iSCSI	B-Protocol
(	O
Internet	O
Small	B-Architecture
Computer	I-Architecture
System	I-Architecture
Interface	I-Architecture
)	O
usually	O
uses	O
Ethernet	O
connectors	O
and	O
cables	O
as	O
its	O
physical	O
transport	O
,	O
but	O
can	O
run	O
over	O
any	O
physical	O
transport	O
capable	O
of	O
transporting	O
IP	B-Protocol
.	O
</s>
<s>
The	O
SCSI	B-Protocol
RDMA	I-Protocol
Protocol	I-Protocol
(	O
SRP	B-Protocol
)	O
is	O
a	O
protocol	O
that	O
specifies	O
how	O
to	O
transport	O
SCSI	B-Architecture
commands	O
over	O
a	O
reliable	O
RDMA	O
connection	O
.	O
</s>
<s>
InfiniBand	B-Architecture
or	O
Ethernet	O
when	O
using	O
RoCE	B-Operating_System
or	O
iWARP	B-Device
.	O
</s>
<s>
USB	B-Protocol
Attached	O
SCSI	B-Architecture
allows	O
SCSI	B-Architecture
devices	O
to	O
use	O
the	O
Universal	B-Protocol
Serial	I-Protocol
Bus	I-Protocol
.	O
</s>
<s>
The	O
second-generation	O
ADT-2	O
standard	O
defines	O
iADT	O
,	O
use	O
of	O
the	O
ADT	O
protocol	O
over	O
IP	B-Protocol
(	O
Internet	B-Protocol
Protocol	I-Protocol
)	O
connections	O
,	O
such	O
as	O
over	O
Ethernet	O
.	O
</s>
<s>
The	O
Automation/Drive	O
Interface	O
−	O
Commands	O
standards	O
(	O
ADC	O
,	O
ADC-2	O
,	O
and	O
ADC-3	O
)	O
define	O
SCSI	B-Architecture
commands	O
for	O
these	O
installations	O
.	O
</s>
<s>
In	O
addition	O
to	O
many	O
different	O
hardware	O
implementations	O
,	O
the	O
SCSI	B-Architecture
standards	O
also	O
include	O
an	O
extensive	O
set	O
of	O
command	O
definitions	O
.	O
</s>
<s>
The	O
SCSI	B-Architecture
command	O
architecture	O
was	O
originally	O
defined	O
for	O
parallel	O
SCSI	B-Architecture
buses	O
but	O
has	O
been	O
carried	O
forward	O
with	O
minimal	O
change	O
for	O
use	O
with	O
iSCSI	B-Protocol
and	O
serial	B-Protocol
SCSI	B-Architecture
.	O
</s>
<s>
Other	O
technologies	O
which	O
use	O
the	O
SCSI	B-Architecture
command	O
set	O
include	O
the	O
ATA	O
Packet	O
Interface	O
,	O
USB	B-Architecture
Mass	I-Architecture
Storage	I-Architecture
class	I-Architecture
and	O
FireWire	B-General_Concept
SBP-2	I-General_Concept
.	O
</s>
<s>
In	O
SCSI	B-Architecture
terminology	O
,	O
communication	O
takes	O
place	O
between	O
an	O
initiator	B-Device
and	O
a	O
target	B-Device
.	O
</s>
<s>
The	O
initiator	B-Device
sends	O
a	O
command	O
to	O
the	O
target	B-Device
,	O
which	O
then	O
responds	O
.	O
</s>
<s>
SCSI	B-Architecture
commands	O
are	O
sent	O
in	O
a	O
Command	O
Descriptor	O
Block	O
(	O
CDB	O
)	O
.	O
</s>
<s>
At	O
the	O
end	O
of	O
the	O
command	O
sequence	O
,	O
the	O
target	B-Device
returns	O
a	O
status	O
code	O
byte	O
,	O
such	O
as	O
00h	O
for	O
success	O
,	O
02h	O
for	O
an	O
error	O
(	O
called	O
a	O
Check	O
Condition	O
)	O
,	O
or	O
08h	O
for	O
busy	O
.	O
</s>
<s>
When	O
the	O
target	B-Device
returns	O
a	O
Check	O
Condition	O
in	O
response	O
to	O
a	O
command	O
,	O
the	O
initiator	B-Device
usually	O
then	O
issues	O
a	O
SCSI	B-Architecture
Request	O
Sense	O
command	O
in	O
order	O
to	O
obtain	O
a	O
key	O
code	O
qualifier	O
(	O
KCQ	O
)	O
from	O
the	O
target	B-Device
.	O
</s>
<s>
The	O
Check	O
Condition	O
and	O
Request	O
Sense	O
sequence	O
involves	O
a	O
special	O
SCSI	B-Architecture
protocol	O
called	O
a	O
Contingent	O
Allegiance	O
Condition	O
.	O
</s>
<s>
There	O
are	O
four	O
categories	O
of	O
SCSI	B-Architecture
commands	O
:	O
N	O
(	O
non-data	O
)	O
,	O
W	O
(	O
writing	O
data	O
from	O
initiator	B-Device
to	O
target	B-Device
)	O
,	O
R	O
(	O
reading	O
data	O
)	O
,	O
and	O
B	O
(	O
bidirectional	O
)	O
.	O
</s>
<s>
There	O
are	O
about	O
60	O
different	O
SCSI	B-Architecture
commands	O
in	O
total	O
,	O
with	O
the	O
most	O
commonly	O
used	O
being	O
:	O
</s>
<s>
Start/Stop	O
unit	O
:	O
Spins	O
disks	O
up	O
and	O
down	O
,	O
or	O
loads/unloads	O
media	O
(	O
CD	B-Device
,	O
tape	O
,	O
etc	O
.	O
</s>
<s>
In	O
a	O
disk	O
,	O
a	O
low	B-Application
level	I-Application
format	I-Application
will	O
occur	O
.	O
</s>
<s>
Each	O
device	O
on	O
the	O
SCSI	B-Architecture
bus	B-Architecture
is	O
assigned	O
a	O
unique	O
SCSI	B-Architecture
identification	O
number	O
or	O
ID	O
.	O
</s>
<s>
disk	O
type	O
)	O
storage	O
device	O
consists	O
of	O
a	O
number	O
of	O
logical	O
blocks	O
,	O
addressed	O
by	O
Logical	B-Device
Block	I-Device
Address	I-Device
(	O
LBA	B-Device
)	O
.	O
</s>
<s>
A	O
typical	O
LBA	B-Device
equates	O
to	O
512	O
bytes	O
of	O
storage	O
.	O
</s>
<s>
The	O
Read(6 )	O
and	O
Write(6 )	O
commands	O
contain	O
a	O
21-bit	O
LBA	B-Device
address	O
.	O
</s>
<s>
The	O
Read(10 )	O
,	O
Read(12 )	O
,	O
Read	O
Long	O
,	O
Write(10 )	O
,	O
Write(12 )	O
,	O
and	O
Write	O
Long	O
commands	O
all	O
contain	O
a	O
32-bit	O
LBA	B-Device
address	O
plus	O
various	O
other	O
parameter	O
options	O
.	O
</s>
<s>
Read	O
and	O
write	O
operations	O
on	O
a	O
sequential	O
access	O
device	O
begin	O
at	O
the	O
current	O
tape	O
position	O
,	O
not	O
at	O
a	O
specific	O
LBA	B-Device
.	O
</s>
<s>
Tape	O
devices	O
such	O
as	O
half-inch	O
9-track	B-Device
tape	I-Device
,	O
DDS	B-Architecture
(	O
4mm	O
tapes	O
physically	O
similar	O
to	O
DAT	O
)	O
,	O
Exabyte	O
,	O
etc.	O
,	O
support	O
variable	O
block	O
sizes	O
.	O
</s>
<s>
On	O
a	O
parallel	O
SCSI	B-Architecture
bus	B-Architecture
,	O
a	O
device	O
(	O
e.g.	O
</s>
<s>
host	B-Architecture
adapter	I-Architecture
,	O
disk	O
drive	O
)	O
is	O
identified	O
by	O
a	O
"	O
SCSI	B-Architecture
ID	O
"	O
,	O
which	O
is	O
a	O
number	O
in	O
the	O
range	O
0	O
–	O
7	O
on	O
a	O
narrow	O
bus	B-Architecture
and	O
in	O
the	O
range	O
0	O
–	O
15	O
on	O
a	O
wide	O
bus	B-Architecture
.	O
</s>
<s>
On	O
earlier	O
models	O
a	O
physical	O
jumper	O
or	O
switch	O
controls	O
the	O
SCSI	B-Architecture
ID	O
of	O
the	O
initiator	B-Device
(	O
host	B-Architecture
adapter	I-Architecture
)	O
.	O
</s>
<s>
On	O
modern	O
host	B-Architecture
adapters	I-Architecture
(	O
since	O
about	O
1997	O
)	O
,	O
doing	O
I/O	O
to	O
the	O
adapter	O
sets	O
the	O
SCSI	B-Architecture
ID	O
;	O
for	O
example	O
,	O
the	O
adapter	O
often	O
contains	O
a	O
Option	B-Device
ROM	I-Device
(	O
SCSI	B-Architecture
BIOS	O
)	O
program	O
that	O
runs	O
when	O
the	O
computer	O
boots	O
up	O
and	O
that	O
program	O
has	O
menus	O
that	O
let	O
the	O
operator	O
choose	O
the	O
SCSI	B-Architecture
ID	O
of	O
the	O
host	B-Architecture
adapter	I-Architecture
.	O
</s>
<s>
Alternatively	O
,	O
the	O
host	B-Architecture
adapter	I-Architecture
may	O
come	O
with	O
software	O
that	O
must	O
be	O
installed	O
on	O
the	O
host	O
computer	O
to	O
configure	O
the	O
SCSI	B-Architecture
ID	O
.	O
</s>
<s>
The	O
traditional	O
SCSI	B-Architecture
ID	O
for	O
a	O
host	B-Architecture
adapter	I-Architecture
is	O
7	O
,	O
as	O
that	O
ID	O
has	O
the	O
highest	O
priority	O
during	O
bus	B-Architecture
arbitration	O
(	O
even	O
on	O
a	O
16bit	O
bus	B-Architecture
)	O
.	O
</s>
<s>
The	O
SCSI	B-Architecture
ID	O
of	O
a	O
device	O
in	O
a	O
drive	O
enclosure	O
that	O
has	O
a	O
back	B-Architecture
plane	I-Architecture
is	O
set	O
either	O
by	O
jumpers	O
or	O
by	O
the	O
slot	O
in	O
the	O
enclosure	O
the	O
device	O
is	O
installed	O
into	O
,	O
depending	O
on	O
the	O
model	O
of	O
the	O
enclosure	O
.	O
</s>
<s>
In	O
the	O
latter	O
case	O
,	O
each	O
slot	O
on	O
the	O
enclosure	O
's	O
back	B-Architecture
plane	I-Architecture
delivers	O
control	O
signals	O
to	O
the	O
drive	O
to	O
select	O
a	O
unique	O
SCSI	B-Architecture
ID	O
.	O
</s>
<s>
A	O
SCSI	B-Architecture
enclosure	O
without	O
a	O
back	B-Architecture
plane	I-Architecture
often	O
has	O
a	O
switch	O
for	O
each	O
drive	O
to	O
choose	O
the	O
drive	O
's	O
SCSI	B-Architecture
ID	O
.	O
</s>
<s>
Setting	O
the	O
bootable	O
(	O
or	O
first	O
)	O
hard	B-Device
disk	I-Device
to	O
SCSI	B-Architecture
ID	O
0	O
is	O
an	O
accepted	O
IT	O
community	O
recommendation	O
.	O
</s>
<s>
SCSI	B-Architecture
ID	O
2	O
is	O
usually	O
set	O
aside	O
for	O
the	O
floppy	O
disk	O
drive	O
while	O
SCSI	B-Architecture
ID	O
3	O
is	O
typically	O
for	O
a	O
CD-ROM	B-Device
drive	O
.	O
</s>
<s>
Note	O
that	O
a	O
SCSI	B-Device
target	I-Device
device	O
(	O
which	O
can	O
be	O
called	O
a	O
"	O
physical	O
unit	O
"	O
)	O
is	O
sometimes	O
divided	O
into	O
smaller	O
"	O
logical	O
units	O
"	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
high-end	O
disk	B-Device
subsystem	I-Device
may	O
be	O
a	O
single	O
SCSI	B-Architecture
device	O
but	O
contain	O
dozens	O
of	O
individual	O
disk	O
drives	O
,	O
each	O
of	O
which	O
is	O
a	O
logical	O
unit	O
.	O
</s>
<s>
Further	O
,	O
a	O
RAID	B-Architecture
array	I-Architecture
may	O
be	O
a	O
single	O
SCSI	B-Architecture
device	O
,	O
but	O
may	O
contain	O
many	O
logical	O
units	O
,	O
each	O
of	O
which	O
is	O
a	O
"	O
virtual	O
"	O
disk	O
—	O
a	O
stripe	O
set	O
or	O
mirror	O
set	O
constructed	O
from	O
portions	O
of	O
real	O
disk	O
drives	O
.	O
</s>
<s>
The	O
SCSI	B-Architecture
ID	O
,	O
WWN	O
,	O
etc	O
.	O
</s>
<s>
In	O
modern	O
SCSI	B-Architecture
transport	O
protocols	O
,	O
there	O
is	O
an	O
automated	O
process	O
for	O
the	O
"	O
discovery	O
"	O
of	O
the	O
IDs	O
.	O
</s>
<s>
The	O
SSA	B-Architecture
initiator	B-Device
(	O
normally	O
the	O
host	O
computer	O
through	O
the	O
'	O
host	B-Architecture
adaptor	I-Architecture
 '	O
)	O
"	O
walk	O
the	O
loop	O
"	O
to	O
determine	O
what	O
devices	O
are	O
connected	O
and	O
then	O
assigns	O
each	O
one	O
a	O
7-bit	O
"	O
hop-count	O
"	O
value	O
.	O
</s>
<s>
Fibre	B-Architecture
Channel	I-Architecture
–	O
Arbitrated	B-Architecture
Loop	I-Architecture
(	O
FC-AL	B-Architecture
)	O
initiators	O
use	O
the	O
LIP	O
(	O
Loop	O
Initialization	O
Protocol	O
)	O
to	O
interrogate	O
each	O
device	O
port	O
for	O
its	O
WWN	O
(	O
World	O
Wide	O
Name	O
)	O
.	O
</s>
<s>
For	O
iSCSI	B-Protocol
,	O
because	O
of	O
the	O
unlimited	O
scope	O
of	O
the	O
(	O
IP	B-Protocol
)	O
network	B-Architecture
,	O
the	O
process	O
is	O
quite	O
complicated	O
.	O
</s>
<s>
These	O
discovery	O
processes	O
occur	O
at	O
power-on/initialization	O
time	O
and	O
also	O
if	O
the	O
bus	B-Architecture
topology	O
changes	O
later	O
,	O
for	O
example	O
if	O
an	O
extra	O
device	O
is	O
added	O
.	O
</s>
<s>
SCSI	B-Architecture
has	O
the	O
CTL	O
(	O
Channel	O
,	O
Target	B-Device
or	O
Physical	O
Unit	O
Number	O
,	O
Logical	O
Unit	O
Number	O
)	O
identification	O
mechanism	O
per	O
host	B-Architecture
bus	I-Architecture
adapter	I-Architecture
,	O
or	O
the	O
HCTL	O
(	O
HBA	O
,	O
Channel	O
,	O
PUN	O
,	O
LUN	O
)	O
identification	O
mechanism	O
,	O
one	O
host	B-Architecture
adapter	I-Architecture
may	O
have	O
more	O
than	O
one	O
channels	O
.	O
</s>
<s>
While	O
all	O
SCSI	B-Architecture
controllers	O
can	O
work	O
with	O
read/write	O
storage	O
devices	O
,	O
i.e.	O
</s>
<s>
disk	O
and	O
tape	O
,	O
some	O
will	O
not	O
work	O
with	O
some	O
other	O
device	O
types	O
;	O
older	O
controllers	O
are	O
likely	O
to	O
be	O
more	O
limited	O
,	O
sometimes	O
by	O
their	O
driver	O
software	O
,	O
and	O
more	O
Device	O
Types	O
were	O
added	O
as	O
SCSI	B-Architecture
evolved	O
.	O
</s>
<s>
Even	O
CD-ROMs	B-Device
are	O
not	O
handled	O
by	O
all	O
controllers	O
.	O
</s>
<s>
Device	O
Type	O
is	O
a	O
5-bit	O
field	O
reported	O
by	O
a	O
SCSI	B-Architecture
Inquiry	O
Command	O
;	O
defined	O
SCSI	B-Architecture
Peripheral	O
Device	O
Types	O
include	O
,	O
in	O
addition	O
to	O
many	O
varieties	O
of	O
storage	O
device	O
,	O
printer	O
,	O
scanner	O
,	O
communications	O
device	O
,	O
and	O
a	O
catch-all	O
"	O
processor	O
"	O
type	O
for	O
devices	O
not	O
otherwise	O
listed	O
.	O
</s>
<s>
In	O
larger	O
SCSI	B-Architecture
servers	O
,	O
the	O
disk-drive	O
devices	O
are	O
housed	O
in	O
an	O
intelligent	O
enclosure	O
that	O
supports	O
SCSI	B-Application
Enclosure	I-Application
Services	I-Application
(	O
SES	O
)	O
.	O
</s>
<s>
The	O
initiator	B-Device
can	O
communicate	O
with	O
the	O
enclosure	O
using	O
a	O
specialized	O
set	O
of	O
SCSI	B-Architecture
commands	O
to	O
access	O
power	O
,	O
cooling	O
,	O
and	O
other	O
non-data	O
characteristics	O
.	O
</s>
