<s>
SCST	B-Application
is	O
a	O
GPL	B-License
licensed	O
SCSI	B-Device
target	I-Device
software	O
stack	O
.	O
</s>
<s>
The	O
design	O
goals	O
of	O
this	O
software	O
stack	O
are	O
high	O
performance	O
,	O
high	O
reliability	O
,	O
strict	O
conformance	O
to	O
existing	O
SCSI	B-Architecture
standards	O
,	O
being	O
easy	O
to	O
extend	O
and	O
easy	O
to	O
use	O
.	O
</s>
<s>
SCST	B-Application
does	O
not	O
only	O
support	O
multiple	O
SCSI	B-Architecture
protocols	O
(	O
iSCSI	B-Protocol
,	O
FC	B-Architecture
,	O
SRP	B-Protocol
,	O
...	O
)	O
but	O
also	O
supports	O
multiple	O
local	O
storage	O
interfaces	O
(	O
SCSI	B-Architecture
pass-through	O
,	O
block	B-General_Concept
I/O	I-General_Concept
and	O
file	O
I/O	O
)	O
and	O
also	O
storage	O
drivers	O
implemented	O
in	O
user-space	O
via	O
the	O
scst_user	O
driver	O
.	O
</s>
<s>
In	O
order	O
to	O
reach	O
maximum	O
performance	O
SCST	B-Application
has	O
been	O
implemented	O
as	O
a	O
set	O
of	O
kernel	O
drivers	O
.	O
</s>
<s>
SCST	B-Application
is	O
often	O
combined	O
with	O
RAID	B-Architecture
,	O
data	B-General_Concept
deduplication	I-General_Concept
and/or	O
high-availability	B-General_Concept
cluster	I-General_Concept
software	O
to	O
augment	O
its	O
functionality	O
.	O
</s>
<s>
The	O
SCST	B-Application
software	O
stack	O
is	O
the	O
basis	O
software	O
of	O
many	O
SAN	O
systems	O
.	O
</s>
<s>
Several	O
world	O
records	O
have	O
been	O
set	O
with	O
SAN	O
systems	O
based	O
on	O
SCST	B-Application
.	O
</s>
<s>
SCST	B-Application
competes	O
with	O
LIO	B-Application
Target	I-Application
for	O
the	O
same	O
purpose	O
of	O
providing	O
a	O
generic	O
SCSI	B-Device
target	I-Device
module	O
inside	O
the	O
Linux	B-Application
kernel	O
.	O
</s>
<s>
For	O
the	O
narrower	O
purpose	O
providing	O
a	O
Linux	B-Application
iSCSI	B-Protocol
target	O
,	O
the	O
older	O
IET	O
and	O
STGT	O
modules	O
also	O
enjoy	O
industry	O
support	O
.	O
</s>
<s>
Between	O
2013	O
and	O
2017	O
SCST	B-Application
was	O
primarily	O
maintained	O
by	O
SanDisk	O
personnel	O
,	O
after	O
Fusion-io	O
acquired	O
the	O
start-up	O
ID7	O
in	O
2013	O
,	O
a	O
driving	O
force	O
behind	O
SCST	B-Application
and	O
after	O
SanDisk	O
acquired	O
Fusion-io	O
in	O
2014	O
.	O
</s>
<s>
SCST	B-Application
consists	O
of	O
three	O
groups	O
of	O
modules	O
:	O
</s>
<s>
The	O
SCST	B-Application
core	O
,	O
a	O
protocol-independent	O
engine	O
for	O
processing	O
SCSI	B-Architecture
commands	O
.	O
</s>
<s>
Target	O
drivers	O
which	O
receive	O
SCSI	B-Architecture
commands	O
from	O
a	O
SCSI	B-Device
initiator	I-Device
,	O
pass	O
these	O
SCSI	B-Architecture
to	O
the	O
SCST	B-Application
core	O
and	O
send	O
back	O
replies	O
to	O
the	O
initiator	O
.	O
</s>
<s>
The	O
supported	O
local	O
storage	O
interfaces	O
are	O
SCSI	B-Architecture
,	O
block	B-General_Concept
device	O
,	O
file	O
and	O
scst_user	O
.	O
</s>
<s>
scst_user	O
is	O
an	O
SCST-specific	O
protocol	O
that	O
allows	O
efficient	O
implementation	O
of	O
storage	O
drivers	O
in	O
user	O
space	O
.	O
</s>
<s>
Configuration	O
of	O
all	O
these	O
modules	O
happens	O
via	O
a	O
sysfs	B-Application
interface	O
.	O
</s>
<s>
Although	O
direct	O
configuration	O
of	O
SCST	B-Application
via	O
its	O
sysfs	B-Application
interface	O
is	O
convenient	O
,	O
the	O
tool	O
called	O
scstadmin	O
allows	O
to	O
control	O
SCST	B-Application
via	O
its	O
sysfs	B-Application
interface	O
and	O
also	O
to	O
save	O
and	O
restore	O
the	O
SCST	B-Application
configuration	O
.	O
</s>
<s>
The	O
following	O
design	O
aspects	O
help	O
SCST	B-Application
to	O
reach	O
the	O
high	O
performance	O
this	O
project	O
is	O
known	O
for	O
:	O
</s>
<s>
The	O
number	O
of	O
threads	O
per	O
storage	O
device	O
that	O
processes	O
SCSI	B-Architecture
commands	O
is	O
configurable	O
.	O
</s>
<s>
All	O
target	O
driver	O
and	O
storage	O
driver	O
functions	O
invoked	O
by	O
the	O
SCST	B-Application
core	O
are	O
asynchronous	O
,	O
at	O
least	O
when	O
the	O
Linux	B-Application
kernel	O
allows	O
this	O
.	O
</s>
<s>
not	O
yet	O
possible	O
to	O
perform	O
asynchronous	O
or	O
direct	O
I/O	O
from	O
inside	O
the	O
Linux	B-Application
kernel	O
.	O
</s>
<s>
One	O
finite-state	B-Architecture
machine	I-Architecture
is	O
associated	O
with	O
each	O
SCSI	B-Architecture
command	O
.	O
</s>
<s>
This	O
allows	O
a	O
single	O
command	O
thread	O
to	O
process	O
multiple	O
SCSI	B-Architecture
commands	O
concurrently	O
.	O
</s>
<s>
the	O
CFQ	B-Application
I/O	O
scheduler	O
.	O
</s>
<s>
This	O
cache	O
avoids	O
having	O
to	O
allocate	O
memory	O
via	O
the	O
kernel	O
for	O
each	O
SCSI	B-Architecture
command	O
.	O
</s>
<s>
Lockless	O
access	O
to	O
certain	O
data	O
structures	O
needed	O
to	O
process	O
SCSI	B-Architecture
commands	O
.	O
</s>
<s>
SCSI	B-Architecture
commands	O
can	O
be	O
submitted	O
by	O
a	O
target	O
driver	O
to	O
the	O
SCST	B-Application
core	O
from	O
softirq	O
context	O
.	O
</s>
<s>
This	O
results	O
in	O
a	O
lower	O
number	O
of	O
context	O
switches	O
compared	O
to	O
having	O
to	O
submit	O
SCSI	B-Architecture
commands	O
from	O
thread	O
context	O
.	O
</s>
<s>
In	O
the	O
year	O
2000	O
a	O
SCSI	B-Device
target	I-Device
framework	O
was	O
developed	O
by	O
the	O
University	O
of	O
New	O
Hampshire	O
InterOperability	O
Laboratory	O
(	O
UNH-IOL	O
)	O
.	O
</s>
<s>
supports	O
the	O
iSCSI	B-Protocol
protocol	O
,	O
it	O
was	O
designed	O
such	O
that	O
support	O
for	O
other	O
storage	O
protocols	O
would	O
be	O
easy	O
to	O
add	O
.	O
</s>
<s>
In	O
2004	O
the	O
SCST	B-Application
project	O
was	O
announced	O
,	O
providing	O
Fibre	B-Architecture
Channel	I-Architecture
and	O
iSCSI	B-Protocol
support	O
.	O
</s>
<s>
The	O
initial	O
iSCSI	B-Protocol
target	O
in	O
SCST	B-Application
was	O
based	O
on	O
the	O
UNH-IOL	O
iSCSI	B-Protocol
target	O
implementation	O
.	O
</s>
<s>
Around	O
that	O
time	O
an	O
alternative	O
iSCSI	B-Protocol
target	O
software	O
stack	O
was	O
developed	O
by	O
Ardis	O
Technologies	O
.	O
</s>
<s>
The	O
iSCSI	B-Protocol
Enterprise	O
Target	O
software	O
,	O
announced	O
in	O
2005	O
,	O
is	O
a	O
further	O
evolution	O
of	O
the	O
Ardis	O
stack	O
.	O
</s>
<s>
Later	O
in	O
2005	O
Chelsio	O
announced	O
an	O
SCST	B-Application
iSCSI	B-Protocol
target	O
driver	O
for	O
the	O
Chelsio	O
T2	O
Ethernet	O
adapter	O
family	O
.	O
</s>
<s>
This	O
tool	O
allows	O
to	O
save	O
and	O
restore	O
the	O
SCST	B-Application
configuration	O
from	O
or	O
to	O
a	O
file	O
.	O
</s>
<s>
In	O
2007	O
the	O
UNH-IOL	O
iSCSI	B-Protocol
target	O
patch	O
in	O
the	O
SCST	B-Application
project	O
was	O
replaced	O
by	O
a	O
fork	O
of	O
IET	O
.	O
</s>
<s>
In	O
2008	O
an	O
SRP	B-Protocol
target	O
driver	O
was	O
donated	O
by	O
Mellanox	O
.	O
</s>
<s>
Also	O
in	O
2009	O
a	O
target	O
driver	O
for	O
Emulex	O
Fibre	O
Channel/FCoE	O
adapter	O
cards	O
was	O
announced	O
by	O
Emulex	O
.	O
</s>
<s>
The	O
same	O
year	O
a	O
sysfs	B-Application
interface	O
has	O
been	O
added	O
to	O
SCST	B-Application
next	O
to	O
the	O
already	O
existing	O
procfs	O
interface	O
.	O
</s>
<s>
In	O
2010	O
an	O
FCoE	B-Protocol
target	O
driver	O
was	O
donated	O
by	O
Cisco	O
and	O
Bart	O
Van	O
Assche	O
ported	O
the	O
IBM	B-Device
System	I-Device
p	I-Device
SCSI	B-Device
target	I-Device
driver	O
to	O
SCST	B-Application
.	O
</s>
<s>
In	O
2011	O
implicit	O
ALUA	O
support	O
was	O
added	O
to	O
the	O
SCST	B-Application
core	O
and	O
to	O
the	O
scstadmin	O
configuration	O
tool	O
.	O
</s>
<s>
The	O
three	O
init	O
scripts	O
(	O
scst	B-Application
,	O
iscsi-scst	O
and	O
qla2x00t	O
)	O
were	O
merged	O
into	O
a	O
single	O
init	O
script	O
,	O
simplifying	O
installation	O
and	O
management	O
of	O
SCST	B-Application
.	O
</s>
<s>
In	O
2013	O
support	O
for	O
QLogic	O
InfiniBand	B-Architecture
HCA	O
's	O
was	O
added	O
in	O
the	O
SRP	B-Protocol
target	O
driver	O
.	O
</s>
<s>
In	O
January	O
2014	O
Mellanox	O
donated	O
an	O
iSER	O
target	O
driver	O
implementation	O
to	O
the	O
SCST	B-Application
project	O
.	O
</s>
<s>
In	O
May	O
2014	O
QLogic	O
made	O
a	O
16	O
Gbit/s	O
QLogic	O
target	O
driver	O
available	O
and	O
also	O
announced	O
to	O
take	O
over	O
maintainership	O
of	O
the	O
SCST	B-Application
QLogic	O
target	O
driver	O
.	O
</s>
<s>
In	O
2019	O
the	O
SCST	B-Application
core	O
was	O
modified	O
such	O
that	O
I/O	O
keeps	O
running	O
at	O
full	O
speed	O
if	O
a	O
LUN	O
or	O
storage	O
device	O
is	O
added	O
or	O
removed	O
.	O
</s>
