<s>
Network-attached	B-Application
storage	I-Application
(	O
NAS	B-Application
)	O
is	O
a	O
file-level	O
(	O
as	O
opposed	O
to	O
block-level	B-General_Concept
storage	I-General_Concept
)	O
computer	B-General_Concept
data	I-General_Concept
storage	I-General_Concept
server	B-Application
connected	O
to	O
a	O
computer	B-Architecture
network	I-Architecture
providing	O
data	O
access	O
to	O
a	O
heterogeneous	O
group	O
of	O
clients	O
.	O
</s>
<s>
The	O
term	O
"	O
NAS	B-Application
"	O
can	O
refer	O
to	O
both	O
the	O
technology	O
and	O
systems	O
involved	O
,	O
or	O
a	O
specialized	O
device	O
built	O
for	O
such	O
functionality	O
(	O
as	O
unlike	O
tangentially	O
related	O
technologies	O
such	O
as	O
local	B-General_Concept
area	I-General_Concept
networks	I-General_Concept
,	O
a	O
NAS	B-Application
device	O
is	O
often	O
a	O
singular	O
unit	O
)	O
.	O
</s>
<s>
A	O
NAS	B-Application
device	O
is	O
optimised	O
for	O
serving	B-Application
files	I-Application
either	O
by	O
its	O
hardware	O
,	O
software	O
,	O
or	O
configuration	O
.	O
</s>
<s>
It	O
is	O
often	O
manufactured	O
as	O
a	O
computer	B-Architecture
appliance	I-Architecture
a	O
purpose-built	O
specialized	O
computer	O
.	O
</s>
<s>
NAS	B-Application
systems	O
are	O
networked	O
appliances	O
that	O
contain	O
one	O
or	O
more	O
storage	B-Device
drives	I-Device
,	O
often	O
arranged	O
into	O
logical	B-General_Concept
,	O
redundant	O
storage	B-General_Concept
containers	O
or	O
RAID	B-Architecture
.	O
</s>
<s>
Network-attached	B-Application
storage	I-Application
typically	O
provide	O
access	O
to	O
files	O
using	O
network	B-Architecture
file	O
sharing	O
protocols	O
such	O
as	O
NFS	B-Protocol
,	O
SMB	B-Operating_System
,	O
or	O
AFP	B-Protocol
.	O
</s>
<s>
From	O
the	O
mid-1990s	O
,	O
NAS	B-Application
devices	O
began	O
gaining	O
popularity	O
as	O
a	O
convenient	O
method	O
of	O
sharing	O
files	O
among	O
multiple	O
computers	O
,	O
as	O
well	O
as	O
to	O
remove	O
the	O
responsibility	O
of	O
file	O
serving	O
from	O
other	O
servers	O
on	O
the	O
network	B-Architecture
;	O
by	O
doing	O
so	O
,	O
a	O
NAS	B-Application
can	O
provide	O
faster	O
data	O
access	O
,	O
easier	O
administration	O
,	O
and	O
simpler	O
configuration	O
as	O
opposed	O
to	O
using	O
general-purpose	O
server	B-Application
to	O
serve	O
files	O
.	O
</s>
<s>
Accompanying	O
a	O
NAS	B-Application
are	O
purpose-built	O
hard	B-Device
disk	I-Device
drives	I-Device
,	O
which	O
are	O
functionally	O
similar	O
to	O
non-NAS	O
drives	O
but	O
may	O
have	O
different	O
firmware	O
,	O
vibration	O
tolerance	O
,	O
or	O
power	O
dissipation	O
to	O
make	O
them	O
more	O
suitable	O
for	O
use	O
in	O
RAID	B-Architecture
arrays	I-Architecture
,	O
a	O
technology	O
often	O
used	O
in	O
NAS	B-Application
implementations	O
.	O
</s>
<s>
For	O
example	O
,	O
some	O
NAS	B-Application
versions	O
of	O
drives	O
support	O
a	O
command	O
extension	O
to	O
allow	O
extended	O
error	O
recovery	O
to	O
be	O
disabled	O
.	O
</s>
<s>
In	O
a	O
non-RAID	O
application	O
,	O
it	O
may	O
be	O
important	O
for	O
a	O
disk	O
drive	O
to	O
go	O
to	O
great	O
lengths	O
to	O
successfully	O
read	O
a	O
problematic	O
storage	B-General_Concept
block	O
,	O
even	O
if	O
it	O
takes	O
several	O
seconds	O
.	O
</s>
<s>
In	O
an	O
appropriately	O
configured	O
RAID	B-Architecture
array	I-Architecture
,	O
a	O
single	O
bad	O
block	O
on	O
a	O
single	O
drive	O
can	O
be	O
recovered	O
completely	O
via	O
the	O
redundancy	O
encoded	O
across	O
the	O
RAID	B-Architecture
set	O
.	O
</s>
<s>
If	O
a	O
drive	O
spends	O
several	O
seconds	O
executing	O
extensive	O
retries	O
it	O
might	O
cause	O
the	O
RAID	B-Architecture
controller	O
to	O
flag	O
the	O
drive	O
as	O
"	O
down	O
"	O
whereas	O
if	O
it	O
simply	O
replied	O
promptly	O
that	O
the	O
block	O
of	O
data	O
had	O
a	O
checksum	O
error	O
,	O
the	O
RAID	B-Architecture
controller	O
would	O
use	O
the	O
redundant	O
data	O
on	O
the	O
other	O
drives	O
to	O
correct	O
the	O
error	O
and	O
continue	O
without	O
any	O
problem	O
.	O
</s>
<s>
Such	O
a	O
"	O
NAS	B-Application
"	O
SATA	O
hard	B-Device
disk	I-Device
drive	I-Device
can	O
be	O
used	O
as	O
an	O
internal	O
PC	O
hard	B-Device
drive	I-Device
,	O
without	O
any	O
problems	O
or	O
adjustments	O
needed	O
,	O
as	O
it	O
simply	O
supports	O
additional	O
options	O
and	O
may	O
possibly	O
be	O
built	O
to	O
a	O
higher	O
quality	O
standard	O
(	O
particularly	O
if	O
accompanied	O
by	O
a	O
higher	O
quoted	O
MTBF	O
figure	O
and	O
higher	O
price	O
)	O
than	O
a	O
regular	O
consumer	O
drive	O
.	O
</s>
<s>
A	O
NAS	B-Application
unit	O
is	O
a	O
computer	O
connected	O
to	O
a	O
network	B-Architecture
that	O
provides	O
only	O
file-based	O
data	O
storage	B-General_Concept
services	O
to	O
other	O
devices	O
on	O
the	O
network	B-Architecture
.	O
</s>
<s>
Although	O
it	O
may	O
technically	O
be	O
possible	O
to	O
run	O
other	O
software	O
on	O
a	O
NAS	B-Application
unit	O
,	O
it	O
is	O
usually	O
not	O
designed	O
to	O
be	O
a	O
general-purpose	O
server	B-Application
.	O
</s>
<s>
For	O
example	O
,	O
NAS	B-Application
units	O
usually	O
do	O
not	O
have	O
a	O
keyboard	O
or	O
display	O
,	O
and	O
are	O
controlled	O
and	O
configured	O
over	O
the	O
network	B-Architecture
,	O
often	O
using	O
a	O
browser	O
.	O
</s>
<s>
A	O
full-featured	O
operating	B-General_Concept
system	I-General_Concept
is	O
not	O
needed	O
on	O
a	O
NAS	B-Application
device	O
,	O
so	O
often	O
a	O
stripped-down	O
operating	B-General_Concept
system	I-General_Concept
is	O
used	O
.	O
</s>
<s>
NAS	B-Application
systems	O
contain	O
one	O
or	O
more	O
hard	B-Device
disk	I-Device
drives	I-Device
,	O
often	O
arranged	O
into	O
logical	B-General_Concept
,	O
redundant	O
storage	B-General_Concept
containers	O
or	O
RAID	B-Architecture
.	O
</s>
<s>
NAS	B-Application
uses	O
file-based	O
protocols	O
such	O
as	O
NFS	B-Protocol
(	O
popular	O
on	O
UNIX	B-Application
systems	I-Application
)	O
,	O
SMB	B-Operating_System
(	O
Server	B-Operating_System
Message	I-Operating_System
Block	I-Operating_System
)	O
(	O
used	O
with	O
Microsoft	B-Application
Windows	I-Application
systems	O
)	O
,	O
AFP	B-Protocol
(	O
used	O
with	O
Apple	B-Device
Macintosh	I-Device
computers	O
)	O
,	O
or	O
NCP	B-Protocol
(	O
used	O
with	O
OES	B-Application
and	O
Novell	B-Operating_System
NetWare	I-Operating_System
)	O
.	O
</s>
<s>
NAS	B-Application
units	O
rarely	O
limit	O
clients	O
to	O
a	O
single	O
protocol	O
.	O
</s>
<s>
The	O
key	O
difference	O
between	O
direct-attached	B-General_Concept
storage	I-General_Concept
(	O
DAS	O
)	O
and	O
NAS	B-Application
is	O
that	O
DAS	O
is	O
simply	O
an	O
extension	O
to	O
an	O
existing	O
server	B-Application
and	O
is	O
not	O
necessarily	O
networked	O
.	O
</s>
<s>
As	O
the	O
name	O
suggests	O
,	O
DAS	O
typically	O
is	O
connected	O
via	O
a	O
USB	B-Protocol
or	O
Thunderbolt	B-Protocol
enabled	O
cable	O
.	O
</s>
<s>
NAS	B-Application
is	O
designed	O
as	O
an	O
easy	O
and	O
self-contained	O
solution	O
for	O
sharing	O
files	O
over	O
the	O
network	B-Architecture
.	O
</s>
<s>
Both	O
DAS	O
and	O
NAS	B-Application
can	O
potentially	O
increase	O
availability	O
of	O
data	O
by	O
using	O
RAID	B-Architecture
or	O
clustering	B-Architecture
.	O
</s>
<s>
When	O
both	O
are	O
served	O
over	O
the	O
network	B-Architecture
,	O
NAS	B-Application
could	O
have	O
better	O
performance	O
than	O
DAS	O
,	O
because	O
the	O
NAS	B-Application
device	O
can	O
be	O
tuned	O
precisely	O
for	O
file	O
serving	O
which	O
is	O
less	O
likely	O
to	O
happen	O
on	O
a	O
server	B-Application
responsible	O
for	O
other	O
processing	O
.	O
</s>
<s>
Both	O
NAS	B-Application
and	O
DAS	O
can	O
have	O
various	O
amount	O
of	O
cache	B-General_Concept
memory	I-General_Concept
,	O
which	O
greatly	O
affects	O
performance	O
.	O
</s>
<s>
When	O
comparing	O
use	O
of	O
NAS	B-Application
with	O
use	O
of	O
local	O
(	O
non-networked	O
)	O
DAS	O
,	O
the	O
performance	O
of	O
NAS	B-Application
depends	O
mainly	O
on	O
the	O
speed	O
of	O
and	O
congestion	O
on	O
the	O
network	B-Architecture
.	O
</s>
<s>
With	O
the	O
introduction	O
of	O
new	O
WiFi	O
standards	O
(	O
like	O
WiFi6	O
)	O
,	O
networking	O
speeds	O
and	O
dramatically	O
increase	O
to	O
allow	O
better	O
performance	O
when	O
using	O
a	O
NAS	B-Application
.	O
</s>
<s>
NAS	B-Application
is	O
generally	O
not	O
as	O
customizable	O
in	O
terms	O
of	O
hardware	O
(	O
CPU	O
,	O
memory	O
,	O
storage	B-General_Concept
components	O
)	O
or	O
low	O
level	O
software	O
(	O
extensions	O
,	O
plug-ins	B-Application
,	O
additional	O
protocols	O
)	O
but	O
most	O
NAS	B-Application
solutions	O
will	O
include	O
the	O
option	O
to	O
install	O
a	O
wide	O
array	O
of	O
software	O
applications	O
to	O
allow	O
better	O
configuration	O
of	O
the	O
system	O
or	O
to	O
include	O
other	O
capabilities	O
outside	O
of	O
storage	B-General_Concept
(	O
like	O
video	O
surveillance	O
,	O
virtualization	O
,	O
media	O
,	O
etc	O
)	O
.	O
</s>
<s>
DAS	O
typically	O
is	O
focused	O
solely	O
on	O
data	O
storage	B-General_Concept
but	O
capabilities	O
can	O
be	O
available	O
based	O
on	O
specific	O
vendor	O
options	O
.	O
</s>
<s>
NAS	B-Application
provides	O
both	O
storage	B-General_Concept
and	O
a	O
file	B-Application
system	I-Application
.	O
</s>
<s>
This	O
is	O
often	O
contrasted	O
with	O
SAN	O
(	O
storage	B-General_Concept
area	O
network	B-Architecture
)	O
,	O
which	O
provides	O
only	O
block-based	O
storage	B-General_Concept
and	O
leaves	O
file	B-Application
system	I-Application
concerns	O
on	O
the	O
"	O
client	B-Protocol
"	O
side	O
.	O
</s>
<s>
SAN	O
protocols	O
include	O
Fibre	B-Architecture
Channel	I-Architecture
,	O
iSCSI	B-Protocol
,	O
ATA	B-Protocol
over	I-Protocol
Ethernet	I-Protocol
(	O
AoE	O
)	O
and	O
HyperSCSI	B-Protocol
.	O
</s>
<s>
One	O
way	O
to	O
loosely	O
conceptualize	O
the	O
difference	O
between	O
a	O
NAS	B-Application
and	O
a	O
SAN	O
is	O
that	O
NAS	B-Application
appears	O
to	O
the	O
client	B-Protocol
OS	O
(	O
operating	B-General_Concept
system	I-General_Concept
)	O
as	O
a	O
file	B-Application
server	I-Application
(	O
the	O
client	B-Protocol
can	O
map	B-General_Concept
network	B-Application
drives	I-Application
to	O
shares	O
on	O
that	O
server	B-Application
)	O
whereas	O
a	O
disk	O
available	O
through	O
a	O
SAN	O
still	O
appears	O
to	O
the	O
client	B-Protocol
OS	O
as	O
a	O
disk	O
,	O
visible	O
in	O
disk	O
and	O
volume	O
management	O
utilities	O
(	O
along	O
with	O
client	B-Protocol
's	O
local	O
disks	O
)	O
,	O
and	O
available	O
to	O
be	O
formatted	O
with	O
a	O
file	B-Application
system	I-Application
and	O
mounted	B-Application
.	O
</s>
<s>
Despite	O
their	O
differences	O
,	O
SAN	O
and	O
NAS	B-Application
are	O
not	O
mutually	O
exclusive	O
and	O
may	O
be	O
combined	O
as	O
a	O
SAN-NAS	O
hybrid	O
,	O
offering	O
both	O
file-level	O
protocols	O
(	O
NAS	B-Application
)	O
and	O
block-level	O
protocols	O
(	O
SAN	O
)	O
from	O
the	O
same	O
system	O
.	O
</s>
<s>
A	O
shared	O
disk	B-Application
file	I-Application
system	I-Application
can	O
also	O
be	O
run	O
on	O
top	O
of	O
a	O
SAN	O
to	O
provide	O
filesystem	B-Application
services	O
.	O
</s>
<s>
In	O
the	O
early	O
1980s	O
,	O
the	O
"	O
Newcastle	B-Protocol
Connection	I-Protocol
"	O
by	O
Brian	O
Randell	O
and	O
his	O
colleagues	O
at	O
Newcastle	O
University	O
demonstrated	O
and	O
developed	O
remote	O
file	B-Application
access	I-Application
across	O
a	O
set	O
of	O
UNIX	B-Application
machines	O
.	O
</s>
<s>
Novell	O
's	O
NetWare	B-Operating_System
server	B-Application
operating	B-General_Concept
system	I-General_Concept
and	O
NCP	B-Protocol
protocol	O
was	O
released	O
in	O
1983	O
.	O
</s>
<s>
Following	O
the	O
Newcastle	B-Protocol
Connection	I-Protocol
,	O
Sun	O
Microsystems	O
 '	O
1984	O
release	O
of	O
NFS	B-Protocol
allowed	O
network	B-Application
servers	I-Application
to	O
share	O
their	O
storage	B-General_Concept
space	O
with	O
networked	O
clients	O
.	O
</s>
<s>
3Com	O
and	O
Microsoft	O
would	O
develop	O
the	O
LAN	B-Algorithm
Manager	I-Algorithm
software	O
and	O
protocol	O
to	O
further	O
this	O
new	O
market	O
.	O
</s>
<s>
3Com	O
's	O
3Server	B-Application
and	O
3+Share	B-General_Concept
software	O
was	O
the	O
first	O
purpose-built	O
server	B-Application
(	O
including	O
proprietary	B-Device
hardware	I-Device
,	O
software	O
,	O
and	O
multiple	O
disks	O
)	O
for	O
open	O
systems	O
servers	O
.	O
</s>
<s>
Inspired	O
by	O
the	O
success	O
of	O
file	B-Application
servers	I-Application
from	O
Novell	O
,	O
IBM	O
,	O
and	O
Sun	O
,	O
several	O
firms	O
developed	O
dedicated	O
file	B-Application
servers	I-Application
.	O
</s>
<s>
While	O
3Com	O
was	O
among	O
the	O
first	O
firms	O
to	O
build	O
a	O
dedicated	O
NAS	B-Application
for	O
desktop	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
,	O
Auspex	O
Systems	O
was	O
one	O
of	O
the	O
first	O
to	O
develop	O
a	O
dedicated	O
NFS	B-Protocol
server	B-Application
for	O
use	O
in	O
the	O
UNIX	B-Application
market	O
.	O
</s>
<s>
A	O
group	O
of	O
Auspex	O
engineers	O
split	O
away	O
in	O
the	O
early	O
1990s	O
to	O
create	O
the	O
integrated	O
NetApp	B-Operating_System
FAS	I-Operating_System
,	O
which	O
supported	O
both	O
the	O
Windows	O
SMB	B-Operating_System
and	O
the	O
UNIX	B-Application
NFS	B-Protocol
protocols	I-Protocol
and	O
had	O
superior	O
scalability	B-Architecture
and	O
ease	O
of	O
deployment	O
.	O
</s>
<s>
This	O
started	O
the	O
market	O
for	O
proprietary	B-Device
NAS	B-Application
devices	O
now	O
led	O
by	O
NetApp	O
and	O
EMC	O
Celerra	O
.	O
</s>
<s>
Starting	O
in	O
the	O
early	O
2000s	O
,	O
a	O
series	O
of	O
startups	O
emerged	O
offering	O
alternative	O
solutions	O
to	O
single	O
filer	O
solutions	O
in	O
the	O
form	O
of	O
clustered	O
NAS	B-Application
Spinnaker	O
Networks	O
(	O
acquired	O
by	O
NetApp	O
in	O
February	O
2004	O
)	O
,	O
Exanet	O
(	O
acquired	O
by	O
Dell	O
in	O
February	O
2010	O
)	O
,	O
Gluster	O
(	O
acquired	O
by	O
RedHat	O
in	O
2011	O
)	O
,	O
ONStor	O
(	O
acquired	O
by	O
LSI	O
in	O
2009	O
)	O
,	O
IBRIX	B-General_Concept
(	O
acquired	O
by	O
HP	O
)	O
,	O
Isilon	O
(	O
acquired	O
by	O
EMC	O
November	O
2010	O
)	O
,	O
PolyServe	O
(	O
acquired	O
by	O
HP	O
in	O
2007	O
)	O
,	O
and	O
Panasas	O
,	O
to	O
name	O
a	O
few	O
.	O
</s>
<s>
In	O
2009	O
,	O
NAS	B-Application
vendors	O
(	O
notably	O
CTERA	O
networks	O
and	O
Netgear	O
)	O
began	O
to	O
introduce	O
online	B-General_Concept
backup	I-General_Concept
solutions	O
integrated	O
in	O
their	O
NAS	B-Application
appliances	O
,	O
for	O
online	O
disaster	O
recovery	O
.	O
</s>
<s>
By	O
2021	O
,	O
three	O
major	O
types	O
of	O
NAS	B-Application
solutions	O
are	O
offered	O
(	O
all	O
with	O
hybrid	O
cloud	O
models	O
where	O
data	O
can	O
be	O
stored	O
both	O
on-premise	O
on	O
the	O
NAS	B-Application
and	O
off	O
site	O
on	O
a	O
separate	O
NAS	B-Application
or	O
through	O
a	O
public	O
cloud	O
service	O
provider	O
)	O
.	O
</s>
<s>
The	O
first	O
type	O
of	O
NAS	B-Application
is	O
focused	O
on	O
consumer	O
needs	O
with	O
lower-cost	O
options	O
that	O
typically	O
support	O
1	O
–	O
5	O
hot	O
plug	O
hard	B-Device
drives	I-Device
.	O
</s>
<s>
The	O
second	O
is	O
focused	O
on	O
small-to-medium-sized	O
businesses	O
–	O
these	O
NAS	B-Application
solutions	O
range	O
from	O
2	O
–	O
24+	O
hard	B-Device
drives	I-Device
and	O
are	O
typically	O
offered	O
in	O
tower	O
or	O
rackmount	B-Application
form	O
factors	O
.	O
</s>
<s>
NAS	B-Application
solutions	O
are	O
typically	O
sold	O
without	O
hard	B-Device
drives	I-Device
installed	O
to	O
allow	O
the	O
buyer	O
(	O
or	O
IT	O
departments	O
)	O
to	O
select	O
the	O
hard	B-Device
drive	I-Device
cost	O
,	O
size	O
,	O
and	O
quality	O
.	O
</s>
<s>
The	O
way	O
manufacturers	O
make	O
NAS	B-Application
devices	O
can	O
be	O
classified	O
into	O
three	O
types	O
:	O
</s>
<s>
Computer-based	O
NAS	B-Application
using	O
a	O
computer	O
(	O
server	B-Application
level	O
or	O
a	O
personal	O
computer	O
)	O
with	O
processors	O
typically	O
from	O
Intel	O
or	O
AMD	O
,	O
installs	O
AFP	B-Protocol
...	O
software	O
server	B-Application
.	O
</s>
<s>
The	O
power	O
consumption	O
of	O
this	O
NAS	B-Application
type	O
is	O
the	O
largest	O
,	O
but	O
its	O
functions	O
are	O
the	O
most	O
powerful	O
.	O
</s>
<s>
Some	O
large	O
NAS	B-Application
manufacturers	O
such	O
as	O
Synology	B-Operating_System
,	O
QNAP	B-Operating_System
systems	I-Operating_System
,	O
and	O
Asus	O
make	O
these	O
types	O
of	O
devices	O
.	O
</s>
<s>
Max	O
FTP	B-Protocol
throughput	O
speed	O
varies	O
by	O
computer	O
CPU	O
and	O
amount	O
of	O
RAM	O
.	O
</s>
<s>
Embedded-system-based	O
NAS	B-Application
using	O
an	O
ARM	B-Architecture
-	O
or	O
MIPS-based	O
processor	O
architecture	O
and	O
a	O
real-time	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
(	O
RTOS	B-Operating_System
)	O
or	O
an	O
embedded	B-Operating_System
operating	I-Operating_System
system	I-Operating_System
to	O
run	O
a	O
NAS	B-Application
server	B-Application
.	O
</s>
<s>
The	O
power	O
consumption	O
of	O
this	O
NAS	B-Application
type	O
is	O
fair	O
,	O
and	O
functions	O
in	O
the	O
NAS	B-Application
can	O
fit	O
most	O
end-user	O
requirements	O
.	O
</s>
<s>
Marvell	O
,	O
Oxford	O
,	O
and	O
Storlink	O
make	O
chipsets	O
for	O
this	O
type	O
of	O
NAS	B-Application
.	O
</s>
<s>
Max	O
FTP	B-Protocol
throughput	O
varies	O
from	O
20MB/s	O
to	O
120MB/s	O
.	O
</s>
<s>
ASIC-based	O
NAS	B-Application
provisioning	O
NAS	B-Application
through	O
the	O
use	O
of	O
a	O
single	O
ASIC	O
chip	O
,	O
using	O
hardware	O
to	O
implement	O
TCP/IP	O
and	O
file	B-Application
system	I-Application
.	O
</s>
<s>
The	O
power	O
consumption	O
of	O
this	O
type	O
of	O
NAS	B-Application
is	O
low	O
,	O
as	O
functions	O
are	O
limited	O
to	O
only	O
support	O
SMB	B-Operating_System
and	O
FTP	B-Protocol
.	O
</s>
<s>
LayerWalker	O
is	O
the	O
only	O
chipset	O
manufacturer	O
for	O
this	O
type	O
of	O
NAS	B-Application
.	O
</s>
<s>
Max	O
FTP	B-Protocol
throughput	O
is	O
40MB/s	O
.	O
</s>
<s>
NAS	B-Application
is	O
useful	O
for	O
more	O
than	O
just	O
general	O
centralized	O
storage	B-General_Concept
provided	O
to	O
client	B-Protocol
computers	I-Protocol
in	O
environments	O
with	O
large	O
amounts	O
of	O
data	O
.	O
</s>
<s>
NAS	B-Application
can	O
enable	O
simpler	O
and	O
lower	O
cost	O
systems	O
such	O
as	O
load-balancing	O
and	O
fault-tolerant	O
email	O
and	O
web	O
server	B-Application
systems	O
by	O
providing	O
storage	B-General_Concept
services	O
.	O
</s>
<s>
The	O
potential	O
emerging	O
market	O
for	O
NAS	B-Application
is	O
the	O
consumer	O
market	O
where	O
there	O
is	O
a	O
large	O
amount	O
of	O
multi-media	O
data	O
.	O
</s>
<s>
The	O
price	O
of	O
NAS	B-Application
appliances	O
has	O
fallen	O
sharply	O
in	O
recent	O
years	O
,	O
offering	O
flexible	O
network-based	O
storage	B-General_Concept
to	O
the	O
home	O
consumer	O
market	O
for	O
little	O
more	O
than	O
the	O
cost	O
of	O
a	O
regular	O
USB	B-Protocol
or	O
FireWire	B-Protocol
external	O
hard	B-Device
disk	I-Device
.	O
</s>
<s>
Many	O
of	O
these	O
home	O
consumer	O
devices	O
are	O
built	O
around	O
ARM	B-Architecture
,	O
x86	O
or	O
MIPS	B-Device
processors	O
running	O
an	O
embedded	B-Application
Linux	I-Application
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Open-source	B-Application
NAS-oriented	O
distributions	O
of	O
Linux	B-Application
and	O
FreeBSD	B-Operating_System
are	O
available	O
.	O
</s>
<s>
They	O
can	O
run	O
from	O
a	O
virtual	B-Architecture
machine	I-Architecture
,	O
Live	O
CD	O
,	O
bootable	B-Operating_System
USB	B-Protocol
flash	O
drive	O
(	O
Live	O
USB	B-Protocol
)	O
,	O
or	O
from	O
one	O
of	O
the	O
mounted	B-Application
hard	B-Device
drives	I-Device
.	O
</s>
<s>
They	O
run	O
Samba	B-Language
(	O
an	O
SMB	B-Operating_System
daemon	O
)	O
,	O
NFS	B-Protocol
daemon	O
,	O
and	O
FTP	B-Protocol
daemons	O
which	O
are	O
freely	O
available	O
for	O
those	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Network-attached	O
secure	O
disks	O
(	O
NASD	O
)	O
is	O
19972001	O
research	O
project	O
of	O
Carnegie	O
Mellon	O
University	O
,	O
with	O
the	O
goal	O
of	O
providing	O
cost-effective	O
scalable	B-Architecture
storage	B-General_Concept
bandwidth	O
.	O
</s>
<s>
NASD	O
reduces	O
the	O
overhead	O
on	O
the	O
file	B-Application
server	I-Application
(	O
file	O
manager	O
)	O
by	O
allowing	O
storage	B-General_Concept
devices	O
to	O
transfer	O
data	O
directly	O
to	O
clients	O
.	O
</s>
<s>
Most	O
of	O
the	O
file	O
manager	O
's	O
work	O
is	O
offloaded	O
to	O
the	O
storage	B-General_Concept
disk	O
without	O
integrating	O
the	O
file	B-Application
system	I-Application
policy	O
into	O
the	O
disk	O
.	O
</s>
<s>
Most	O
client	B-Protocol
operations	O
like	O
Read/Write	O
go	O
directly	O
to	O
the	O
disks	O
;	O
less	O
frequent	O
operations	O
like	O
authentication	O
go	O
to	O
the	O
file	O
manager	O
.	O
</s>
<s>
The	O
File	O
Manager	O
provides	O
a	O
time-limited	O
cachable	O
capability	O
for	O
clients	O
to	O
access	O
the	O
storage	B-General_Concept
objects	O
.	O
</s>
<s>
A	O
file	B-Application
access	I-Application
from	O
the	O
client	B-Protocol
to	O
the	O
disks	O
has	O
the	O
following	O
sequence	O
:	O
</s>
<s>
The	O
client	B-Protocol
authenticates	O
itself	O
with	O
the	O
file	O
manager	O
and	O
requests	O
for	O
the	O
file	B-Application
access	I-Application
.	O
</s>
<s>
If	O
the	O
client	B-Protocol
can	O
be	O
granted	O
access	O
to	O
the	O
file	O
requested	O
,	O
the	O
client	B-Protocol
receives	O
the	O
network	B-Architecture
location	O
of	O
NASD	O
disks	O
and	O
their	O
capability	O
.	O
</s>
<s>
If	O
the	O
client	B-Protocol
is	O
accessing	O
the	O
disk	O
for	O
the	O
first	O
time	O
,	O
it	O
receives	O
a	O
time-limited	O
key	O
for	O
the	O
establishment	O
of	O
secure	O
communication	O
to	O
the	O
disk	O
.	O
</s>
<s>
From	O
now	O
on	O
,	O
the	O
client	B-Protocol
directly	O
accesses	O
the	O
NASD	O
disks	O
by	O
giving	O
the	O
capability	O
it	O
received	O
and	O
further	O
data	O
transfers	O
go	O
through	O
the	O
network	B-Architecture
,	O
bypassing	O
the	O
file	O
manager	O
.	O
</s>
<s>
A	O
clustered	O
NAS	B-Application
is	O
a	O
NAS	B-Application
that	O
is	O
using	O
a	O
distributed	O
file	B-Application
system	I-Application
running	O
simultaneously	O
on	O
multiple	O
servers	O
.	O
</s>
<s>
The	O
key	O
difference	O
between	O
a	O
clustered	O
and	O
traditional	O
NAS	B-Application
is	O
the	O
ability	O
to	O
distribute	O
(	O
e.g.	O
</s>
<s>
stripe	O
)	O
data	O
and	O
metadata	O
across	O
the	O
cluster	O
nodes	O
or	O
storage	B-General_Concept
devices	O
.	O
</s>
<s>
Clustered	O
NAS	B-Application
,	O
like	O
a	O
traditional	O
one	O
,	O
still	O
provides	O
unified	O
access	O
to	O
the	O
files	O
from	O
any	O
of	O
the	O
cluster	O
nodes	O
,	O
unrelated	O
to	O
the	O
actual	O
location	O
of	O
the	O
data	O
.	O
</s>
