<s>
Computer	B-General_Concept
data	I-General_Concept
storage	I-General_Concept
is	O
a	O
technology	O
consisting	O
of	O
computer	B-Architecture
components	I-Architecture
and	O
recording	B-General_Concept
media	I-General_Concept
that	O
are	O
used	O
to	O
retain	O
digital	O
data	B-General_Concept
.	O
</s>
<s>
The	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	B-Device
)	O
of	O
a	O
computer	O
is	O
what	O
manipulates	O
data	B-General_Concept
by	O
performing	O
computations	O
.	O
</s>
<s>
In	O
practice	O
,	O
almost	O
all	O
computers	O
use	O
a	O
storage	B-General_Concept
hierarchy	I-General_Concept
,	O
which	O
puts	O
fast	O
but	O
expensive	O
and	O
small	O
storage	O
options	O
close	O
to	O
the	O
CPU	B-Device
and	O
slower	O
but	O
less	O
expensive	O
and	O
larger	O
options	O
further	O
away	O
.	O
</s>
<s>
Even	O
the	O
first	B-Architecture
computer	I-Architecture
designs	O
,	O
Charles	O
Babbage	O
's	O
Analytical	B-Device
Engine	I-Device
and	O
Percy	O
Ludgate	O
's	O
Analytical	O
Machine	O
,	O
clearly	O
distinguished	O
between	O
processing	O
and	O
memory	O
(	O
Babbage	O
stored	O
numbers	O
as	O
rotations	O
of	O
gears	O
,	O
while	O
Ludgate	O
stored	O
numbers	O
as	O
displacements	O
of	O
rods	O
in	O
shuttles	O
)	O
.	O
</s>
<s>
This	O
distinction	O
was	O
extended	O
in	O
the	O
Von	B-Architecture
Neumann	I-Architecture
architecture	I-Architecture
,	O
where	O
the	O
CPU	B-Device
consists	O
of	O
two	O
main	O
parts	O
:	O
The	O
control	B-General_Concept
unit	I-General_Concept
and	O
the	O
arithmetic	B-General_Concept
logic	I-General_Concept
unit	I-General_Concept
(	O
ALU	O
)	O
.	O
</s>
<s>
The	O
former	O
controls	O
the	O
flow	O
of	O
data	B-General_Concept
between	O
the	O
CPU	B-Device
and	O
memory	O
,	O
while	O
the	O
latter	O
performs	O
arithmetic	O
and	O
logical	O
operations	O
on	O
data	B-General_Concept
.	O
</s>
<s>
This	O
is	O
acceptable	O
for	O
devices	O
such	O
as	O
desk	O
calculators	B-Application
,	O
digital	B-General_Concept
signal	I-General_Concept
processors	I-General_Concept
,	O
and	O
other	O
specialized	O
devices	O
.	O
</s>
<s>
Von	B-Architecture
Neumann	I-Architecture
machines	O
differ	O
in	O
having	O
a	O
memory	O
in	O
which	O
they	O
store	O
their	O
operating	O
instructions	O
and	O
data	B-General_Concept
.	O
</s>
<s>
Such	O
computers	O
are	O
more	O
versatile	O
in	O
that	O
they	O
do	O
not	O
need	O
to	O
have	O
their	O
hardware	B-Architecture
reconfigured	O
for	O
each	O
new	O
program	O
,	O
but	O
can	O
simply	O
be	O
reprogrammed	B-General_Concept
with	O
new	O
in-memory	O
instructions	O
;	O
they	O
also	O
tend	O
to	O
be	O
simpler	O
to	O
design	O
,	O
in	O
that	O
a	O
relatively	O
simple	O
processor	O
may	O
keep	O
state	B-Application
between	O
successive	O
computations	O
to	O
build	O
up	O
complex	O
procedural	O
results	O
.	O
</s>
<s>
Most	O
modern	O
computers	O
are	O
von	B-Architecture
Neumann	I-Architecture
machines	O
.	O
</s>
<s>
A	O
modern	O
digital	O
computer	O
represents	O
data	B-General_Concept
using	O
the	O
binary	O
numeral	O
system	O
.	O
</s>
<s>
The	O
most	O
common	O
unit	O
of	O
storage	O
is	O
the	O
byte	B-Application
,	O
equal	O
to	O
8	O
bits	O
.	O
</s>
<s>
A	O
piece	O
of	O
information	O
can	O
be	O
handled	O
by	O
any	O
computer	O
or	O
device	O
whose	O
storage	O
space	O
is	O
large	O
enough	O
to	O
accommodate	O
the	O
binary	O
representation	O
of	O
the	O
piece	O
of	O
information	O
,	O
or	O
simply	O
data	B-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
the	O
complete	O
works	O
of	O
Shakespeare	O
,	O
about	O
1250pages	O
in	O
print	O
,	O
can	O
be	O
stored	O
in	O
about	O
five	O
megabytes	O
(	O
40million	O
bits	O
)	O
with	O
one	O
byte	B-Application
per	O
character	O
.	O
</s>
<s>
Data	B-General_Concept
are	O
encoded	O
by	O
assigning	O
a	O
bit	O
pattern	O
to	O
each	O
character	O
,	O
digit	O
,	O
or	O
multimedia	O
object	O
.	O
</s>
<s>
character	B-General_Concept
encodings	I-General_Concept
like	O
ASCII	B-Protocol
,	O
image	O
encodings	O
like	O
JPEG	O
,	O
and	O
video	O
encodings	O
like	O
MPEG-4	B-Algorithm
)	O
.	O
</s>
<s>
By	O
adding	O
bits	O
to	O
each	O
encoded	O
unit	O
,	O
redundancy	O
allows	O
the	O
computer	O
to	O
both	O
detect	O
errors	O
in	O
coded	O
data	B-General_Concept
and	O
correct	O
them	O
based	O
on	O
mathematical	O
algorithms	O
.	O
</s>
<s>
A	O
random	O
bit	B-General_Concept
flip	I-General_Concept
(	O
e.g.	O
</s>
<s>
The	O
cyclic	O
redundancy	O
check	O
(	O
CRC	O
)	O
method	O
is	O
typically	O
used	O
in	O
communications	O
and	O
storage	O
for	O
error	B-Error_Name
detection	I-Error_Name
.	O
</s>
<s>
Data	B-General_Concept
compression	I-General_Concept
methods	O
allow	O
in	O
many	O
cases	O
(	O
such	O
as	O
a	O
database	O
)	O
to	O
represent	O
a	O
string	O
of	O
bits	O
by	O
a	O
shorter	O
bit	O
string	O
(	O
"	O
compress	O
"	O
)	O
and	O
reconstruct	O
the	O
original	O
string	O
(	O
"	O
decompress	O
"	O
)	O
when	O
needed	O
.	O
</s>
<s>
This	O
utilizes	O
substantially	O
less	O
storage	O
(	O
tens	O
of	O
percents	O
)	O
for	O
many	O
types	O
of	O
data	B-General_Concept
at	O
the	O
cost	O
of	O
more	O
computation	O
(	O
compress	O
and	O
decompress	O
when	O
needed	O
)	O
.	O
</s>
<s>
Analysis	O
of	O
the	O
trade-off	O
between	O
storage	O
cost	O
saving	O
and	O
costs	O
of	O
related	O
computations	O
and	O
possible	O
delays	O
in	O
data	B-General_Concept
availability	O
is	O
done	O
before	O
deciding	O
whether	O
to	O
keep	O
certain	O
data	B-General_Concept
compressed	O
or	O
not	O
.	O
</s>
<s>
For	O
security	B-General_Concept
reasons	I-General_Concept
,	O
certain	O
types	O
of	O
data	B-General_Concept
(	O
e.g.	O
</s>
<s>
Generally	O
,	O
the	O
lower	O
a	O
storage	O
is	O
in	O
the	O
hierarchy	O
,	O
the	O
lesser	O
its	O
bandwidth	O
and	O
the	O
greater	O
its	O
access	O
latency	O
is	O
from	O
the	O
CPU	B-Device
.	O
</s>
<s>
In	O
contemporary	O
usage	O
,	O
memory	O
is	O
usually	O
semiconductor	O
storage	O
read-write	O
random-access	B-Architecture
memory	I-Architecture
,	O
typically	O
DRAM	O
(	O
dynamic	O
RAM	B-Architecture
)	O
or	O
other	O
forms	O
of	O
fast	O
but	O
temporary	O
storage	O
.	O
</s>
<s>
Storage	O
consists	O
of	O
storage	O
devices	O
and	O
their	O
media	O
not	O
directly	O
accessible	O
by	O
the	O
CPU	B-Device
(	O
secondary	O
or	O
tertiary	O
storage	O
)	O
,	O
typically	O
hard	B-Device
disk	I-Device
drives	I-Device
,	O
optical	B-Device
disc	I-Device
drives	O
,	O
and	O
other	O
devices	O
slower	O
than	O
RAM	B-Architecture
but	O
non-volatile	B-General_Concept
(	O
retaining	O
contents	O
when	O
powered	O
down	O
)	O
.	O
</s>
<s>
Historically	O
,	O
memory	O
has	O
,	O
depending	O
on	O
technology	O
,	O
been	O
called	O
central	O
memory	O
,	O
core	O
memory	O
,	O
core	O
storage	O
,	O
drum	O
,	O
main	O
memory	O
,	O
real	O
storage	O
,	O
or	O
internal	B-Device
memory	O
.	O
</s>
<s>
Primary	O
storage	O
(	O
also	O
known	O
as	O
main	O
memory	O
,	O
internal	B-Device
memory	O
,	O
or	O
prime	O
memory	O
)	O
,	O
often	O
referred	O
to	O
simply	O
as	O
memory	O
,	O
is	O
the	O
only	O
one	O
directly	O
accessible	O
to	O
the	O
CPU	B-Device
.	O
</s>
<s>
The	O
CPU	B-Device
continuously	O
reads	O
instructions	O
stored	O
there	O
and	O
executes	O
them	O
as	O
required	O
.	O
</s>
<s>
Any	O
data	B-General_Concept
actively	O
operated	O
on	O
is	O
also	O
stored	O
there	O
in	O
uniform	O
manner	O
.	O
</s>
<s>
Historically	O
,	O
early	B-Architecture
computers	I-Architecture
used	O
delay	O
lines	O
,	O
Williams	B-General_Concept
tubes	I-General_Concept
,	O
or	O
rotating	O
magnetic	B-General_Concept
drums	I-General_Concept
as	O
primary	O
storage	O
.	O
</s>
<s>
Core	O
memory	O
remained	O
dominant	O
until	O
the	O
1970s	O
,	O
when	O
advances	O
in	O
integrated	O
circuit	O
technology	O
allowed	O
semiconductor	B-Architecture
memory	I-Architecture
to	O
become	O
economically	O
competitive	O
.	O
</s>
<s>
This	O
led	O
to	O
modern	O
random-access	B-Architecture
memory	I-Architecture
(	O
RAM	B-Architecture
)	O
.	O
</s>
<s>
The	O
particular	O
types	O
of	O
RAM	B-Architecture
used	O
for	O
primary	O
storage	O
are	O
volatile	B-General_Concept
,	O
meaning	O
that	O
they	O
lose	O
the	O
information	O
when	O
not	O
powered	O
.	O
</s>
<s>
Besides	O
storing	O
opened	O
programs	O
,	O
it	O
serves	O
as	O
disk	B-General_Concept
cache	I-General_Concept
and	O
write	B-General_Concept
buffer	I-General_Concept
to	O
improve	O
both	O
reading	O
and	O
writing	O
performance	O
.	O
</s>
<s>
Operating	B-General_Concept
systems	I-General_Concept
borrow	O
RAM	B-Architecture
capacity	O
for	O
caching	O
so	O
long	O
as	O
not	O
needed	O
by	O
running	O
software	O
.	O
</s>
<s>
Spare	O
memory	O
can	O
be	O
utilized	O
as	O
RAM	B-Application
drive	I-Application
for	O
temporary	O
high-speed	O
data	B-General_Concept
storage	I-General_Concept
.	O
</s>
<s>
As	O
shown	O
in	O
the	O
diagram	O
,	O
traditionally	O
there	O
are	O
two	O
more	O
sub-layers	O
of	O
the	O
primary	O
storage	O
,	O
besides	O
main	O
large-capacity	O
RAM	B-Architecture
:	O
</s>
<s>
Processor	B-General_Concept
registers	I-General_Concept
are	O
located	O
inside	O
the	O
processor	O
.	O
</s>
<s>
Each	O
register	O
typically	O
holds	O
a	O
word	O
of	O
data	B-General_Concept
(	O
often	O
32	O
or	O
64bits	O
)	O
.	O
</s>
<s>
CPU	B-Device
instructions	O
instruct	O
the	O
arithmetic	B-General_Concept
logic	I-General_Concept
unit	I-General_Concept
to	O
perform	O
various	O
calculations	O
or	O
other	O
operations	O
on	O
this	O
data	B-General_Concept
(	O
or	O
with	O
the	O
help	O
of	O
it	O
)	O
.	O
</s>
<s>
Registers	O
are	O
the	O
fastest	O
of	O
all	O
forms	O
of	O
computer	B-General_Concept
data	I-General_Concept
storage	I-General_Concept
.	O
</s>
<s>
Processor	B-General_Concept
cache	I-General_Concept
is	O
an	O
intermediate	O
stage	O
between	O
ultra-fast	O
registers	O
and	O
much	O
slower	O
main	O
memory	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
main	O
memory	O
is	O
much	O
slower	O
,	O
but	O
has	O
a	O
much	O
greater	O
storage	B-General_Concept
capacity	I-General_Concept
than	O
processor	B-General_Concept
registers	I-General_Concept
.	O
</s>
<s>
Multi-level	O
hierarchical	B-General_Concept
cache	I-General_Concept
setup	O
is	O
also	O
commonly	O
used	O
—	O
primary	O
cache	O
being	O
smallest	O
,	O
fastest	O
and	O
located	O
inside	O
the	O
processor	O
;	O
secondary	B-General_Concept
cache	I-General_Concept
being	O
somewhat	O
larger	O
and	O
slower	O
.	O
</s>
<s>
Main	O
memory	O
is	O
directly	O
or	O
indirectly	O
connected	O
to	O
the	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
via	O
a	O
memory	O
bus	O
.	O
</s>
<s>
It	O
is	O
actually	O
two	O
buses	O
(	O
not	O
on	O
the	O
diagram	O
)	O
:	O
an	O
address	O
bus	O
and	O
a	O
data	B-General_Concept
bus	I-General_Concept
.	O
</s>
<s>
The	O
CPU	B-Device
firstly	O
sends	O
a	O
number	O
through	O
an	O
address	O
bus	O
,	O
a	O
number	O
called	O
memory	B-General_Concept
address	I-General_Concept
,	O
that	O
indicates	O
the	O
desired	O
location	O
of	O
data	B-General_Concept
.	O
</s>
<s>
Then	O
it	O
reads	O
or	O
writes	O
the	O
data	B-General_Concept
in	O
the	O
memory	B-Algorithm
cells	I-Algorithm
using	O
the	O
data	B-General_Concept
bus	I-General_Concept
.	O
</s>
<s>
Additionally	O
,	O
a	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
(	O
MMU	O
)	O
is	O
a	O
small	O
device	O
between	O
CPU	B-Device
and	O
RAM	B-Architecture
recalculating	O
the	O
actual	O
memory	B-General_Concept
address	I-General_Concept
,	O
for	O
example	O
to	O
provide	O
an	O
abstraction	B-Application
of	O
virtual	B-Architecture
memory	I-Architecture
or	O
other	O
tasks	O
.	O
</s>
<s>
As	O
the	O
RAM	B-Architecture
types	O
used	O
for	O
primary	O
storage	O
are	O
volatile	B-General_Concept
(	O
uninitialized	O
at	O
start	O
up	O
)	O
,	O
a	O
computer	O
containing	O
only	O
such	O
storage	O
would	O
not	O
have	O
a	O
source	O
to	O
read	O
instructions	O
from	O
,	O
in	O
order	O
to	O
start	O
the	O
computer	O
.	O
</s>
<s>
Hence	O
,	O
non-volatile	B-General_Concept
primary	I-General_Concept
storage	I-General_Concept
containing	O
a	O
small	O
startup	O
program	O
(	O
BIOS	B-Operating_System
)	O
is	O
used	O
to	O
bootstrap	O
the	O
computer	O
,	O
that	O
is	O
,	O
to	O
read	O
a	O
larger	O
program	O
from	O
non-volatile	B-General_Concept
secondary	O
storage	O
to	O
RAM	B-Architecture
and	O
start	O
to	O
execute	O
it	O
.	O
</s>
<s>
A	O
non-volatile	B-General_Concept
technology	O
used	O
for	O
this	O
purpose	O
is	O
called	O
ROM	B-Device
,	O
for	O
read-only	B-Device
memory	I-Device
(	O
the	O
terminology	O
may	O
be	O
somewhat	O
confusing	O
as	O
most	O
ROM	B-Device
types	O
are	O
also	O
capable	O
of	O
random	B-General_Concept
access	I-General_Concept
)	O
.	O
</s>
<s>
Many	O
types	O
of	O
"	O
ROM	B-Device
"	O
are	O
not	O
literally	O
read	O
only	O
,	O
as	O
updates	O
to	O
them	O
are	O
possible	O
;	O
however	O
it	O
is	O
slow	O
and	O
memory	O
must	O
be	O
erased	O
in	O
large	O
portions	O
before	O
it	O
can	O
be	O
re-written	O
.	O
</s>
<s>
Some	O
embedded	B-Architecture
systems	I-Architecture
run	O
programs	O
directly	O
from	O
ROM	B-Device
(	O
or	O
similar	O
)	O
,	O
because	O
such	O
programs	O
are	O
rarely	O
changed	O
.	O
</s>
<s>
Standard	O
computers	O
do	O
not	O
store	O
non-rudimentary	O
programs	O
in	O
ROM	B-Device
,	O
and	O
rather	O
,	O
use	O
large	O
capacities	O
of	O
secondary	O
storage	O
,	O
which	O
is	O
non-volatile	B-General_Concept
as	O
well	O
,	O
and	O
not	O
as	O
costly	O
.	O
</s>
<s>
Secondary	O
storage	O
(	O
also	O
known	O
as	O
external	O
memory	O
or	O
auxiliary	O
storage	O
)	O
differs	O
from	O
primary	O
storage	O
in	O
that	O
it	O
is	O
not	O
directly	O
accessible	O
by	O
the	O
CPU	B-Device
.	O
</s>
<s>
The	O
computer	O
usually	O
uses	O
its	O
input/output	B-General_Concept
channels	O
to	O
access	O
secondary	O
storage	O
and	O
transfer	O
the	O
desired	O
data	B-General_Concept
to	O
primary	O
storage	O
.	O
</s>
<s>
Secondary	O
storage	O
is	O
non-volatile	B-General_Concept
(	O
retaining	O
data	B-General_Concept
when	O
its	O
power	O
is	O
shut	O
off	O
)	O
.	O
</s>
<s>
In	O
modern	O
computers	O
,	O
hard	B-Device
disk	I-Device
drives	I-Device
(	O
HDDs	O
)	O
or	O
solid-state	B-Device
drives	I-Device
(	O
SSDs	B-Device
)	O
are	O
usually	O
used	O
as	O
secondary	O
storage	O
.	O
</s>
<s>
The	O
access	B-General_Concept
time	I-General_Concept
per	O
byte	B-Application
for	O
HDDs	O
or	O
SSDs	B-Device
is	O
typically	O
measured	O
in	O
milliseconds	O
(	O
thousandths	O
of	O
a	O
second	O
)	O
,	O
while	O
the	O
access	B-General_Concept
time	I-General_Concept
per	O
byte	B-Application
for	O
primary	O
storage	O
is	O
measured	O
in	O
nanoseconds	O
(	O
billionths	O
of	O
a	O
second	O
)	O
.	O
</s>
<s>
Rotating	O
optical	B-General_Concept
storage	I-General_Concept
devices	I-General_Concept
,	O
such	O
as	O
CD	O
and	O
DVD	O
drives	O
,	O
have	O
even	O
longer	O
access	B-General_Concept
times	I-General_Concept
.	O
</s>
<s>
Other	O
examples	O
of	O
secondary	O
storage	O
technologies	O
include	O
USB	O
flash	O
drives	O
,	O
floppy	B-Device
disks	I-Device
,	O
magnetic	B-Architecture
tape	I-Architecture
,	O
paper	O
tape	O
,	O
punched	B-Architecture
cards	I-Architecture
,	O
and	O
RAM	B-Application
disks	I-Application
.	O
</s>
<s>
Once	O
the	O
disk	B-Device
read/write	I-Device
head	I-Device
on	O
HDDs	O
reaches	O
the	O
proper	O
placement	O
and	O
the	O
data	B-General_Concept
,	O
subsequent	O
data	B-General_Concept
on	O
the	O
track	O
are	O
very	O
fast	O
to	O
access	O
.	O
</s>
<s>
To	O
reduce	O
the	O
seek	O
time	O
and	O
rotational	O
latency	O
,	O
data	B-General_Concept
are	O
transferred	O
to	O
and	O
from	O
disks	B-Device
in	O
large	O
contiguous	O
blocks	O
.	O
</s>
<s>
Sequential	O
or	O
block	O
access	O
on	O
disks	B-Device
is	O
orders	O
of	O
magnitude	O
faster	O
than	O
random	B-General_Concept
access	I-General_Concept
,	O
and	O
many	O
sophisticated	O
paradigms	O
have	O
been	O
developed	O
to	O
design	O
efficient	O
algorithms	O
based	O
upon	O
sequential	O
and	O
block	O
access	O
.	O
</s>
<s>
Another	O
way	O
to	O
reduce	O
the	O
I/O	B-General_Concept
bottleneck	O
is	O
to	O
use	O
multiple	O
disks	B-Device
in	O
parallel	O
in	O
order	O
to	O
increase	O
the	O
bandwidth	O
between	O
primary	O
and	O
secondary	O
memory	O
.	O
</s>
<s>
Secondary	O
storage	O
is	O
often	O
formatted	O
according	O
to	O
a	O
file	B-Application
system	I-Application
format	O
,	O
which	O
provides	O
the	O
abstraction	B-Application
necessary	O
to	O
organize	O
data	B-General_Concept
into	O
files	B-Operating_System
and	O
directories	B-Application
,	O
while	O
also	O
providing	O
metadata	O
describing	O
the	O
owner	O
of	O
a	O
certain	O
file	O
,	O
the	O
access	B-General_Concept
time	I-General_Concept
,	O
the	O
access	O
permissions	O
,	O
and	O
other	O
information	O
.	O
</s>
<s>
Most	O
computer	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
use	O
the	O
concept	O
of	O
virtual	B-Architecture
memory	I-Architecture
,	O
allowing	O
utilization	O
of	O
more	O
primary	O
storage	B-General_Concept
capacity	I-General_Concept
than	O
is	O
physically	O
available	O
in	O
the	O
system	O
.	O
</s>
<s>
As	O
the	O
primary	O
memory	O
fills	O
up	O
,	O
the	O
system	O
moves	O
the	O
least-used	O
chunks	O
(	O
pages	B-General_Concept
)	O
to	O
a	O
swap	O
file	O
or	O
page	O
file	O
on	O
secondary	O
storage	O
,	O
retrieving	O
them	O
later	O
when	O
needed	O
.	O
</s>
<s>
If	O
a	O
lot	O
of	O
pages	B-General_Concept
are	O
moved	O
to	O
slower	O
secondary	O
storage	O
,	O
the	O
system	O
performance	O
is	O
degraded	O
.	O
</s>
<s>
Typically	O
,	O
it	O
involves	O
a	O
robotic	O
mechanism	O
which	O
will	O
mount	O
(	O
insert	O
)	O
and	O
dismount	O
removable	O
mass	B-Device
storage	I-Device
media	O
into	O
a	O
storage	O
device	O
according	O
to	O
the	O
system	O
's	O
demands	O
;	O
such	O
data	B-General_Concept
are	O
often	O
copied	O
to	O
secondary	O
storage	O
before	O
use	O
.	O
</s>
<s>
This	O
is	O
primarily	O
useful	O
for	O
extraordinarily	O
large	O
data	B-General_Concept
stores	O
,	O
accessed	O
without	O
human	O
operators	O
.	O
</s>
<s>
Typical	O
examples	O
include	O
tape	B-Device
libraries	I-Device
and	O
optical	B-Device
jukeboxes	I-Device
.	O
</s>
<s>
Tertiary	O
storage	O
is	O
also	O
known	O
as	O
nearline	B-General_Concept
storage	I-General_Concept
because	O
it	O
is	O
"	O
near	O
to	O
online	O
"	O
.	O
</s>
<s>
The	O
formal	O
distinction	O
between	O
online	O
,	O
nearline	B-General_Concept
,	O
and	O
offline	O
storage	O
is	O
:	O
</s>
<s>
Online	O
storage	O
is	O
immediately	O
available	O
for	O
I/O	B-General_Concept
.	O
</s>
<s>
Nearline	B-General_Concept
storage	I-General_Concept
is	O
not	O
immediately	O
available	O
,	O
but	O
can	O
be	O
made	O
online	O
quickly	O
without	O
human	O
intervention	O
.	O
</s>
<s>
For	O
example	O
,	O
always-on	O
spinning	O
hard	B-Device
disk	I-Device
drives	I-Device
are	O
online	O
storage	O
,	O
while	O
spinning	O
drives	O
that	O
spin	O
down	O
automatically	O
,	O
such	O
as	O
in	O
massive	O
arrays	O
of	O
idle	O
disks	B-Device
(	O
MAID	O
)	O
,	O
are	O
nearline	B-General_Concept
storage	I-General_Concept
.	O
</s>
<s>
Removable	B-Device
media	I-Device
such	O
as	O
tape	O
cartridges	O
that	O
can	O
be	O
automatically	O
loaded	O
,	O
as	O
in	O
tape	B-Device
libraries	I-Device
,	O
are	O
nearline	B-General_Concept
storage	I-General_Concept
,	O
while	O
tape	O
cartridges	O
that	O
must	O
be	O
manually	O
loaded	O
are	O
offline	O
storage	O
.	O
</s>
<s>
Off-line	O
storage	O
is	O
a	O
computer	B-General_Concept
data	I-General_Concept
storage	I-General_Concept
on	O
a	O
medium	O
or	O
a	O
device	O
that	O
is	O
not	O
under	O
the	O
control	O
of	O
a	O
processing	B-General_Concept
unit	I-General_Concept
.	O
</s>
<s>
Additionally	O
,	O
it	O
is	O
useful	O
for	O
cases	O
of	O
disaster	O
,	O
where	O
,	O
for	O
example	O
,	O
a	O
fire	O
destroys	O
the	O
original	O
data	B-General_Concept
,	O
a	O
medium	O
in	O
a	O
remote	O
location	O
will	O
be	O
unaffected	O
,	O
enabling	O
disaster	B-General_Concept
recovery	I-General_Concept
.	O
</s>
<s>
Off-line	O
storage	O
increases	O
general	O
information	O
security	O
,	O
since	O
it	O
is	O
physically	O
inaccessible	O
from	O
a	O
computer	O
,	O
and	O
data	B-General_Concept
confidentiality	O
or	O
integrity	O
cannot	O
be	O
affected	O
by	O
computer-based	O
attack	O
techniques	O
.	O
</s>
<s>
In	O
modern	O
personal	O
computers	O
,	O
most	O
secondary	O
and	O
tertiary	O
storage	B-General_Concept
media	I-General_Concept
are	O
also	O
used	O
for	O
off-line	O
storage	O
.	O
</s>
<s>
Optical	B-Device
discs	I-Device
and	O
flash	B-Device
memory	I-Device
devices	O
are	O
the	O
most	O
popular	O
,	O
and	O
to	O
a	O
much	O
lesser	O
extent	O
removable	O
hard	B-Device
disk	I-Device
drives	I-Device
.	O
</s>
<s>
In	O
enterprise	O
uses	O
,	O
magnetic	B-Architecture
tape	I-Architecture
is	O
predominant	O
.	O
</s>
<s>
Older	O
examples	O
are	O
floppy	B-Device
disks	I-Device
,	O
Zip	O
disks	B-Device
,	O
or	O
punched	B-Architecture
cards	I-Architecture
.	O
</s>
<s>
Storage	O
technologies	O
at	O
all	O
levels	O
of	O
the	O
storage	B-General_Concept
hierarchy	I-General_Concept
can	O
be	O
differentiated	O
by	O
evaluating	O
certain	O
core	O
characteristics	O
as	O
well	O
as	O
measuring	O
characteristics	O
specific	O
to	O
a	O
particular	O
implementation	O
.	O
</s>
<s>
+	O
Overview	O
Characteristic	O
Hard	B-Device
disk	I-Device
drive	I-Device
Optical	B-Device
disc	I-Device
Flash	B-Device
memory	I-Device
Random-access	B-Architecture
memory	I-Architecture
Linear	B-Operating_System
tape-open	I-Operating_System
Technology	O
Magnetic	B-Device
disk	I-Device
Laser	O
beam	O
Semiconductor	O
Magnetic	B-Architecture
tape	I-Architecture
Volatility	O
No	O
No	O
No	O
Volatile	B-General_Concept
No	O
Random	B-General_Concept
access	I-General_Concept
Yes	O
Yes	O
Yes	O
Yes	O
No	O
Latency	O
(	O
access	B-General_Concept
time	I-General_Concept
)	O
~	O
15	O
ms	O
(	O
swift	O
)	O
~	O
150	O
ms	O
(	O
moderate	O
)	O
None	O
(	O
instant	O
)	O
None	O
(	O
instant	O
)	O
Lack	O
of	O
random	B-General_Concept
access	I-General_Concept
(	O
very	O
slow	O
)	O
Controller	O
Internal	B-Device
External	O
Internal	B-Device
Internal	B-Device
External	O
Failure	O
with	O
imminent	O
data	B-General_Concept
loss	I-General_Concept
Head	O
crash	O
—	O
Circuitry	O
—	O
Error	B-Error_Name
detection	I-Error_Name
Diagnostic	O
(	O
S.M.A.R.T.	O
)	O
</s>
<s>
Non-volatile	B-General_Concept
memory	I-General_Concept
retains	O
the	O
stored	O
information	O
even	O
if	O
not	O
constantly	O
supplied	O
with	O
electric	O
power	O
.	O
</s>
<s>
Volatile	B-General_Concept
memory	I-General_Concept
requires	O
constant	O
power	O
to	O
maintain	O
the	O
stored	O
information	O
.	O
</s>
<s>
The	O
fastest	O
memory	O
technologies	O
are	O
volatile	B-General_Concept
ones	O
,	O
although	O
that	O
is	O
not	O
a	O
universal	O
rule	O
.	O
</s>
<s>
Since	O
the	O
primary	O
storage	O
is	O
required	O
to	O
be	O
very	O
fast	O
,	O
it	O
predominantly	O
uses	O
volatile	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
Dynamic	O
random-access	B-Architecture
memory	I-Architecture
is	O
a	O
form	O
of	O
volatile	B-General_Concept
memory	I-General_Concept
that	O
also	O
requires	O
the	O
stored	O
information	O
to	O
be	O
periodically	O
reread	O
and	O
rewritten	O
,	O
or	O
refreshed	B-General_Concept
,	O
otherwise	O
it	O
would	O
vanish	O
.	O
</s>
<s>
Static	B-Architecture
random-access	I-Architecture
memory	I-Architecture
is	O
a	O
form	O
of	O
volatile	B-General_Concept
memory	I-General_Concept
similar	O
to	O
DRAM	O
with	O
the	O
exception	O
that	O
it	O
never	O
needs	O
to	O
be	O
refreshed	B-General_Concept
as	O
long	O
as	O
power	O
is	O
applied	O
;	O
it	O
loses	O
its	O
content	O
when	O
the	O
power	O
supply	O
is	O
lost	O
.	O
</s>
<s>
An	O
uninterruptible	O
power	O
supply	O
(	O
UPS	O
)	O
can	O
be	O
used	O
to	O
give	O
a	O
computer	O
a	O
brief	O
window	O
of	O
time	O
to	O
move	O
information	O
from	O
primary	O
volatile	B-General_Concept
storage	I-General_Concept
into	O
non-volatile	B-General_Concept
storage	I-General_Concept
before	O
the	O
batteries	O
are	O
exhausted	O
.	O
</s>
<s>
Some	O
systems	O
,	O
for	O
example	O
EMC	B-Device
Symmetrix	I-Device
,	O
have	O
integrated	O
batteries	O
that	O
maintain	O
volatile	B-General_Concept
storage	I-General_Concept
for	O
several	O
minutes	O
.	O
</s>
<s>
Examples	O
include	O
CD-RW	O
and	O
SSD	B-Device
.	O
</s>
<s>
Write	O
once	O
storage	O
Write	B-Device
once	I-Device
read	I-Device
many	I-Device
(	O
WORM	B-Device
)	O
allows	O
the	O
information	O
to	O
be	O
written	O
only	O
once	O
at	O
some	O
point	O
after	O
manufacture	O
.	O
</s>
<s>
Examples	O
include	O
semiconductor	O
programmable	B-General_Concept
read-only	I-General_Concept
memory	I-General_Concept
and	O
CD-R	O
.	O
</s>
<s>
Examples	O
include	O
mask	B-Device
ROM	I-Device
ICs	O
and	O
CD-ROM	B-Device
.	O
</s>
<s>
Random	B-General_Concept
access	I-General_Concept
Any	O
location	O
in	O
storage	O
can	O
be	O
accessed	O
at	O
any	O
moment	O
in	O
approximately	O
the	O
same	O
amount	O
of	O
time	O
.	O
</s>
<s>
Most	O
semiconductor	B-Architecture
memories	I-Architecture
,	O
flash	B-Device
memories	I-Device
and	O
hard	B-Device
disk	I-Device
drives	I-Device
provide	O
random	B-General_Concept
access	I-General_Concept
,	O
though	O
both	O
semiconductor	O
and	O
flash	B-Device
memories	I-Device
have	O
minimal	O
latency	O
when	O
compared	O
to	O
hard	B-Device
disk	I-Device
drives	I-Device
,	O
as	O
no	O
mechanical	O
parts	O
need	O
to	O
be	O
moved	O
.	O
</s>
<s>
Sequential	B-General_Concept
access	I-General_Concept
The	O
accessing	O
of	O
pieces	O
of	O
information	O
will	O
be	O
in	O
a	O
serial	O
order	O
,	O
one	O
after	O
the	O
other	O
;	O
therefore	O
the	O
time	O
to	O
access	O
a	O
particular	O
piece	O
of	O
information	O
depends	O
upon	O
which	O
piece	O
of	O
information	O
was	O
last	O
accessed	O
.	O
</s>
<s>
Location-addressable	O
Each	O
individually	O
accessible	O
unit	O
of	O
information	O
in	O
storage	O
is	O
selected	O
with	O
its	O
numerical	O
memory	B-General_Concept
address	I-General_Concept
.	O
</s>
<s>
File	B-Application
addressable	I-Application
Information	O
is	O
divided	O
into	O
files	B-Operating_System
of	O
variable	O
length	O
,	O
and	O
a	O
particular	O
file	O
is	O
selected	O
with	O
human-readable	B-General_Concept
directory	B-Application
and	O
file	O
names	O
.	O
</s>
<s>
The	O
underlying	O
device	O
is	O
still	O
location-addressable	O
,	O
but	O
the	O
operating	B-General_Concept
system	I-General_Concept
of	O
a	O
computer	O
provides	O
the	O
file	B-Application
system	I-Application
abstraction	B-Application
to	O
make	O
the	O
operation	O
more	O
understandable	O
.	O
</s>
<s>
In	O
modern	O
computers	O
,	O
secondary	O
,	O
tertiary	O
and	O
off-line	O
storage	O
use	O
file	B-Application
systems	I-Application
.	O
</s>
<s>
Content-addressable	B-Data_Structure
Each	O
individually	O
accessible	O
unit	O
of	O
information	O
is	O
selected	O
based	O
on	O
the	O
basis	O
of	O
(	O
part	O
of	O
)	O
the	O
contents	O
stored	O
there	O
.	O
</s>
<s>
Content-addressable	B-Data_Structure
storage	I-Data_Structure
can	O
be	O
implemented	O
using	O
software	O
(	O
computer	O
program	O
)	O
or	O
hardware	B-Architecture
(	O
computer	B-Architecture
device	I-Architecture
)	O
,	O
with	O
hardware	B-Architecture
being	O
faster	O
but	O
more	O
expensive	O
option	O
.	O
</s>
<s>
Hardware	B-Architecture
content	B-Data_Structure
addressable	I-Data_Structure
memory	I-Data_Structure
is	O
often	O
used	O
in	O
a	O
computer	O
's	O
CPU	B-General_Concept
cache	I-General_Concept
.	O
</s>
<s>
It	O
is	O
expressed	O
as	O
a	O
quantity	O
of	O
bits	O
or	O
bytes	B-Application
(	O
e.g.	O
</s>
<s>
Memory	B-Device
storage	I-Device
density	I-Device
The	O
compactness	O
of	O
stored	O
information	O
.	O
</s>
<s>
It	O
is	O
the	O
storage	B-General_Concept
capacity	I-General_Concept
of	O
a	O
medium	O
divided	O
with	O
a	O
unit	O
of	O
length	O
,	O
area	O
or	O
volume	O
(	O
e.g.	O
</s>
<s>
It	O
may	O
make	O
sense	O
to	O
separate	O
read	O
latency	O
and	O
write	O
latency	O
(	O
especially	O
for	O
non-volatile	B-General_Concept
memory	I-General_Concept
)	O
and	O
in	O
case	O
of	O
sequential	B-General_Concept
access	I-General_Concept
storage	O
,	O
minimum	O
,	O
maximum	O
and	O
average	O
latency	O
.	O
</s>
<s>
In	O
computer	B-General_Concept
data	I-General_Concept
storage	I-General_Concept
,	O
throughput	O
is	O
usually	O
expressed	O
in	O
terms	O
of	O
megabytes	O
per	O
second	O
(	O
MB/s	O
)	O
,	O
though	O
bit	O
rate	O
may	O
also	O
be	O
used	O
.	O
</s>
<s>
Granularity	O
The	O
size	O
of	O
the	O
largest	O
"	O
chunk	O
"	O
of	O
data	B-General_Concept
that	O
can	O
be	O
efficiently	O
accessed	O
as	O
a	O
single	O
unit	O
,	O
e.g.	O
</s>
<s>
Utilities	O
such	O
as	O
hdparm	B-Application
and	O
sar	B-Application
can	O
be	O
used	O
to	O
measure	O
IO	O
performance	O
in	O
Linux	O
.	O
</s>
<s>
Storage	O
devices	O
that	O
reduce	O
fan	O
usage	O
automatically	O
shut-down	O
during	O
inactivity	O
,	O
and	O
low	O
power	O
hard	B-Device
drives	I-Device
can	O
reduce	O
energy	O
consumption	O
by	O
90	O
percent	O
.	O
</s>
<s>
2.5-inch	O
hard	B-Device
disk	I-Device
drives	I-Device
often	O
consume	O
less	O
power	O
than	O
larger	O
ones	O
.	O
</s>
<s>
Low	O
capacity	O
solid-state	B-Device
drives	I-Device
have	O
no	O
moving	O
parts	O
and	O
consume	O
less	O
power	O
than	O
hard	B-Device
disks	I-Device
.	O
</s>
<s>
Also	O
,	O
memory	O
may	O
use	O
more	O
power	O
than	O
hard	B-Device
disks	I-Device
.	O
</s>
<s>
Full	O
disk	B-Device
encryption	O
,	O
volume	O
and	O
virtual	O
disk	B-Device
encryption	O
,	O
andor	O
file/folder	O
encryption	O
is	O
readily	O
available	O
for	O
most	O
storage	O
devices	O
.	O
</s>
<s>
Hardware	B-Architecture
memory	O
encryption	O
is	O
available	O
in	O
Intel	O
Architecture	O
,	O
supporting	O
Total	O
Memory	O
Encryption	O
(	O
TME	O
)	O
and	O
page	O
granular	O
memory	O
encryption	O
with	O
multiple	O
keys	O
(	O
MKTME	O
)	O
.	O
</s>
<s>
and	O
in	O
SPARC	B-Architecture
M7	O
generation	O
since	O
October	O
2015	O
.	O
</s>
<s>
Distinct	O
types	O
of	O
data	B-General_Concept
storage	I-General_Concept
have	O
different	O
points	O
of	O
failure	O
and	O
various	O
methods	O
of	O
predictive	B-Device
failure	I-Device
analysis	I-Device
.	O
</s>
<s>
Vulnerabilities	O
that	O
can	O
instantly	O
lead	O
to	O
total	O
loss	O
are	O
head	O
crashing	O
on	O
mechanical	O
hard	B-Device
drives	I-Device
and	O
failure	O
of	O
electronic	O
components	O
on	O
flash	B-Device
storage	I-Device
.	O
</s>
<s>
Impending	O
failure	O
on	O
hard	B-Device
disk	I-Device
drives	I-Device
is	O
estimable	O
using	O
S.M.A.R.T.	O
</s>
<s>
diagnostic	O
data	B-General_Concept
that	O
includes	O
the	O
hours	O
of	O
operation	O
and	O
the	O
count	O
of	O
spin-ups	O
,	O
though	O
its	O
reliability	O
is	O
disputed	O
.	O
</s>
<s>
Flash	B-Device
storage	I-Device
may	O
experience	O
downspiking	O
transfer	O
rates	O
as	O
a	O
result	O
of	O
accumulating	O
errors	O
,	O
which	O
the	O
flash	B-Device
memory	I-Device
controller	I-Device
attempts	O
to	O
correct	O
.	O
</s>
<s>
The	O
health	O
of	O
optical	B-Device
media	I-Device
can	O
be	O
determined	O
by	O
measuring	O
correctable	O
minor	O
errors	O
,	O
of	O
which	O
high	O
counts	O
signify	O
deteriorating	O
and/or	O
low-quality	O
media	O
.	O
</s>
<s>
Too	O
many	O
consecutive	O
minor	O
errors	O
can	O
lead	O
to	O
data	B-General_Concept
corruption	O
.	O
</s>
<s>
,	O
the	O
most	O
commonly	O
used	O
data	B-General_Concept
storage	I-General_Concept
media	I-General_Concept
are	O
semiconductor	O
,	O
magnetic	O
,	O
and	O
optical	O
,	O
while	O
paper	O
still	O
sees	O
some	O
limited	O
usage	O
.	O
</s>
<s>
Semiconductor	B-Architecture
memory	I-Architecture
uses	O
semiconductor-based	O
integrated	O
circuit	O
(	O
IC	O
)	O
chips	O
to	O
store	O
information	O
.	O
</s>
<s>
Data	B-General_Concept
are	O
typically	O
stored	O
in	O
metal	B-Architecture
–	I-Architecture
oxide	I-Architecture
–	I-Architecture
semiconductor	I-Architecture
(	O
MOS	O
)	O
memory	B-Algorithm
cells	I-Algorithm
.	O
</s>
<s>
A	O
semiconductor	B-Architecture
memory	I-Architecture
chip	O
may	O
contain	O
millions	O
of	O
memory	B-Algorithm
cells	I-Algorithm
,	O
consisting	O
of	O
tiny	O
MOS	B-Architecture
field-effect	I-Architecture
transistors	I-Architecture
(	O
MOSFETs	B-Architecture
)	O
and/or	O
MOS	O
capacitors	O
.	O
</s>
<s>
Both	O
volatile	B-General_Concept
and	O
non-volatile	B-General_Concept
forms	O
of	O
semiconductor	B-Architecture
memory	I-Architecture
exist	O
,	O
the	O
former	O
using	O
standard	O
MOSFETs	B-Architecture
and	O
the	O
latter	O
using	O
floating-gate	B-Algorithm
MOSFETs	I-Algorithm
.	O
</s>
<s>
In	O
modern	O
computers	O
,	O
primary	O
storage	O
almost	O
exclusively	O
consists	O
of	O
dynamic	O
volatile	B-General_Concept
semiconductor	O
random-access	B-Architecture
memory	I-Architecture
(	O
RAM	B-Architecture
)	O
,	O
particularly	O
dynamic	O
random-access	B-Architecture
memory	I-Architecture
(	O
DRAM	O
)	O
.	O
</s>
<s>
Since	O
the	O
turn	O
of	O
the	O
century	O
,	O
a	O
type	O
of	O
non-volatile	B-General_Concept
floating-gate	B-Algorithm
semiconductor	B-Architecture
memory	I-Architecture
known	O
as	O
flash	B-Device
memory	I-Device
has	O
steadily	O
gained	O
share	O
as	O
off-line	O
storage	O
for	O
home	O
computers	O
.	O
</s>
<s>
Non-volatile	B-General_Concept
semiconductor	B-Architecture
memory	I-Architecture
is	O
also	O
used	O
for	O
secondary	O
storage	O
in	O
various	O
advanced	O
electronic	O
devices	O
and	O
specialized	O
computers	O
that	O
are	O
designed	O
for	O
them	O
.	O
</s>
<s>
As	O
early	O
as	O
2006	O
,	O
notebook	B-Device
and	O
desktop	B-Device
computer	I-Device
manufacturers	O
started	O
using	O
flash-based	O
solid-state	B-Device
drives	I-Device
(	O
SSDs	B-Device
)	O
as	O
default	O
configuration	O
options	O
for	O
the	O
secondary	O
storage	O
either	O
in	O
addition	O
to	O
or	O
instead	O
of	O
the	O
more	O
traditional	O
HDD	O
.	O
</s>
<s>
Magnetic	B-Architecture
storage	I-Architecture
uses	O
different	O
patterns	O
of	O
magnetization	O
on	O
a	O
magnetically	O
coated	O
surface	O
to	O
store	O
information	O
.	O
</s>
<s>
Magnetic	B-Architecture
storage	I-Architecture
is	O
non-volatile	B-General_Concept
.	O
</s>
<s>
The	O
information	O
is	O
accessed	O
using	O
one	O
or	O
more	O
read/write	B-Device
heads	I-Device
which	O
may	O
contain	O
one	O
or	O
more	O
recording	O
transducers	O
.	O
</s>
<s>
A	O
read/write	B-Device
head	I-Device
only	O
covers	O
a	O
part	O
of	O
the	O
surface	O
so	O
that	O
the	O
head	O
or	O
medium	O
or	O
both	O
must	O
be	O
moved	O
relative	O
to	O
another	O
in	O
order	O
to	O
access	O
data	B-General_Concept
.	O
</s>
<s>
In	O
modern	O
computers	O
,	O
magnetic	B-Architecture
storage	I-Architecture
will	O
take	O
these	O
forms	O
:	O
</s>
<s>
Magnetic	B-Device
disk	I-Device
;	O
</s>
<s>
Floppy	B-Device
disk	I-Device
,	O
used	O
for	O
off-line	O
storage	O
;	O
</s>
<s>
Hard	B-Device
disk	I-Device
drive	I-Device
,	O
used	O
for	O
secondary	O
storage	O
.	O
</s>
<s>
Magnetic	B-Architecture
tape	I-Architecture
,	O
used	O
for	O
tertiary	O
and	O
off-line	O
storage	O
;	O
</s>
<s>
In	O
early	B-Architecture
computers	I-Architecture
,	O
magnetic	B-Architecture
storage	I-Architecture
was	O
also	O
used	O
as	O
:	O
</s>
<s>
Primary	O
storage	O
in	O
a	O
form	O
of	O
magnetic	B-General_Concept
memory	I-General_Concept
,	O
or	O
core	O
memory	O
,	O
core	B-General_Concept
rope	I-General_Concept
memory	I-General_Concept
,	O
thin-film	B-General_Concept
memory	I-General_Concept
and/or	O
twistor	B-General_Concept
memory	I-General_Concept
;	O
</s>
<s>
NCR	B-Device
CRAM	I-Device
)	O
or	O
off	O
line	O
storage	O
in	O
the	O
form	O
of	O
magnetic	O
cards	O
;	O
</s>
<s>
Magnetic	B-Architecture
tape	I-Architecture
was	O
then	O
often	O
used	O
for	O
secondary	O
storage	O
.	O
</s>
<s>
Magnetic	B-Architecture
storage	I-Architecture
does	O
not	O
have	O
a	O
definite	O
limit	O
of	O
rewriting	O
cycles	O
like	O
flash	B-Device
storage	I-Device
and	O
re-writeable	O
optical	B-Device
media	I-Device
,	O
as	O
altering	O
magnetic	O
fields	O
causes	O
no	O
physical	O
wear	O
.	O
</s>
<s>
Optical	B-General_Concept
storage	I-General_Concept
,	O
the	O
typical	O
optical	B-Device
disc	I-Device
,	O
stores	O
information	O
in	O
deformities	O
on	O
the	O
surface	O
of	O
a	O
circular	O
disc	O
and	O
reads	O
this	O
information	O
by	O
illuminating	O
the	O
surface	O
with	O
a	O
laser	O
diode	O
and	O
observing	O
the	O
reflection	O
.	O
</s>
<s>
Optical	B-Device
disc	I-Device
storage	O
is	O
non-volatile	B-General_Concept
.	O
</s>
<s>
CD	O
,	O
CD-ROM	B-Device
,	O
DVD	O
,	O
BD-ROM	O
:	O
Read	O
only	O
storage	O
,	O
used	O
for	O
mass	O
distribution	O
of	O
digital	O
information	O
(	O
music	O
,	O
video	O
,	O
computer	O
programs	O
)	O
;	O
</s>
<s>
CD-R	O
,	O
DVD-R	O
,	O
DVD+R	O
,	O
BD-R	B-Operating_System
:	O
Write	O
once	O
storage	O
,	O
used	O
for	O
tertiary	O
and	O
off-line	O
storage	O
;	O
</s>
<s>
CD-RW	O
,	O
DVD-RW	O
,	O
DVD+RW	O
,	O
DVD-RAM	O
,	O
BD-RE	B-Operating_System
:	O
Slow	O
write	O
,	O
fast	O
read	O
storage	O
,	O
used	O
for	O
tertiary	O
and	O
off-line	O
storage	O
;	O
</s>
<s>
Ultra	B-Application
Density	I-Application
Optical	I-Application
or	O
UDO	O
is	O
similar	O
in	O
capacity	O
to	O
BD-R	B-Operating_System
or	I-Operating_System
BD-RE	I-Operating_System
and	O
is	O
slow	O
write	O
,	O
fast	O
read	O
storage	O
used	O
for	O
tertiary	O
and	O
off-line	O
storage	O
.	O
</s>
<s>
Magneto-optical	B-Device
disc	I-Device
storage	I-Device
is	O
optical	B-Device
disc	I-Device
storage	O
where	O
the	O
magnetic	O
state	B-Application
on	O
a	O
ferromagnetic	O
surface	O
stores	O
information	O
.	O
</s>
<s>
Magneto-optical	B-Device
disc	I-Device
storage	I-Device
is	O
non-volatile	B-General_Concept
,	O
sequential	B-General_Concept
access	I-General_Concept
,	O
slow	O
write	O
,	O
fast	O
read	O
storage	O
used	O
for	O
tertiary	O
and	O
off-line	O
storage	O
.	O
</s>
<s>
3D	O
optical	B-General_Concept
data	I-General_Concept
storage	I-General_Concept
has	O
also	O
been	O
proposed	O
.	O
</s>
<s>
Light	O
induced	O
magnetization	O
melting	O
in	O
magnetic	O
photoconductors	O
has	O
also	O
been	O
proposed	O
for	O
high-speed	O
low-energy	O
consumption	O
magneto-optical	B-Device
storage	I-Device
.	O
</s>
<s>
Paper	B-General_Concept
data	I-General_Concept
storage	I-General_Concept
,	O
typically	O
in	O
the	O
form	O
of	O
paper	O
tape	O
or	O
punched	B-Architecture
cards	I-Architecture
,	O
has	O
long	O
been	O
used	O
to	O
store	O
information	O
for	O
automatic	O
processing	O
,	O
particularly	O
before	O
general-purpose	O
computers	O
existed	O
.	O
</s>
<s>
Relatively	O
small	O
amounts	O
of	O
digital	O
data	B-General_Concept
(	O
compared	O
to	O
other	O
digital	O
data	B-General_Concept
storage	I-General_Concept
)	O
may	O
be	O
backed	O
up	O
on	O
paper	O
as	O
a	O
matrix	O
barcode	O
for	O
very	O
long-term	O
storage	O
,	O
as	O
the	O
longevity	O
of	O
paper	O
typically	O
exceeds	O
even	O
magnetic	B-Architecture
data	I-Architecture
storage	I-Architecture
.	O
</s>
<s>
Vacuum-tube	O
memory	O
A	O
Williams	B-General_Concept
tube	I-General_Concept
used	O
a	O
cathode-ray	B-Device
tube	I-Device
,	O
and	O
a	O
Selectron	O
tube	O
used	O
a	O
large	O
vacuum	O
tube	O
to	O
store	O
information	O
.	O
</s>
<s>
These	O
primary	O
storage	O
devices	O
were	O
short-lived	O
in	O
the	O
market	O
,	O
since	O
the	O
Williams	B-General_Concept
tube	I-General_Concept
was	O
unreliable	O
,	O
and	O
the	O
Selectron	O
tube	O
was	O
expensive	O
.	O
</s>
<s>
Delay-line	O
memory	O
was	O
dynamic	O
volatile	B-General_Concept
,	O
cycle	O
sequential	O
read/write	O
storage	O
,	O
and	O
was	O
used	O
for	O
primary	O
storage	O
.	O
</s>
<s>
Optical	B-Device
tape	I-Device
is	O
a	O
medium	O
for	O
optical	B-General_Concept
storage	I-General_Concept
,	O
generally	O
consisting	O
of	O
a	O
long	O
and	O
narrow	O
strip	O
of	O
plastic	O
,	O
onto	O
which	O
patterns	O
can	O
be	O
written	O
and	O
from	O
which	O
the	O
patterns	O
can	O
be	O
read	O
back	O
.	O
</s>
<s>
It	O
shares	O
some	O
technologies	O
with	O
cinema	O
film	O
stock	O
and	O
optical	B-Device
discs	I-Device
,	O
but	O
is	O
compatible	O
with	O
neither	O
.	O
</s>
<s>
The	O
motivation	O
behind	O
developing	O
this	O
technology	O
was	O
the	O
possibility	O
of	O
far	O
greater	O
storage	B-General_Concept
capacities	I-General_Concept
than	O
either	O
magnetic	B-Architecture
tape	I-Architecture
or	O
optical	B-Device
discs	I-Device
.	O
</s>
<s>
Phase-change	B-Device
memory	I-Device
uses	O
different	O
mechanical	O
phases	O
of	O
phase-change	O
material	O
to	O
store	O
information	O
in	O
an	O
X	O
–	O
Y	O
addressable	O
matrix	O
and	O
reads	O
the	O
information	O
by	O
observing	O
the	O
varying	O
electrical	O
resistance	O
of	O
the	O
material	O
.	O
</s>
<s>
Phase-change	B-Device
memory	I-Device
would	O
be	O
non-volatile	B-General_Concept
,	O
random-access	B-General_Concept
read/write	O
storage	O
,	O
and	O
might	O
be	O
used	O
for	O
primary	O
,	O
secondary	O
and	O
off-line	O
storage	O
.	O
</s>
<s>
Most	O
rewritable	O
and	O
many	O
write-once	O
optical	B-Device
disks	I-Device
already	O
use	O
phase-change	O
material	O
to	O
store	O
information	O
.	O
</s>
<s>
Holographic	B-Algorithm
data	I-Algorithm
storage	I-Algorithm
stores	O
information	O
optically	O
inside	O
crystals	O
or	O
photopolymers	O
.	O
</s>
<s>
Holographic	B-Algorithm
storage	I-Algorithm
can	O
utilize	O
the	O
whole	O
volume	O
of	O
the	O
storage	B-General_Concept
medium	I-General_Concept
,	O
unlike	O
optical	B-Device
disc	I-Device
storage	O
,	O
which	O
is	O
limited	O
to	O
a	O
small	O
number	O
of	O
surface	O
layers	O
.	O
</s>
<s>
Holographic	B-Algorithm
storage	I-Algorithm
would	O
be	O
non-volatile	B-General_Concept
,	O
sequential-access	O
,	O
and	O
either	O
write-once	O
or	O
read/write	O
storage	O
.	O
</s>
<s>
See	O
Holographic	B-Device
Versatile	I-Device
Disc	I-Device
(	O
HVD	O
)	O
.	O
</s>
<s>
Molecular	B-Algorithm
memory	I-Algorithm
stores	O
information	O
in	O
polymer	B-Language
that	O
can	O
store	O
electric	O
charge	O
.	O
</s>
<s>
Molecular	B-Algorithm
memory	I-Algorithm
might	O
be	O
especially	O
suited	O
for	O
primary	O
storage	O
.	O
</s>
<s>
The	O
theoretical	O
storage	B-General_Concept
capacity	I-General_Concept
of	O
molecular	B-Algorithm
memory	I-Algorithm
is	O
10terabits	O
per	O
square	O
inch	O
(	O
16Gbit/mm2	O
)	O
.	O
</s>
<s>
While	O
a	O
group	O
of	O
bits	O
malfunction	O
may	O
be	O
resolved	O
by	O
error	B-Error_Name
detection	I-Error_Name
and	I-Error_Name
correction	I-Error_Name
mechanisms	O
(	O
see	O
above	O
)	O
,	O
storage	O
device	O
malfunction	O
requires	O
different	O
solutions	O
.	O
</s>
<s>
Device	O
mirroring	B-General_Concept
(	O
replication	O
)	O
–	O
A	O
common	O
solution	O
to	O
the	O
problem	O
is	O
constantly	O
maintaining	O
an	O
identical	O
copy	O
of	O
device	O
content	O
on	O
another	O
device	O
(	O
typically	O
of	O
a	O
same	O
type	O
)	O
.	O
</s>
<s>
The	O
upside	O
is	O
possible	O
concurrent	O
read	O
of	O
a	O
same	O
data	B-General_Concept
group	O
by	O
two	O
independent	O
processes	O
,	O
which	O
increases	O
performance	O
.	O
</s>
<s>
Redundant	B-Architecture
array	I-Architecture
of	I-Architecture
independent	I-Architecture
disks	I-Architecture
(	O
RAID	B-Architecture
)	O
–	O
This	O
method	O
generalizes	O
the	O
device	O
mirroring	B-General_Concept
above	O
by	O
allowing	O
one	O
device	O
in	O
a	O
group	O
of	O
ndevices	O
to	O
fail	O
and	O
be	O
replaced	O
with	O
the	O
content	O
restored	O
(	O
Device	O
mirroring	B-General_Concept
is	O
RAID	B-Architecture
with	O
n	O
=	O
2	O
)	O
.	O
</s>
<s>
RAID	B-Architecture
groups	O
of	O
n	O
=	O
5	O
or	O
n	O
=	O
6	O
are	O
common	O
.	O
</s>
<s>
Device	O
mirroring	B-General_Concept
and	O
typical	O
RAID	B-Architecture
are	O
designed	O
to	O
handle	O
a	O
single	O
device	O
failure	O
in	O
the	O
RAID	B-Architecture
group	O
of	O
devices	O
.	O
</s>
<s>
However	O
,	O
if	O
a	O
second	O
failure	O
occurs	O
before	O
the	O
RAID	B-Architecture
group	O
is	O
completely	O
repaired	O
from	O
the	O
first	O
failure	O
,	O
then	O
data	B-General_Concept
can	O
be	O
lost	O
.	O
</s>
<s>
Thus	O
the	O
probability	O
of	O
two	O
failures	O
in	O
a	O
same	O
RAID	B-Architecture
group	O
in	O
time	O
proximity	O
is	O
much	O
smaller	O
(	O
approximately	O
the	O
probability	O
squared	O
,	O
i.e.	O
,	O
multiplied	O
by	O
itself	O
)	O
.	O
</s>
<s>
If	O
a	O
database	O
cannot	O
tolerate	O
even	O
such	O
smaller	O
probability	O
of	O
data	B-General_Concept
loss	I-General_Concept
,	O
then	O
the	O
RAID	B-Architecture
group	O
itself	O
is	O
replicated	O
(	O
mirrored	O
)	O
.	O
</s>
<s>
In	O
many	O
cases	O
such	O
mirroring	B-General_Concept
is	O
done	O
geographically	O
remotely	O
,	O
in	O
a	O
different	O
storage	O
array	O
,	O
to	O
handle	O
also	O
recovery	O
from	O
disasters	O
(	O
see	O
disaster	B-General_Concept
recovery	I-General_Concept
above	O
)	O
.	O
</s>
<s>
A	O
secondary	O
or	O
tertiary	O
storage	O
may	O
connect	O
to	O
a	O
computer	O
utilizing	O
computer	B-Architecture
networks	I-Architecture
.	O
</s>
<s>
Direct-attached	B-General_Concept
storage	I-General_Concept
(	O
DAS	O
)	O
is	O
a	O
traditional	O
mass	B-Device
storage	I-Device
,	O
that	O
does	O
not	O
use	O
any	O
network	O
.	O
</s>
<s>
This	O
retronym	O
was	O
coined	O
recently	O
,	O
together	O
with	O
NAS	B-Application
and	O
SAN	O
.	O
</s>
<s>
Network-attached	B-Application
storage	I-Application
(	O
NAS	B-Application
)	O
is	O
mass	B-Device
storage	I-Device
attached	O
to	O
a	O
computer	O
which	O
another	O
computer	O
can	O
access	O
at	O
file	O
level	O
over	O
a	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
,	O
a	O
private	O
wide	O
area	O
network	O
,	O
or	O
in	O
the	O
case	O
of	O
online	B-Application
file	I-Application
storage	I-Application
,	O
over	O
the	O
Internet	O
.	O
</s>
<s>
NAS	B-Application
is	O
commonly	O
associated	O
with	O
the	O
NFS	B-Protocol
and	O
CIFS/SMB	B-Operating_System
protocols	O
.	O
</s>
<s>
Storage	O
area	O
network	O
(	O
SAN	O
)	O
is	O
a	O
specialized	O
network	O
,	O
that	O
provides	O
other	O
computers	O
with	O
storage	B-General_Concept
capacity	I-General_Concept
.	O
</s>
<s>
The	O
crucial	O
difference	O
between	O
NAS	B-Application
and	O
SAN	O
,	O
is	O
that	O
NAS	B-Application
presents	O
and	O
manages	O
file	B-Application
systems	I-Application
to	O
client	O
computers	O
,	O
while	O
SAN	O
provides	O
access	O
at	O
block-addressing	O
(	O
raw	O
)	O
level	O
,	O
leaving	O
it	O
to	O
attaching	O
systems	O
to	O
manage	O
data	B-General_Concept
or	O
file	B-Application
systems	I-Application
within	O
the	O
provided	O
capacity	O
.	O
</s>
<s>
SAN	O
is	O
commonly	O
associated	O
with	O
Fibre	B-Architecture
Channel	I-Architecture
networks	O
.	O
</s>
<s>
Large	O
quantities	O
of	O
individual	O
magnetic	O
tapes	O
,	O
and	O
optical	O
or	O
magneto-optical	B-Device
discs	I-Device
may	O
be	O
stored	O
in	O
robotic	O
tertiary	O
storage	O
devices	O
.	O
</s>
<s>
In	O
tape	B-Architecture
storage	I-Architecture
field	O
they	O
are	O
known	O
as	O
tape	B-Device
libraries	I-Device
,	O
and	O
in	O
optical	B-General_Concept
storage	I-General_Concept
field	O
optical	B-Device
jukeboxes	I-Device
,	O
or	O
optical	B-Device
disk	I-Device
libraries	I-Device
per	O
analogy	O
.	O
</s>
<s>
Tape	B-Device
libraries	I-Device
may	O
have	O
from	O
10	O
to	O
more	O
than	O
100,000	O
slots	O
,	O
and	O
provide	O
terabytes	O
or	O
petabytes	O
of	O
near-line	O
information	O
.	O
</s>
<s>
Optical	B-Device
jukeboxes	I-Device
are	O
somewhat	O
smaller	O
solutions	O
,	O
up	O
to	O
1,000	O
slots	O
.	O
</s>
<s>
Robotic	O
storage	O
is	O
used	O
for	O
backups	B-Protocol
,	O
and	O
for	O
high-capacity	O
archives	O
in	O
imaging	O
,	O
medical	O
,	O
and	O
video	O
industries	O
.	O
</s>
<s>
Hierarchical	B-Application
storage	I-Application
management	I-Application
is	O
a	O
most	O
known	O
archiving	O
strategy	O
of	O
automatically	O
migrating	O
long-unused	O
files	B-Operating_System
from	O
fast	O
hard	B-Device
disk	I-Device
storage	I-Device
to	O
libraries	O
or	O
jukeboxes	O
.	O
</s>
<s>
If	O
the	O
files	B-Operating_System
are	O
needed	O
,	O
they	O
are	O
retrieved	O
back	O
to	O
disk	B-Device
.	O
</s>
