<s>
Disk	B-Device
storage	I-Device
(	O
also	O
sometimes	O
called	O
drive	O
storage	O
)	O
is	O
a	O
general	O
category	O
of	O
storage	O
mechanisms	O
where	O
data	O
is	O
recorded	O
by	O
various	O
electronic	O
,	O
magnetic	O
,	O
optical	O
,	O
or	O
mechanical	O
changes	O
to	O
a	O
surface	O
layer	O
of	O
one	O
or	O
more	O
rotating	O
disks	B-Device
.	O
</s>
<s>
A	O
disk	B-Device
drive	I-Device
is	O
a	O
device	O
implementing	O
such	O
a	O
storage	O
mechanism	O
.	O
</s>
<s>
Notable	O
types	O
are	O
the	O
hard	B-Device
disk	I-Device
drive	I-Device
(	O
HDD	O
)	O
containing	O
a	O
non-removable	O
disk	B-Device
,	O
the	O
floppy	B-Device
disk	I-Device
drive	I-Device
(	O
FDD	O
)	O
and	O
its	O
removable	O
floppy	B-Device
disk	I-Device
,	O
and	O
various	O
optical	B-Device
disc	I-Device
drives	O
(	O
ODD	O
)	O
and	O
associated	O
optical	B-Device
disc	I-Device
media	I-Device
.	O
</s>
<s>
(	O
The	O
spelling	O
disk	B-Device
and	O
disc	O
are	O
used	O
interchangeably	O
except	O
where	O
trademarks	O
preclude	O
one	O
usage	O
,	O
e.g.	O
</s>
<s>
The	O
choice	O
of	O
a	O
particular	O
form	O
is	O
frequently	O
historical	O
,	O
as	O
in	O
IBM	O
's	O
usage	O
of	O
the	O
disk	B-Device
form	O
beginning	O
in	O
1956	O
with	O
the	O
"	O
IBM	B-Device
350	I-Device
disk	B-Device
storage	I-Device
unit	O
"	O
.	O
)	O
</s>
<s>
The	O
first	O
commercial	O
digital	O
disk	B-Device
storage	I-Device
device	O
was	O
the	O
IBM	B-Device
350	I-Device
which	O
shipped	O
in	O
1956	O
as	O
a	O
part	O
of	O
the	O
IBM	B-Device
305	I-Device
RAMAC	I-Device
computing	O
system	O
.	O
</s>
<s>
The	O
random-access	B-Architecture
,	O
low-density	O
storage	O
of	O
disks	B-Device
was	O
developed	O
to	O
complement	O
the	O
already	O
used	O
sequential-access	B-General_Concept
,	O
high-density	O
storage	O
provided	O
by	O
tape	O
drives	O
using	O
magnetic	O
tape	O
.	O
</s>
<s>
Vigorous	O
innovation	O
in	O
disk	B-Device
storage	I-Device
technology	O
,	O
coupled	O
with	O
less	O
vigorous	O
innovation	O
in	O
tape	O
storage	O
,	O
has	O
reduced	O
the	O
difference	O
in	O
acquisition	O
cost	O
per	O
terabyte	O
between	O
disk	B-Device
storage	I-Device
and	O
tape	O
storage	O
;	O
however	O
,	O
the	O
total	O
cost	O
of	O
ownership	O
of	O
data	O
on	O
disk	B-Device
including	O
power	O
and	O
management	O
remains	O
larger	O
than	O
that	O
of	O
tape	O
.	O
</s>
<s>
Disk	B-Device
storage	I-Device
is	O
now	O
used	O
in	O
both	O
computer	B-General_Concept
storage	I-General_Concept
and	O
consumer	O
electronic	O
storage	O
,	O
e.g.	O
,	O
audio	O
CDs	O
and	O
video	O
discs	O
(	O
VCD	O
,	O
DVD	O
and	O
Blu-ray	O
)	O
.	O
</s>
<s>
Data	O
on	O
modern	O
disks	B-Device
is	O
stored	O
in	O
fixed	O
length	O
blocks	B-General_Concept
,	O
usually	O
called	O
sectors	B-Application
and	O
varying	O
in	O
length	O
from	O
a	O
few	O
hundred	O
to	O
many	O
thousands	O
of	O
bytes	O
.	O
</s>
<s>
Gross	O
disk	B-Device
drive	I-Device
capacity	O
is	O
simply	O
the	O
number	O
of	O
disk	B-Device
surfaces	O
times	O
the	O
number	O
of	O
blocks/surface	O
times	O
the	O
number	O
of	O
bytes/block	O
.	O
</s>
<s>
In	O
certain	O
legacy	O
IBM	O
CKD	O
drives	O
the	O
data	O
was	O
stored	O
on	O
magnetic	B-Architecture
disks	I-Architecture
with	O
variable	O
length	O
blocks	B-General_Concept
,	O
called	O
records	O
;	O
record	O
length	O
could	O
vary	O
on	O
and	O
between	O
disks	B-Device
.	O
</s>
<s>
Capacity	O
decreased	O
as	O
record	O
length	O
decreased	O
due	O
to	O
the	O
necessary	O
gaps	O
between	O
blocks	B-General_Concept
.	O
</s>
<s>
Digital	O
disk	B-Device
drives	I-Device
are	O
block	B-General_Concept
storage	I-General_Concept
devices	I-General_Concept
.	O
</s>
<s>
Each	O
disk	B-Device
is	O
divided	O
into	O
logical	O
blocks	B-General_Concept
(	O
collection	O
of	O
sectors	B-Application
)	O
.	O
</s>
<s>
Blocks	B-General_Concept
are	O
addressed	O
using	O
their	O
logical	B-Device
block	I-Device
addresses	I-Device
(	O
LBA	O
)	O
.	O
</s>
<s>
Read	O
from	O
or	O
writing	O
to	O
disk	B-Device
happens	O
at	O
the	O
granularity	O
of	O
blocks	B-General_Concept
.	O
</s>
<s>
Originally	O
the	O
disk	B-General_Concept
capacity	I-General_Concept
was	O
quite	O
low	O
and	O
has	O
been	O
improved	O
in	O
one	O
of	O
several	O
ways	O
.	O
</s>
<s>
Improvements	O
in	O
mechanical	O
design	O
and	O
manufacture	O
allowed	O
smaller	O
and	O
more	O
precise	O
heads	O
,	O
meaning	O
that	O
more	O
tracks	O
could	O
be	O
stored	O
on	O
each	O
of	O
the	O
disks	B-Device
.	O
</s>
<s>
Advancements	O
in	O
data	O
compression	O
methods	O
permitted	O
more	O
information	O
to	O
be	O
stored	O
in	O
each	O
of	O
the	O
individual	O
sectors	B-Application
.	O
</s>
<s>
The	O
drive	O
stores	O
data	O
onto	O
cylinders	B-Device
,	I-Device
heads	I-Device
,	I-Device
and	I-Device
sectors	I-Device
.	O
</s>
<s>
The	O
sectors	B-Application
unit	O
is	O
the	O
smallest	O
size	O
of	O
data	O
to	O
be	O
stored	O
in	O
a	O
hard	B-Device
disk	I-Device
drive	I-Device
and	O
each	O
file	O
will	O
have	O
many	O
sectors	B-Application
units	O
assigned	O
to	O
it	O
.	O
</s>
<s>
This	O
is	O
then	O
sent	O
out	O
to	O
the	O
hard	B-Device
drive	I-Device
via	O
a	O
multi-wire	O
connector	O
.	O
</s>
<s>
Once	O
the	O
data	O
is	O
received	O
onto	O
the	O
circuit	O
board	O
of	O
the	O
drive	O
,	O
they	O
are	O
translated	O
and	O
compressed	O
into	O
a	O
format	O
that	O
the	O
individual	O
drive	O
can	O
use	O
to	O
store	O
onto	O
the	O
disk	B-Device
itself	O
.	O
</s>
<s>
The	O
drive	O
is	O
divided	O
into	O
sectors	B-Application
of	O
data	O
stored	O
onto	O
one	O
of	O
the	O
sides	O
of	O
one	O
of	O
the	O
internal	O
disks	B-Device
.	O
</s>
<s>
An	O
HDD	O
with	O
two	O
disks	B-Device
internally	O
will	O
typically	O
store	O
data	O
on	O
all	O
four	O
surfaces	O
.	O
</s>
<s>
One	O
is	O
the	O
rotation	O
of	O
the	O
disks	B-Device
inside	O
the	O
device	O
.	O
</s>
<s>
The	O
other	O
is	O
the	O
side-to-side	O
motion	O
of	O
the	O
head	O
across	O
the	O
disk	B-Device
as	O
it	O
moves	O
between	O
tracks	O
.	O
</s>
<s>
There	O
are	O
two	O
types	O
of	O
disk	B-Device
rotation	O
methods	O
:	O
</s>
<s>
constant	B-Device
angular	I-Device
velocity	I-Device
(	O
used	O
in	O
HDDs	O
,	O
standard	O
FDDs	O
,	O
a	O
few	O
optical	B-Device
disc	I-Device
systems	O
,	O
and	O
vinyl	O
audio	O
records	O
)	O
spins	O
the	O
media	O
at	O
one	O
constant	O
speed	O
regardless	O
of	O
where	O
the	O
head	O
is	O
positioned	O
.	O
</s>
<s>
Track	O
positioning	O
also	O
follows	O
two	O
different	O
methods	O
across	O
disk	B-Device
storage	I-Device
devices	O
.	O
</s>
<s>
Storage	O
devices	O
focused	O
on	O
holding	O
computer	O
data	O
,	O
e.g.	O
,	O
HDDs	O
,	O
FDDs	O
,	O
Iomega	B-Operating_System
zip	I-Operating_System
drives	I-Operating_System
,	O
use	O
concentric	O
tracks	O
to	O
store	O
data	O
.	O
</s>
<s>
During	O
a	O
sequential	B-General_Concept
read	I-General_Concept
or	O
write	O
operation	O
,	O
after	O
the	O
drive	O
accesses	O
all	O
the	O
sectors	B-Application
in	O
a	O
track	O
it	O
repositions	O
the	O
head(s )	O
to	O
the	O
next	O
track	O
.	O
</s>
<s>
The	O
disk	B-Device
drive	I-Device
interface	O
is	O
the	O
mechanism/protocol	O
of	O
communication	O
between	O
the	O
rest	O
of	O
the	O
system	O
and	O
the	O
disk	B-Device
drive	I-Device
itself	O
.	O
</s>
<s>
Storage	O
devices	O
intended	O
for	O
desktop	O
and	O
mobile	O
computers	O
typically	O
use	O
ATA	B-Protocol
(	O
PATA	B-Protocol
)	O
and	O
SATA	O
interfaces	O
.	O
</s>
<s>
Enterprise	O
systems	O
and	O
high-end	O
storage	O
devices	O
will	O
typically	O
use	O
SCSI	B-Architecture
,	O
SAS	O
,	O
and	O
FC	B-Architecture
interfaces	O
in	O
addition	O
to	O
some	O
use	O
of	O
SATA	O
.	O
</s>
<s>
Disk	B-Device
Generally	O
refers	O
to	O
magnetic	B-Architecture
media	I-Architecture
and	O
devices	O
.	O
</s>
<s>
Disc	O
Required	O
by	O
trademarks	O
for	O
certain	O
optical	B-Device
media	I-Device
and	O
devices	O
.	O
</s>
<s>
Platter	O
An	O
individual	O
recording	O
disk	B-Device
.	O
</s>
<s>
A	O
hard	B-Device
disk	I-Device
drive	I-Device
contains	O
a	O
set	O
of	O
platters	O
.	O
</s>
<s>
Constant	B-Device
angular	I-Device
velocity	I-Device
(	O
CAV	O
)	O
keeps	O
the	O
disk	B-Device
spinning	O
at	O
a	O
fixed	O
rate	O
,	O
measured	O
in	O
revolutions	O
per	O
minute	O
(	O
RPM	O
)	O
.	O
</s>
<s>
This	O
method	O
is	O
typical	O
with	O
computer	O
hard	B-Device
drives	I-Device
.	O
</s>
<s>
Constant	B-Device
linear	I-Device
velocity	I-Device
(	O
CLV	O
)	O
keeps	O
the	O
distance	O
covered	O
by	O
the	O
heads	O
per	O
unit	O
time	O
fixed	O
.	O
</s>
<s>
Thus	O
the	O
disk	B-Device
has	O
to	O
slow	O
down	O
as	O
the	O
arm	O
moves	O
to	O
the	O
outer	O
tracks	O
.	O
</s>
<s>
Low	O
level	O
formatting	B-Application
Establishing	O
the	O
tracks	O
and	O
sectors	B-Application
.	O
</s>
<s>
Head	O
The	O
device	O
that	O
reads	O
and	O
writes	O
the	O
information	O
—	O
magnetic	O
or	O
optical	O
—	O
on	O
the	O
disk	B-Device
surface	O
.	O
</s>
<s>
Rotational	O
latency	O
Average	O
time	O
,	O
once	O
the	O
arm	O
is	O
on	O
the	O
right	O
track	O
,	O
before	O
a	O
head	O
is	O
over	O
a	O
desired	O
sector	B-Application
.	O
</s>
