<s>
A	O
logical	B-General_Concept
disk	I-General_Concept
,	O
logical	B-Application
volume	I-Application
or	O
virtual	O
disk	B-Device
(	O
VD	O
or	O
vdisk	B-General_Concept
for	O
short	O
)	O
is	O
a	O
virtual	B-Application
device	I-Application
that	O
provides	O
an	O
area	O
of	O
usable	O
storage	O
capacity	O
on	O
one	O
or	O
more	O
physical	O
disk	B-Device
drive(s )	O
in	O
a	O
computer	O
system	O
.	O
</s>
<s>
The	O
disk	B-Device
is	O
described	O
as	O
logical	O
or	O
virtual	O
because	O
it	O
does	O
not	O
actually	O
exist	O
as	O
a	O
single	O
physical	O
entity	O
in	O
its	O
own	O
right	O
.	O
</s>
<s>
The	O
goal	O
of	O
the	O
logical	B-General_Concept
disk	I-General_Concept
is	O
to	O
provide	O
computer	O
software	O
with	O
what	O
seems	O
a	O
contiguous	O
storage	O
area	O
,	O
sparing	O
them	O
the	O
burden	O
of	O
dealing	O
with	O
the	O
intricacies	O
of	O
storing	O
files	O
on	O
multiple	O
physical	O
units	O
.	O
</s>
<s>
Most	O
modern	O
operating	B-General_Concept
systems	I-General_Concept
provide	O
some	O
form	O
of	O
logical	B-Application
volume	I-Application
management	I-Application
.	O
</s>
<s>
Logical	B-General_Concept
disks	I-General_Concept
can	O
be	O
defined	O
at	O
various	O
levels	O
in	O
the	O
storage	O
infrastructure	O
.	O
</s>
<s>
An	O
operating	B-General_Concept
system	I-General_Concept
may	O
define	O
volumes	O
or	O
logical	B-General_Concept
disks	I-General_Concept
and	O
assign	O
each	O
to	O
one	O
physical	O
disk	B-Device
,	O
more	O
than	O
one	O
physical	O
disk	B-Device
or	O
part	O
of	O
the	O
storage	O
area	O
of	O
a	O
physical	O
disk	B-Device
.	O
</s>
<s>
For	O
example	O
,	O
Windows	B-Device
NT	I-Device
can	O
create	B-Application
several	I-Application
partitions	I-Application
on	O
a	O
hard	B-Device
disk	I-Device
drive	I-Device
,	O
each	O
of	O
which	O
a	O
separate	O
volume	O
with	O
its	O
own	O
file	B-Application
system	I-Application
.	O
</s>
<s>
Each	O
floppy	B-Device
disk	I-Device
drive	I-Device
,	O
optical	O
disc	B-Device
drive	I-Device
or	O
USB	O
flash	O
drive	O
in	O
Windows	B-Device
NT	I-Device
becomes	O
one	O
volume	O
.	O
</s>
<s>
Windows	B-Device
NT	I-Device
can	O
also	O
create	O
partitions	B-Application
that	O
span	O
multiple	O
hard	B-Device
disks	I-Device
drives	O
.	O
</s>
<s>
As	O
such	O
logical	B-General_Concept
disks	I-General_Concept
or	O
vdisks	B-General_Concept
allow	O
computer	O
programs	O
to	O
access	O
files	O
stored	O
on	O
a	O
SAN	O
.	O
</s>
<s>
A	O
hardware-level	O
redundant	B-Architecture
array	I-Architecture
of	I-Architecture
independent	I-Architecture
disks	I-Architecture
(	O
RAID	B-Architecture
)	O
exposes	O
itself	O
to	O
the	O
operating	B-General_Concept
system	I-General_Concept
as	O
one	O
logical	B-General_Concept
disk	I-General_Concept
while	O
the	O
array	O
itself	O
consists	O
of	O
several	O
disks	B-Device
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
either	O
does	O
not	O
know	O
that	O
the	O
hardware	O
with	O
which	O
it	O
is	O
interfacing	O
is	O
a	O
RAID	B-Architecture
,	O
or	O
knows	O
but	O
still	O
does	O
not	O
concern	O
itself	O
with	O
intricate	O
details	O
of	O
storage	O
.	O
</s>
<s>
In	O
case	O
of	O
the	O
latter	O
,	O
specialized	O
management	O
,	O
maintenance	O
and	O
diagnostics	O
software	O
dedicated	O
to	O
that	O
specific	O
RAID	B-Architecture
may	O
run	O
on	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
When	O
IBM	O
first	O
released	O
the	O
magnetic	B-Architecture
disk	I-Architecture
drive	I-Device
in	O
the	O
1956	O
IBM	B-Device
305	I-Device
,	O
a	O
single	O
disk	B-Device
drive	I-Device
would	O
be	O
directly	O
attached	O
to	O
each	O
system	O
,	O
managed	O
as	O
a	O
single	O
entity	O
.	O
</s>
<s>
As	O
the	O
development	O
of	O
drives	O
continued	O
,	O
it	O
became	O
apparent	O
that	O
reliability	O
was	O
a	O
problem	O
and	O
systems	O
using	O
RAID	B-Architecture
technology	O
evolved	O
,	O
so	O
that	O
more	O
than	O
one	O
physical	O
disk	B-Device
is	O
used	O
to	O
produce	O
a	O
single	O
logical	B-General_Concept
disk	I-General_Concept
.	O
</s>
<s>
Many	O
modern	O
business	O
information	B-General_Concept
technology	I-General_Concept
environments	O
use	O
a	O
SAN	O
.	O
</s>
<s>
Here	O
,	O
many	O
storage	O
devices	O
are	O
connected	O
to	O
many	O
host	O
server	B-Application
devices	O
in	O
a	O
network	O
.	O
</s>
<s>
A	O
single	O
RAID	B-Architecture
array	I-Architecture
may	O
provide	O
some	O
capacity	O
to	O
one	O
server	B-Application
,	O
and	O
some	O
capacity	O
to	O
another	O
.	O
</s>
<s>
Therefore	O
,	O
logical	B-General_Concept
disks	I-General_Concept
are	O
used	O
to	O
partition	B-Application
the	O
available	O
capacity	O
and	O
provide	O
the	O
amount	O
of	O
storage	O
needed	O
by	O
each	O
host	O
from	O
a	O
common	O
pool	O
of	O
logical	B-General_Concept
disks	I-General_Concept
.	O
</s>
<s>
The	O
IBM	B-Application
SAN	I-Application
Volume	I-Application
Controller	I-Application
uses	O
the	O
term	O
"	O
vdisk	B-General_Concept
"	O
to	O
refer	O
to	O
these	O
logical	B-General_Concept
disks	I-General_Concept
.	O
</s>
<s>
Today	O
,	O
the	O
rationale	O
for	O
the	O
logical	B-General_Concept
disk	I-General_Concept
approach	O
starts	O
to	O
be	O
questioned	O
and	O
solutions	O
that	O
offer	O
more	O
flexibility	O
and	O
better	O
abstraction	O
are	O
increasingly	O
needed	O
.	O
</s>
