<s>
Hardware	O
virtualization	B-General_Concept
is	O
the	O
virtualization	B-General_Concept
of	O
computers	O
as	O
complete	O
hardware	O
platforms	O
,	O
certain	O
logical	O
abstractions	O
of	O
their	O
componentry	O
,	O
or	O
only	O
the	O
functionality	O
required	O
to	O
run	O
various	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Virtualization	B-General_Concept
hides	O
the	O
physical	O
characteristics	O
of	O
a	O
computing	O
platform	O
from	O
the	O
users	O
,	O
presenting	O
instead	O
an	O
abstract	O
computing	O
platform	O
.	O
</s>
<s>
At	O
its	O
origins	O
,	O
the	O
software	O
that	O
controlled	O
virtualization	B-General_Concept
was	O
called	O
a	O
"	O
control	O
program	O
"	O
,	O
but	O
the	O
terms	O
"	O
hypervisor	B-Operating_System
"	O
or	O
"	O
virtual	B-Operating_System
machine	I-Operating_System
monitor	I-Operating_System
"	O
became	O
preferred	O
over	O
time	O
.	O
</s>
<s>
The	O
term	O
"	O
virtualization	B-General_Concept
"	O
was	O
coined	O
in	O
the	O
1960s	O
to	O
refer	O
to	O
a	O
virtual	B-Architecture
machine	I-Architecture
(	O
sometimes	O
called	O
"	O
pseudo	O
machine	O
"	O
)	O
,	O
a	O
term	O
which	O
itself	O
dates	O
from	O
the	O
experimental	O
IBM	B-Device
M44/44X	I-Device
system	O
.	O
</s>
<s>
The	O
creation	O
and	O
management	O
of	O
virtual	B-Architecture
machines	I-Architecture
has	O
been	O
called	O
"	O
platform	O
virtualization	B-General_Concept
"	O
,	O
or	O
"	O
server	B-Application
virtualization	B-General_Concept
"	O
,	O
more	O
recently	O
.	O
</s>
<s>
Platform	O
virtualization	B-General_Concept
is	O
performed	O
on	O
a	O
given	O
hardware	O
platform	O
by	O
host	O
software	O
(	O
a	O
control	O
program	O
)	O
,	O
which	O
creates	O
a	O
simulated	O
computer	O
environment	O
,	O
a	O
virtual	B-Architecture
machine	I-Architecture
(	O
VM	B-Application
)	O
,	O
for	O
its	O
guest	O
software	O
.	O
</s>
<s>
The	O
guest	O
software	O
is	O
not	O
limited	O
to	O
user	O
applications	O
;	O
many	O
hosts	O
allow	O
the	O
execution	O
of	O
complete	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Access	O
to	O
physical	O
system	O
resources	O
(	O
such	O
as	O
the	O
network	B-Protocol
access	I-Protocol
,	O
display	O
,	O
keyboard	O
,	O
and	O
disk	B-Device
storage	I-Device
)	O
is	O
generally	O
managed	O
at	O
a	O
more	O
restrictive	O
level	O
than	O
the	O
host	O
processor	O
and	O
system-memory	O
.	O
</s>
<s>
Guests	O
are	O
often	O
restricted	O
from	O
accessing	O
specific	O
peripheral	B-Device
devices	I-Device
,	O
or	O
may	O
be	O
limited	O
to	O
a	O
subset	O
of	O
the	O
device	O
's	O
native	O
capabilities	O
,	O
depending	O
on	O
the	O
hardware	O
access	O
policy	O
implemented	O
by	O
the	O
virtualization	B-General_Concept
host	O
.	O
</s>
<s>
Virtualization	B-General_Concept
often	O
exacts	O
performance	O
penalties	O
,	O
both	O
in	O
resources	O
required	O
to	O
run	O
the	O
hypervisor	B-Operating_System
,	O
and	O
as	O
well	O
as	O
in	O
reduced	O
performance	O
on	O
the	O
virtual	B-Architecture
machine	I-Architecture
compared	O
to	O
running	O
native	O
on	O
the	O
physical	O
machine	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
server	B-Application
consolidation	O
,	O
many	O
small	O
physical	O
servers	O
are	O
replaced	O
by	O
one	O
larger	O
physical	O
server	B-Application
to	O
decrease	O
the	O
need	O
for	O
more	O
(	O
costly	O
)	O
hardware	O
resources	O
such	O
as	O
CPUs	O
,	O
and	O
hard	B-Device
drives	I-Device
.	O
</s>
<s>
Instead	O
,	O
each	O
OS	O
running	O
on	O
a	O
physical	O
server	B-Application
is	O
converted	O
to	O
a	O
distinct	O
OS	O
running	O
inside	O
a	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
Thereby	O
,	O
the	O
large	O
server	B-Application
can	O
"	O
host	O
"	O
many	O
such	O
"	O
guest	O
"	O
virtual	B-Architecture
machines	I-Architecture
.	O
</s>
<s>
This	O
is	O
known	O
as	O
Physical-to-Virtual	B-General_Concept
(	O
P2V	O
)	O
transformation	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
typical	O
server	B-Application
runs	O
at	O
425W	O
and	O
VMware	O
estimates	O
a	O
hardware	O
reduction	O
ratio	O
of	O
up	O
to	O
15:1	O
.	O
</s>
<s>
A	O
virtual	B-Architecture
machine	I-Architecture
(	O
VM	B-Application
)	O
can	O
be	O
more	O
easily	O
controlled	O
and	O
inspected	O
from	O
a	O
remote	O
site	O
than	O
a	O
physical	O
machine	O
,	O
and	O
the	O
configuration	O
of	O
a	O
VM	B-Application
is	O
more	O
flexible	O
.	O
</s>
<s>
This	O
is	O
very	O
useful	O
in	O
kernel	B-Operating_System
development	O
and	O
for	O
teaching	O
operating	B-General_Concept
system	I-General_Concept
courses	O
,	O
including	O
running	O
legacy	O
operating	B-General_Concept
systems	I-General_Concept
that	O
do	O
not	O
support	O
modern	O
hardware	O
.	O
</s>
<s>
A	O
new	O
virtual	B-Architecture
machine	I-Architecture
can	O
be	O
provisioned	O
as	O
required	O
without	O
the	O
need	O
for	O
an	O
up-front	O
hardware	O
purchase	O
.	O
</s>
<s>
A	O
virtual	B-Architecture
machine	I-Architecture
can	O
easily	O
be	O
relocated	O
from	O
one	O
physical	O
machine	O
to	O
another	O
as	O
needed	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
salesperson	O
going	O
to	O
a	O
customer	O
can	O
copy	O
a	O
virtual	B-Architecture
machine	I-Architecture
with	O
the	O
demonstration	O
software	O
to	O
their	O
laptop	O
,	O
without	O
the	O
need	O
to	O
transport	O
the	O
physical	O
computer	O
.	O
</s>
<s>
Likewise	O
,	O
an	O
error	O
inside	O
a	O
virtual	B-Architecture
machine	I-Architecture
does	O
not	O
harm	O
the	O
host	O
system	O
,	O
so	O
there	O
is	O
no	O
risk	O
of	O
the	O
OS	O
crashing	O
on	O
the	O
laptop	O
.	O
</s>
<s>
Because	O
of	O
this	O
ease	O
of	O
relocation	O
,	O
virtual	B-Architecture
machines	I-Architecture
can	O
be	O
readily	O
used	O
in	O
disaster	B-General_Concept
recovery	I-General_Concept
scenarios	O
without	O
concerns	O
with	O
impact	O
of	O
refurbished	O
and	O
faulty	O
energy	O
sources	O
.	O
</s>
<s>
However	O
,	O
when	O
multiple	O
VMs	B-Application
are	O
concurrently	O
running	O
on	O
the	O
same	O
physical	O
host	O
,	O
each	O
VM	B-Application
may	O
exhibit	O
varying	O
and	O
unstable	O
performance	O
which	O
highly	O
depends	O
on	O
the	O
workload	O
imposed	O
on	O
the	O
system	O
by	O
other	O
VMs	B-Application
.	O
</s>
<s>
This	O
issue	O
can	O
be	O
addressed	O
by	O
appropriate	O
installation	O
techniques	O
for	O
temporal	B-General_Concept
isolation	I-General_Concept
among	I-General_Concept
virtual	I-General_Concept
machines	I-General_Concept
.	O
</s>
<s>
There	O
are	O
several	O
approaches	O
to	O
platform	O
virtualization	B-General_Concept
.	O
</s>
<s>
Examples	O
of	O
virtualization	B-General_Concept
use	O
cases	O
:	O
</s>
<s>
Running	O
one	O
or	O
more	O
applications	O
that	O
are	O
not	O
supported	O
by	O
the	O
host	O
OS	O
:	O
A	O
virtual	B-Architecture
machine	I-Architecture
running	O
the	O
required	O
guest	O
OS	O
could	O
permit	O
the	O
desired	O
applications	O
to	O
run	O
,	O
without	O
altering	O
the	O
host	O
OS	O
.	O
</s>
<s>
Evaluating	O
an	O
alternate	O
operating	B-General_Concept
system	I-General_Concept
:	O
The	O
new	O
OS	O
could	O
be	O
run	O
within	O
a	O
VM	B-Application
,	O
without	O
altering	O
the	O
host	O
OS	O
.	O
</s>
<s>
Server	B-Application
virtualization	B-General_Concept
:	O
Multiple	O
virtual	O
servers	O
could	O
be	O
run	O
on	O
a	O
single	O
physical	O
server	B-Application
,	O
in	O
order	O
to	O
more	O
fully	O
utilize	O
the	O
hardware	O
resources	O
of	O
the	O
physical	O
server	B-Application
.	O
</s>
<s>
Duplicating	O
specific	O
environments	O
:	O
A	O
virtual	B-Architecture
machine	I-Architecture
could	O
,	O
depending	O
on	O
the	O
virtualization	B-General_Concept
software	O
used	O
,	O
be	O
duplicated	O
and	O
installed	O
on	O
multiple	O
hosts	O
,	O
or	O
restored	O
to	O
a	O
previously	O
backed-up	O
system	O
state	O
.	O
</s>
<s>
Creating	O
a	O
protected	O
environment	O
:	O
If	O
a	O
guest	O
OS	O
running	O
on	O
a	O
VM	B-Application
becomes	O
damaged	O
in	O
a	O
way	O
that	O
is	O
not	O
cost-effective	O
to	O
repair	O
,	O
such	O
as	O
may	O
occur	O
when	O
studying	O
malware	O
or	O
installing	O
badly	O
behaved	O
software	O
,	O
the	O
VM	B-Application
may	O
simply	O
be	O
discarded	O
without	O
harm	O
to	O
the	O
host	O
system	O
,	O
and	O
a	O
clean	O
copy	O
used	O
upon	O
rebooting	O
the	O
guest	O
.	O
</s>
<s>
In	O
full	O
virtualization	B-General_Concept
,	O
the	O
virtual	B-Architecture
machine	I-Architecture
simulates	O
enough	O
hardware	O
to	O
allow	O
an	O
unmodified	O
"	O
guest	O
"	O
OS	O
designed	O
for	O
the	O
same	O
instruction	B-General_Concept
set	I-General_Concept
to	O
be	O
run	O
in	O
isolation	O
.	O
</s>
<s>
This	O
approach	O
was	O
pioneered	O
in	O
1966	O
with	O
the	O
IBM	B-Application
CP-40	I-Application
and	O
CP-67	B-Application
,	O
predecessors	O
of	O
the	O
VM	B-Application
family	O
.	O
</s>
<s>
In	O
hardware-assisted	O
virtualization	B-General_Concept
,	O
the	O
hardware	O
provides	O
architectural	O
support	O
that	O
facilitates	O
building	O
a	O
virtual	B-Operating_System
machine	I-Operating_System
monitor	I-Operating_System
and	O
allows	O
guest	O
OSs	O
to	O
be	O
run	O
in	O
isolation	O
.	O
</s>
<s>
Hardware-assisted	O
virtualization	B-General_Concept
was	O
first	O
introduced	O
on	O
the	O
IBM	O
System/370	O
in	O
1972	O
,	O
for	O
use	O
with	O
VM/370	B-Application
,	O
the	O
first	O
virtual	B-Architecture
machine	I-Architecture
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
In	O
2005	O
and	O
2006	O
,	O
Intel	O
and	O
AMD	O
provided	O
additional	O
hardware	O
to	O
support	O
virtualization	B-General_Concept
.	O
</s>
<s>
Sun	O
Microsystems	O
(	O
now	O
Oracle	B-Application
Corporation	I-Application
)	O
added	O
similar	O
features	O
in	O
their	O
UltraSPARC	B-Device
T-Series	I-Device
processors	O
in	O
2005	O
.	O
</s>
<s>
In	O
2006	O
,	O
first-generation	O
32	O
-	O
and	O
64-bit	O
x86	O
hardware	O
support	O
was	O
found	O
to	O
rarely	O
offer	O
performance	O
advantages	O
over	O
software	O
virtualization	B-General_Concept
.	O
</s>
<s>
In	O
paravirtualization	O
,	O
the	O
virtual	B-Architecture
machine	I-Architecture
does	O
not	O
necessarily	O
simulate	O
hardware	O
,	O
but	O
instead	O
(	O
or	O
in	O
addition	O
)	O
offers	O
a	O
special	O
API	O
that	O
can	O
only	O
be	O
used	O
by	O
modifying	O
the	O
"	O
guest	O
"	O
OS	O
.	O
</s>
<s>
This	O
system	O
call	O
to	O
the	O
hypervisor	B-Operating_System
is	O
called	O
a	O
"	O
hypercall	B-Operating_System
"	O
in	O
TRANGO	O
and	O
Xen	B-Operating_System
;	O
it	O
is	O
implemented	O
via	O
a	O
DIAG	O
(	O
"	O
diagnose	O
"	O
)	O
hardware	O
instruction	O
in	O
IBM	O
's	O
CMS	B-Operating_System
under	O
VM	B-Application
(	O
which	O
was	O
the	O
origin	O
of	O
the	O
term	O
hypervisor	B-Operating_System
)	O
..	O
</s>
<s>
In	O
operating-system-level	O
virtualization	B-General_Concept
,	O
a	O
physical	O
server	B-Application
is	O
virtualized	B-General_Concept
at	O
the	O
operating	B-General_Concept
system	I-General_Concept
level	O
,	O
enabling	O
multiple	O
isolated	O
and	O
secure	O
virtualized	B-General_Concept
servers	O
to	O
run	O
on	O
a	O
single	O
physical	O
server	B-Application
.	O
</s>
<s>
The	O
"	O
guest	O
"	O
operating	B-General_Concept
system	I-General_Concept
environments	O
share	O
the	O
same	O
running	O
instance	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
as	O
the	O
host	O
system	O
.	O
</s>
<s>
Thus	O
,	O
the	O
same	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
is	O
also	O
used	O
to	O
implement	O
the	O
"	O
guest	O
"	O
environments	O
,	O
and	O
applications	O
running	O
in	O
a	O
given	O
"	O
guest	O
"	O
environment	O
view	O
it	O
as	O
a	O
stand-alone	O
system	O
.	O
</s>
<s>
A	O
disaster	B-General_Concept
recovery	I-General_Concept
(	O
DR	O
)	O
plan	O
is	O
often	O
considered	O
good	O
practice	O
for	O
a	O
hardware	O
virtualization	B-General_Concept
platform	O
.	O
</s>
<s>
DR	O
of	O
a	O
virtualization	B-General_Concept
environment	O
can	O
ensure	O
high	O
rate	O
of	O
availability	O
during	O
a	O
wide	O
range	O
of	O
situations	O
that	O
disrupt	O
normal	O
business	O
operations	O
.	O
</s>
<s>
In	O
situations	O
where	O
continued	O
operations	O
of	O
hardware	O
virtualization	B-General_Concept
platforms	O
is	O
important	O
,	O
a	O
disaster	B-General_Concept
recovery	I-General_Concept
plan	O
can	O
ensure	O
hardware	O
performance	O
and	O
maintenance	O
requirements	O
are	O
met	O
.	O
</s>
<s>
A	O
hardware	O
virtualization	B-General_Concept
disaster	B-General_Concept
recovery	I-General_Concept
plan	O
involves	O
both	O
hardware	O
and	O
software	O
protection	O
by	O
various	O
methods	O
,	O
including	O
those	O
described	O
below	O
.	O
</s>
<s>
This	O
method	O
ensures	O
the	O
highest	O
level	O
of	O
disaster	B-General_Concept
recovery	I-General_Concept
protection	O
for	O
a	O
hardware	O
virtualization	B-General_Concept
solution	O
,	O
by	O
providing	O
duplicate	O
hardware	O
and	O
software	O
replication	O
in	O
two	O
distinct	O
geographic	O
areas	O
.	O
</s>
