<s>
In	O
the	O
context	O
of	O
virtualization	B-General_Concept
,	O
where	O
a	O
guest	O
simulation	O
of	O
an	O
entire	O
computer	O
is	O
actually	O
merely	O
a	O
software	O
virtual	B-Architecture
machine	I-Architecture
(	O
VM	O
)	O
running	O
on	O
a	O
host	O
computer	O
under	O
a	O
hypervisor	B-Operating_System
,	O
migration	O
(	O
also	O
known	O
as	O
teleportation	B-General_Concept
)	O
is	O
the	O
process	O
by	O
which	O
a	O
running	O
virtual	B-Architecture
machine	I-Architecture
is	O
moved	O
from	O
one	O
physical	O
host	O
to	O
another	O
,	O
with	O
little	O
or	O
no	O
disruption	O
in	O
service	O
.	O
</s>
<s>
Virtualization	B-General_Concept
is	O
far	O
more	O
frequently	O
used	O
with	O
network	O
services	O
and	O
user	O
applications	O
,	O
and	O
these	O
can	O
generally	O
tolerate	O
the	O
brief	O
delays	O
which	O
may	O
be	O
involved	O
.	O
</s>
<s>
The	O
perceived	O
impact	O
,	O
if	O
any	O
,	O
is	O
similar	O
to	O
a	O
longer-than-usual	O
kernel	B-Operating_System
delay	O
.	O
</s>
<s>
The	O
actual	O
process	O
is	O
heavily	O
dependent	O
on	O
the	O
particular	O
virtualization	B-General_Concept
package	O
in	O
use	O
,	O
but	O
in	O
general	O
,	O
the	O
process	O
is	O
as	O
follows	O
:	O
</s>
<s>
Regular	O
snapshots	B-Application
of	O
the	O
VM	O
(	O
its	O
simulated	O
hard	B-Device
disk	I-Device
storage	I-Device
,	O
its	O
memory	B-Architecture
,	O
and	O
its	O
virtual	O
peripherals	O
)	O
are	O
taken	O
in	O
the	O
background	O
by	O
the	O
hypervisor	B-Operating_System
,	O
or	O
by	O
a	O
set	O
of	O
administrative	O
scripts	O
.	O
</s>
<s>
Each	O
new	O
snapshot	B-Application
adds	O
a	O
differential	O
overlay	O
file	O
to	O
the	O
top	O
of	O
a	O
stack	O
that	O
,	O
as	O
a	O
whole	O
,	O
fully	O
describes	O
the	O
machine	O
.	O
</s>
<s>
When	O
a	O
migration	O
operation	O
is	O
requested	O
,	O
the	O
virtual	B-Architecture
machine	I-Architecture
is	O
paused	O
,	O
and	O
its	O
current	O
state	O
is	O
saved	O
to	O
disk	O
.	O
</s>
<s>
The	O
hypervisor	B-Operating_System
on	O
the	O
new	O
host	O
resumes	O
the	O
guest	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
Note	O
that	O
in	O
practice	O
,	O
regular	O
maintenance	O
operations	O
are	O
required	O
to	O
"	O
merge	O
down	O
"	O
the	O
snapshot	B-Application
stack	O
into	O
a	O
more	O
manageable	O
number	O
of	O
files	O
,	O
to	O
improve	O
performance	O
and	O
optimize	O
storage	O
(	O
redundant	O
overwrites	O
are	O
merged	O
out	O
)	O
.	O
</s>
<s>
Migration	O
is	O
similar	O
to	O
the	O
failover	B-General_Concept
capability	O
some	O
virtualization	B-General_Concept
suites	O
provide	O
.	O
</s>
<s>
In	O
true	O
failover	B-General_Concept
,	O
the	O
host	O
may	O
have	O
suddenly	O
completely	O
failed	O
,	O
which	O
precludes	O
the	O
latest	O
state	O
of	O
the	O
VM	O
having	O
been	O
copied	O
to	O
the	O
backup	O
host	O
.	O
</s>
