<s>
OS-level	B-Application
virtualization	I-Application
is	O
an	O
operating	B-General_Concept
system	I-General_Concept
(	O
OS	O
)	O
paradigm	O
in	O
which	O
the	O
kernel	B-Operating_System
allows	O
the	O
existence	O
of	O
multiple	O
isolated	O
user	B-Operating_System
space	I-Operating_System
instances	O
,	O
called	O
containers	B-Application
(	O
LXC	B-Application
,	O
Solaris	B-Application
containers	B-Application
,	O
Docker	B-Language
,	O
Podman	O
)	O
,	O
zones	O
(	O
Solaris	B-Application
containers	B-Application
)	O
,	O
virtual	B-General_Concept
private	I-General_Concept
servers	I-General_Concept
(	O
OpenVZ	B-Language
)	O
,	O
partitions	O
,	O
virtual	O
environments	O
(	O
VEs	O
)	O
,	O
virtual	B-Language
kernels	I-Language
(	O
DragonFly	B-Application
BSD	I-Application
)	O
,	O
or	O
jails	O
(	O
FreeBSD	B-Language
jail	I-Language
or	O
chroot	B-Application
jail	I-Application
)	O
.	O
</s>
<s>
A	O
computer	O
program	O
running	O
on	O
an	O
ordinary	O
operating	B-General_Concept
system	I-General_Concept
can	O
see	O
all	O
resources	O
(	O
connected	O
devices	O
,	O
files	O
and	O
folders	O
,	O
network	B-General_Concept
shares	I-General_Concept
,	O
CPU	B-General_Concept
power	O
,	O
quantifiable	O
hardware	O
capabilities	O
)	O
of	O
that	O
computer	O
.	O
</s>
<s>
However	O
,	O
programs	O
running	O
inside	O
of	O
a	O
container	B-General_Concept
can	O
only	O
see	O
the	O
container	B-General_Concept
's	O
contents	O
and	O
devices	O
assigned	O
to	O
the	O
container	B-General_Concept
.	O
</s>
<s>
On	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
,	O
this	O
feature	O
can	O
be	O
seen	O
as	O
an	O
advanced	O
implementation	O
of	O
the	O
standard	O
chroot	B-Application
mechanism	O
,	O
which	O
changes	O
the	O
apparent	O
root	O
folder	O
for	O
the	O
current	O
running	O
process	O
and	O
its	O
children	O
.	O
</s>
<s>
In	O
addition	O
to	O
isolation	O
mechanisms	O
,	O
the	O
kernel	B-Operating_System
often	O
provides	O
resource-management	O
features	O
to	O
limit	O
the	O
impact	O
of	O
one	O
container	B-General_Concept
's	O
activities	O
on	O
other	O
containers	B-Application
.	O
</s>
<s>
Linux	B-Application
containers	B-Application
are	O
all	O
based	O
on	O
the	O
virtualization	B-General_Concept
,	O
isolation	O
,	O
and	O
resource	O
management	O
mechanisms	O
provided	O
by	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
,	O
notably	O
Linux	B-Architecture
namespaces	I-Architecture
and	O
cgroups	B-Application
.	O
</s>
<s>
The	O
term	O
container	B-General_Concept
,	O
while	O
most	O
popularly	O
referring	O
to	O
OS-level	B-Application
virtualization	I-Application
systems	O
,	O
is	O
sometimes	O
ambiguously	O
used	O
to	O
refer	O
to	O
fuller	O
virtual	B-Architecture
machine	I-Architecture
environments	O
operating	O
in	O
varying	O
degrees	O
of	O
concert	O
with	O
the	O
host	O
OS	O
,	O
e.g.	O
</s>
<s>
Microsoft	O
's	O
Hyper-V	O
containers	B-Application
.	O
</s>
<s>
A	O
more	O
historic	O
overview	O
of	O
virtualization	B-General_Concept
in	O
general	O
since	O
1960	O
can	O
be	O
found	O
in	O
the	O
Timeline	O
of	O
virtualization	B-General_Concept
development	O
.	O
</s>
<s>
On	O
ordinary	O
operating	B-General_Concept
systems	I-General_Concept
for	O
personal	O
computers	O
,	O
a	O
computer	O
program	O
can	O
see	O
(	O
even	O
though	O
it	O
might	O
not	O
be	O
able	O
to	O
access	O
)	O
all	O
the	O
system	O
's	O
resources	O
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
may	O
be	O
able	O
to	O
allow	O
or	O
deny	O
access	O
to	O
such	O
resources	O
based	O
on	O
which	O
program	O
requests	O
them	O
and	O
the	O
user	O
account	O
in	O
the	O
context	O
of	O
which	O
it	O
runs	O
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
may	O
also	O
hide	O
those	O
resources	O
,	O
so	O
that	O
when	O
the	O
computer	O
program	O
enumerates	O
them	O
,	O
they	O
do	O
not	O
appear	O
in	O
the	O
enumeration	O
results	O
.	O
</s>
<s>
Nevertheless	O
,	O
from	O
a	O
programming	O
point	O
of	O
view	O
,	O
the	O
computer	O
program	O
has	O
interacted	O
with	O
those	O
resources	O
and	O
the	O
operating	B-General_Concept
system	I-General_Concept
has	O
managed	O
an	O
act	O
of	O
interaction	O
.	O
</s>
<s>
With	O
operating-system-virtualization	O
,	O
or	O
containerization	B-General_Concept
,	O
it	O
is	O
possible	O
to	O
run	O
programs	O
within	O
containers	B-Application
,	O
to	O
which	O
only	O
parts	O
of	O
these	O
resources	O
are	O
allocated	O
.	O
</s>
<s>
A	O
program	O
expecting	O
to	O
see	O
the	O
whole	O
computer	O
,	O
once	O
run	O
inside	O
a	O
container	B-General_Concept
,	O
can	O
only	O
see	O
the	O
allocated	O
resources	O
and	O
believes	O
them	O
to	O
be	O
all	O
that	O
is	O
available	O
.	O
</s>
<s>
Several	O
containers	B-Application
can	O
be	O
created	O
on	O
each	O
operating	B-General_Concept
system	I-General_Concept
,	O
to	O
each	O
of	O
which	O
a	O
subset	O
of	O
the	O
computer	O
's	O
resources	O
is	O
allocated	O
.	O
</s>
<s>
Each	O
container	B-General_Concept
may	O
contain	O
any	O
number	O
of	O
computer	O
programs	O
.	O
</s>
<s>
Containerization	B-General_Concept
has	O
similarities	O
to	O
application	B-Operating_System
virtualization	I-Operating_System
:	O
In	O
the	O
latter	O
,	O
only	O
one	O
computer	O
program	O
is	O
placed	O
in	O
an	O
isolated	O
container	B-General_Concept
and	O
the	O
isolation	O
applies	O
to	O
file	O
system	O
only	O
.	O
</s>
<s>
Operating-system-level	B-Application
virtualization	I-Application
is	O
commonly	O
used	O
in	O
virtual	B-Architecture
hosting	I-Architecture
environments	O
,	O
where	O
it	O
is	O
useful	O
for	O
securely	O
allocating	O
finite	O
hardware	O
resources	O
among	O
a	O
large	O
number	O
of	O
mutually-distrusting	O
users	O
.	O
</s>
<s>
System	O
administrators	O
may	O
also	O
use	O
it	O
for	O
consolidating	O
server	O
hardware	O
by	O
moving	O
services	O
on	O
separate	O
hosts	O
into	O
containers	B-Application
on	O
the	O
one	O
server	O
.	O
</s>
<s>
Other	O
typical	O
scenarios	O
include	O
separating	O
several	O
programs	O
to	O
separate	O
containers	B-Application
for	O
improved	O
security	O
,	O
hardware	O
independence	O
,	O
and	O
added	O
resource	O
management	O
features	O
.	O
</s>
<s>
The	O
improved	O
security	O
provided	O
by	O
the	O
use	O
of	O
a	O
chroot	B-Application
mechanism	O
,	O
however	O
,	O
is	O
not	O
perfect	O
.	O
</s>
<s>
Operating-system-level	B-Application
virtualization	I-Application
implementations	O
capable	O
of	O
live	O
migration	O
can	O
also	O
be	O
used	O
for	O
dynamic	O
load	O
balancing	O
of	O
containers	B-Application
between	O
nodes	O
in	O
a	O
cluster	O
.	O
</s>
<s>
Operating-system-level	B-Application
virtualization	I-Application
usually	O
imposes	O
less	O
overhead	O
than	O
full	B-Architecture
virtualization	I-Architecture
because	O
programs	O
in	O
OS-level	O
virtual	O
partitions	O
use	O
the	O
operating	B-General_Concept
system	I-General_Concept
's	O
normal	O
system	B-Operating_System
call	I-Operating_System
interface	O
and	O
do	O
not	O
need	O
to	O
be	O
subjected	O
to	O
emulation	B-Application
or	O
be	O
run	O
in	O
an	O
intermediate	O
virtual	B-Architecture
machine	I-Architecture
,	O
as	O
is	O
the	O
case	O
with	O
full	B-Architecture
virtualization	I-Architecture
(	O
such	O
as	O
VMware	B-Operating_System
ESXi	I-Operating_System
,	O
QEMU	B-Application
,	O
or	O
Hyper-V	O
)	O
and	O
paravirtualization	O
(	O
such	O
as	O
Xen	B-Operating_System
or	O
User-mode	B-Application
Linux	I-Application
)	O
.	O
</s>
<s>
This	O
form	O
of	O
virtualization	B-General_Concept
also	O
does	O
not	O
require	O
hardware	O
support	O
for	O
efficient	O
performance	O
.	O
</s>
<s>
Operating-system-level	B-Application
virtualization	I-Application
is	O
not	O
as	O
flexible	O
as	O
other	O
virtualization	B-General_Concept
approaches	O
since	O
it	O
cannot	O
host	O
a	O
guest	O
operating	B-General_Concept
system	I-General_Concept
different	O
from	O
the	O
host	O
one	O
,	O
or	O
a	O
different	O
guest	O
kernel	B-Operating_System
.	O
</s>
<s>
For	O
example	O
,	O
with	O
Linux	B-Application
,	O
different	O
distributions	O
are	O
fine	O
,	O
but	O
other	O
operating	B-General_Concept
systems	I-General_Concept
such	O
as	O
Windows	B-Application
cannot	O
be	O
hosted	O
.	O
</s>
<s>
Operating	B-General_Concept
systems	I-General_Concept
using	O
variable	O
input	O
systematics	O
are	O
subject	O
to	O
limitations	O
within	O
the	O
virtualized	B-General_Concept
architecture	O
.	O
</s>
<s>
Solaris	B-Application
partially	O
overcomes	O
the	O
limitation	O
described	O
above	O
with	O
its	O
branded	O
zones	O
feature	O
,	O
which	O
provides	O
the	O
ability	O
to	O
run	O
an	O
environment	O
within	O
a	O
container	B-General_Concept
that	O
emulates	B-Application
an	O
older	O
Solaris	B-Application
8	I-Application
or	O
9	O
version	O
in	O
a	O
Solaris	B-Application
10	I-Application
host	O
.	O
</s>
<s>
Linux	B-Application
branded	O
zones	O
(	O
referred	O
to	O
as	O
"	O
lx	O
"	O
branded	O
zones	O
)	O
are	O
also	O
available	O
on	O
x86-based	O
Solaris	B-Application
systems	O
,	O
providing	O
a	O
complete	O
Linux	B-Application
user	B-Operating_System
space	I-Operating_System
and	O
support	O
for	O
the	O
execution	O
of	O
Linux	B-Application
applications	O
;	O
additionally	O
,	O
Solaris	B-Application
provides	O
utilities	O
needed	O
to	O
install	O
Red	O
Hat	O
Enterprise	O
Linux3.x	O
or	O
CentOS3.x	O
Linux	B-Application
distributions	I-Application
inside	O
"	O
lx	O
"	O
zones	O
.	O
</s>
<s>
However	O
,	O
in	O
2010	O
Linux	B-Application
branded	O
zones	O
were	O
removed	O
from	O
Solaris	B-Application
;	O
in	O
2014	O
they	O
were	O
reintroduced	O
in	O
Illumos	B-Application
,	O
which	O
is	O
the	O
open	O
source	O
Solaris	B-Application
fork	O
,	O
supporting	O
32-bit	O
Linux	B-Operating_System
kernels	I-Operating_System
.	O
</s>
<s>
Some	O
implementations	O
provide	O
file-level	O
copy-on-write	B-Language
(	O
CoW	O
)	O
mechanisms	O
.	O
</s>
<s>
This	O
is	O
easier	O
to	O
back	O
up	O
,	O
more	O
space-efficient	O
and	O
simpler	O
to	O
cache	O
than	O
the	O
block-level	O
copy-on-write	B-Language
schemes	O
common	O
on	O
whole-system	O
virtualizers	O
.	O
</s>
<s>
Mechanism	O
Operating	B-General_Concept
system	I-General_Concept
License	O
Actively	O
developed	O
since	O
or	O
between	O
FeaturesFile	O
system	O
isolationCopy	O
on	O
writeDisk	O
quotasI/O	O
rate	O
limitingMemory	O
limitsCPU	O
quotasNetwork	O
isolationNested	O
virtualizationPartition	O
checkpointing	O
and	O
live	O
migrationRoot	O
privilege	O
isolationchroot	O
Most	O
UNIX-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
Varies	O
by	O
operating	B-General_Concept
system	I-General_Concept
1982	O
DockerLinux	O
,	O
FreeBSD	B-Operating_System
,	O
Windows	B-Application
x64	O
macOS	B-Application
2013	O
Linux-VServer(security context )	O
Linux	B-Application
,	O
Windows	B-Device
Server	I-Device
2016	I-Device
2001	O
lmctfy	B-Application
Linux	B-Application
20132015	O
LXC	B-Application
Linux	B-Application
2008	O
Singularity	B-Language
Linux	B-Application
2015	O
OpenVZLinux	O
2005	O
VirtuozzoLinux	O
,	O
Windows	B-Application
2000	O
Solaris	B-Application
Containers	B-Application
(	O
Zones	O
)	O
illumos	B-Application
(	O
OpenSolaris	B-Operating_System
)	O
,	O
Solaris	B-Application
,	O
Proprietary	B-Application
2004	O
(	O
ZFS	O
)	O
Network	O
virtualization	B-General_Concept
and	O
resource	O
control	O
(	O
Crossbow	O
)	O
FAQ	O
FreeBSD	B-Operating_System
jailFreeBSD	O
,	O
DragonFly	B-Application
BSD	I-Application
2000	O
(	O
ZFS	O
)	O
vkernelDragonFly	O
BSD	O
2006	O
?	O
</s>
<s>
Linux	B-Application
containers	B-Application
not	O
listed	O
above	O
include	O
:	O
</s>
