<s>
Linux-VServer	B-Application
is	O
a	O
virtual	B-General_Concept
private	I-General_Concept
server	I-General_Concept
implementation	O
that	O
was	O
created	O
by	O
adding	O
operating	B-Application
system-level	I-Application
virtualization	I-Application
capabilities	O
to	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
It	O
is	O
developed	O
and	O
distributed	O
as	O
open-source	B-Application
software	I-Application
.	O
</s>
<s>
It	O
is	O
not	O
related	O
to	O
the	O
Linux	B-Operating_System
Virtual	I-Operating_System
Server	I-Operating_System
project	O
,	O
which	O
implements	O
network	O
load	B-Application
balancing	I-Application
.	O
</s>
<s>
Linux-VServer	B-Application
is	O
a	O
jail	B-Application
mechanism	I-Application
in	O
that	O
it	O
can	O
be	O
used	O
to	O
securely	O
partition	O
resources	O
on	O
a	O
computer	O
system	O
(	O
such	O
as	O
the	O
file	B-Application
system	I-Application
,	O
CPU	O
time	O
,	O
network	O
addresses	O
and	O
memory	O
)	O
in	O
such	O
a	O
way	O
that	O
processes	B-Operating_System
cannot	O
mount	O
a	O
denial-of-service	O
attack	O
on	O
anything	O
outside	O
their	O
partition	O
.	O
</s>
<s>
Each	O
partition	O
is	O
called	O
a	O
security	O
context	O
,	O
and	O
the	O
virtualized	O
system	O
within	O
it	O
is	O
the	O
virtual	B-General_Concept
private	I-General_Concept
server	I-General_Concept
.	O
</s>
<s>
A	O
chroot-like	O
utility	O
for	O
descending	O
into	O
security	O
contexts	O
is	O
provided	O
.	O
</s>
<s>
Booting	O
a	O
virtual	B-General_Concept
private	I-General_Concept
server	I-General_Concept
is	O
then	O
simply	O
a	O
matter	O
of	O
kickstarting	O
init	B-Operating_System
in	O
a	O
new	O
security	O
context	O
;	O
likewise	O
,	O
shutting	O
it	O
down	O
simply	O
entails	O
killing	O
all	O
processes	B-Operating_System
with	O
that	O
security	O
context	O
.	O
</s>
<s>
The	O
contexts	O
themselves	O
are	O
robust	O
enough	O
to	O
boot	O
many	O
Linux	B-Application
distributions	I-Application
unmodified	O
,	O
including	O
Debian	O
and	O
Fedora	O
.	O
</s>
<s>
Virtual	B-General_Concept
private	I-General_Concept
servers	I-General_Concept
are	O
commonly	O
used	O
in	O
web	O
hosting	O
services	O
,	O
where	O
they	O
are	O
useful	O
for	O
segregating	O
customer	O
accounts	O
,	O
pooling	O
resources	O
and	O
containing	O
any	O
potential	O
security	O
breaches	O
.	O
</s>
<s>
To	O
save	O
space	O
on	O
such	O
installations	O
,	O
each	O
virtual	O
server	O
's	O
file	B-Application
system	I-Application
can	O
be	O
created	O
as	O
a	O
tree	O
of	O
copy-on-write	B-Language
hard	B-Application
links	I-Application
to	O
a	O
"	O
template	O
"	O
file	B-Application
system	I-Application
.	O
</s>
<s>
The	O
hard	B-Application
link	I-Application
is	O
marked	O
with	O
a	O
special	O
filesystem	B-Application
attribute	O
and	O
when	O
modified	O
,	O
is	O
securely	O
and	O
transparently	O
replaced	O
with	O
a	O
real	O
copy	O
of	O
the	O
file	O
.	O
</s>
<s>
Linux-VServer	B-Application
provides	O
two	O
branches	O
,	O
stable	O
(	O
2.2.x	O
)	O
,	O
and	O
devel	O
(	O
2.3.x	O
)	O
for	O
2.6-series	O
kernels	B-Operating_System
and	O
a	O
single	O
stable	O
branch	O
for	O
2.4-series	O
.	O
</s>
<s>
Virtual	O
servers	O
share	O
the	O
same	O
system	B-Operating_System
call	I-Operating_System
interface	O
and	O
do	O
not	O
have	O
any	O
emulation	B-Application
overhead	O
.	O
</s>
<s>
Virtual	O
servers	O
do	O
not	O
have	O
to	O
be	O
backed	O
by	O
opaque	O
disk	B-Application
images	I-Application
,	O
but	O
can	O
share	O
a	O
common	O
file	B-Application
system	I-Application
and	O
common	O
sets	O
of	O
files	O
(	O
through	O
copy-on-write	B-Language
hard	B-Application
links	I-Application
)	O
.	O
</s>
<s>
Processes	B-Operating_System
within	O
the	O
virtual	O
server	O
run	O
as	O
regular	O
processes	B-Operating_System
on	O
the	O
host	O
system	O
.	O
</s>
<s>
This	O
is	O
somewhat	O
more	O
memory-efficient	O
and	O
I/O	O
-efficient	O
than	O
whole-system	O
emulation	B-Application
,	O
although	O
memory	O
ballooning	O
and	O
modern	O
VMs	O
allow	O
returning	O
unused	O
memory	O
and	O
sharing	O
disk	O
cache	O
with	O
the	O
host	O
and	O
other	O
virtual	O
servers	O
.	O
</s>
<s>
Processes	B-Operating_System
within	O
the	O
virtual	O
server	O
are	O
queued	O
on	O
the	O
same	O
scheduler	O
as	O
on	O
the	O
host	O
,	O
allowing	O
guest	O
's	O
processes	B-Operating_System
to	O
run	O
concurrently	O
on	O
SMP	B-Operating_System
systems	O
.	O
</s>
<s>
This	O
is	O
not	O
trivial	O
to	O
implement	O
with	O
whole-system	O
emulation	B-Application
.	O
</s>
<s>
Only	O
one	O
kernel	B-Operating_System
with	O
small	O
additional	O
code-base	O
compared	O
to	O
2+	O
kernels	B-Operating_System
and	O
large	O
interfaces	O
between	O
them	O
.	O
</s>
<s>
Rich	O
Linux	B-Operating_System
scheduling	O
features	O
such	O
as	O
real-time	O
priorities	O
.	O
</s>
<s>
Requires	O
that	O
the	O
host	O
kernel	B-Operating_System
be	O
patched	O
.	O
</s>
<s>
No	O
clustering	B-Architecture
or	O
process	B-Operating_System
migration	I-Operating_System
capability	O
is	O
included	O
,	O
so	O
the	O
host	O
kernel	B-Operating_System
and	O
host	O
computer	O
is	O
still	O
a	O
single	O
point	O
of	O
failure	O
for	O
all	O
virtual	O
servers	O
.	O
</s>
<s>
Some	O
system	B-Operating_System
calls	I-Operating_System
(	O
mostly	O
hardware-related	O
:	O
e.g.	O
</s>
<s>
real-time	O
clock	O
)	O
and	O
parts	O
of	O
the	O
/proc	B-Application
and	O
/sys	B-Application
filesystems	B-Application
are	O
left	O
unvirtualized	O
.	O
</s>
