<s>
Checkpoint/Restore	O
In	O
Userspace	B-Operating_System
(	O
CRIU	B-Application
)	O
(	O
pronounced	O
kree-oo	O
,	O
)	O
,	O
is	O
a	O
software	O
tool	O
for	O
the	O
Linux	B-Application
operating	I-Application
system	I-Application
.	O
</s>
<s>
Using	O
this	O
tool	O
,	O
it	O
is	O
possible	O
to	O
freeze	O
a	O
running	O
application	O
(	O
or	O
part	O
of	O
it	O
)	O
and	O
checkpoint	B-General_Concept
it	I-General_Concept
to	I-General_Concept
persistent	I-General_Concept
storage	I-General_Concept
as	O
a	O
collection	O
of	O
files	O
.	O
</s>
<s>
The	O
distinctive	O
feature	O
of	O
the	O
CRIU	B-Application
project	O
is	O
that	O
it	O
is	O
mainly	O
implemented	O
in	O
user	B-Operating_System
space	I-Operating_System
,	O
rather	O
than	O
in	O
the	O
kernel	O
.	O
</s>
<s>
The	O
initial	O
version	O
of	O
CRIU	B-Application
software	O
was	O
presented	O
to	O
the	O
Linux	B-Application
developers	O
community	O
by	O
Pavel	O
Emelyanov	O
,	O
the	O
OpenVZ	B-Language
kernel	O
team	O
leader	O
,	O
on	O
15	O
July	O
2011	O
.	O
</s>
<s>
In	O
September	O
2011	O
,	O
the	O
project	O
was	O
presented	O
at	O
the	O
Linux	B-Application
Plumbers	O
Conference	O
.	O
</s>
<s>
The	O
CRIU	B-Application
tool	O
is	O
being	O
developed	O
as	O
part	O
of	O
the	O
OpenVZ	B-Language
project	O
,	O
with	O
the	O
aim	O
of	O
replacing	O
the	O
in-kernel	O
checkpoint/restore	O
.	O
</s>
<s>
Though	O
its	O
main	O
focus	O
is	O
to	O
support	O
the	O
migration	O
of	O
containers	O
,	O
allowing	O
users	O
to	O
check-point	O
and	O
restore	O
the	O
current	O
state	O
of	O
running	O
processes	B-Operating_System
and	O
process	B-Operating_System
groups	I-Operating_System
.	O
</s>
<s>
The	O
tool	O
can	O
currently	O
be	O
used	O
on	O
x86-64	B-Device
and	O
ARM	B-Architecture
systems	O
and	O
supports	O
the	O
following	O
features	O
:	O
</s>
<s>
Processes	B-Operating_System
:	O
their	O
hierarchy	B-Operating_System
,	O
PIDs	B-Operating_System
,	O
user	O
and	O
group	O
authenticators	O
(	O
UID	B-Application
,	O
GID	B-Application
,	O
SID	B-Protocol
,	O
etc	O
.	O
</s>
<s>
,	O
no	O
kernel	O
patching	O
is	O
required	O
because	O
all	O
of	O
the	O
required	O
functionality	O
has	O
already	O
been	O
merged	O
into	O
the	O
Linux	B-Application
kernel	O
mainline	O
since	O
kernel	O
version	O
3.11	O
,	O
which	O
was	O
released	O
on	O
September	O
2	O
,	O
2013	O
.	O
</s>
<s>
One	O
of	O
the	O
initial	O
project	O
goals	O
was	O
to	O
support	O
the	O
migration	O
of	O
TCP	B-Protocol
connections	O
,	O
the	O
biggest	O
challenge	O
being	O
to	O
suspend	O
and	O
then	O
restore	O
only	O
one	O
side	O
of	O
a	O
connection	O
.	O
</s>
<s>
This	O
was	O
necessary	O
for	O
performing	O
the	O
live	O
migration	O
of	O
containers	O
(	O
along	O
with	O
all	O
their	O
active	O
network	O
connections	O
)	O
between	O
physical	O
servers	O
,	O
the	O
main	O
scenario	O
of	O
using	O
the	O
checkpoint/restore	O
feature	O
in	O
OpenVZ	B-Language
.	O
</s>
<s>
To	O
cope	O
with	O
this	O
problem	O
,	O
a	O
new	O
feature	O
,	O
"	O
TCP	B-Protocol
repair	O
mode	O
"	O
,	O
was	O
implemented	O
.	O
</s>
<s>
The	O
feature	O
was	O
included	O
in	O
version	O
3.5	O
of	O
the	O
Linux	B-Application
kernel	O
mainline	O
and	O
provides	O
users	O
with	O
additional	O
means	O
to	O
disassemble	O
and	O
reconstruct	O
TCP	B-Protocol
sockets	I-Protocol
without	O
the	O
necessity	O
of	O
exchanging	O
network	O
packets	O
with	O
the	O
opposite	O
side	O
of	O
the	O
connection	O
.	O
</s>
