<s>
In	O
computer	O
systems	O
,	O
a	O
snapshot	B-Application
is	O
the	O
state	B-Application
of	O
a	O
system	O
at	O
a	O
particular	O
point	O
in	O
time	O
.	O
</s>
<s>
A	O
full	B-Protocol
backup	I-Protocol
of	O
a	O
large	O
data	O
set	O
may	O
take	O
a	O
long	O
time	O
to	O
complete	O
.	O
</s>
<s>
On	O
multi-tasking	B-Operating_System
or	O
multi-user	B-Operating_System
systems	I-Operating_System
,	O
there	O
may	O
be	O
writes	O
to	O
that	O
data	O
while	O
it	O
is	O
being	O
backed	O
up	O
.	O
</s>
<s>
This	O
prevents	O
the	O
backup	B-Protocol
from	O
being	O
atomic	B-General_Concept
and	O
introduces	O
a	O
version	O
skew	O
that	O
may	O
result	O
in	O
data	O
corruption	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
a	O
user	O
moves	O
a	O
file	O
into	O
a	O
directory	O
that	O
has	O
already	O
been	O
backed	O
up	O
,	O
then	O
that	O
file	O
would	O
be	O
completely	O
missing	O
on	O
the	O
backup	B-General_Concept
media	I-General_Concept
,	O
since	O
the	O
backup	B-Protocol
operation	O
had	O
already	O
taken	O
place	O
before	O
the	O
addition	O
of	O
the	O
file	O
.	O
</s>
<s>
One	O
approach	O
to	O
safely	O
backing	O
up	O
live	O
data	O
is	O
to	O
temporarily	O
disable	O
write	O
access	O
to	O
data	O
during	O
the	O
backup	B-Protocol
,	O
either	O
by	O
stopping	O
the	O
accessing	O
applications	O
or	O
by	O
using	O
the	O
locking	B-Operating_System
API	B-General_Concept
provided	O
by	O
the	O
operating	O
system	O
to	O
enforce	O
exclusive	O
read	O
access	O
.	O
</s>
<s>
To	O
avoid	O
downtime	O
,	O
high-availability	O
systems	O
may	O
instead	O
perform	O
the	O
backup	B-Protocol
on	O
a	O
snapshot	B-Application
—	O
a	O
read-only	O
copy	O
of	O
the	O
data	O
set	O
frozen	O
at	O
a	O
point	O
in	O
time	O
—	O
and	O
allow	O
applications	O
to	O
continue	O
writing	O
to	O
their	O
data	O
.	O
</s>
<s>
Most	O
snapshot	B-Application
implementations	O
are	O
efficient	O
and	O
can	O
create	O
snapshots	B-Application
in	O
O(1 )	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
the	O
time	O
and	O
I/O	O
needed	O
to	O
create	O
the	O
snapshot	B-Application
does	O
not	O
increase	O
with	O
the	O
size	O
of	O
the	O
data	O
set	O
;	O
by	O
contrast	O
,	O
the	O
time	O
and	O
I/O	O
required	O
for	O
a	O
direct	O
backup	B-Protocol
is	O
proportional	O
to	O
the	O
size	O
of	O
the	O
data	O
set	O
.	O
</s>
<s>
In	O
some	O
systems	O
once	O
the	O
initial	O
snapshot	B-Application
is	O
taken	O
of	O
a	O
data	O
set	O
,	O
subsequent	O
snapshots	B-Application
copy	O
the	O
changed	O
data	O
only	O
,	O
and	O
use	O
a	O
system	O
of	O
pointers	O
to	O
reference	O
the	O
initial	O
snapshot	B-Application
.	O
</s>
<s>
This	O
method	O
of	O
pointer-based	O
snapshots	B-Application
consumes	O
less	O
disk	B-General_Concept
capacity	I-General_Concept
than	O
if	O
the	O
data	O
set	O
was	O
repeatedly	O
cloned	O
.	O
</s>
<s>
Some	O
Unix	B-Application
systems	I-Application
have	O
snapshot-capable	O
logical	B-Application
volume	I-Application
managers	I-Application
.	O
</s>
<s>
These	O
implement	O
copy-on-write	B-Language
on	O
entire	O
block	O
devices	O
by	O
copying	O
changed	O
blocksjust	O
before	O
they	O
are	O
to	O
be	O
overwritten	O
within	O
"	O
parent	O
"	O
volumesto	O
other	O
storage	O
,	O
thus	O
preserving	O
a	O
self-consistent	O
past	O
image	O
of	O
the	O
block	O
device	O
.	O
</s>
<s>
Filesystems	O
on	O
such	O
snapshot	B-Application
images	O
can	O
later	O
be	O
mounted	O
as	O
if	O
they	O
were	O
on	O
a	O
read-only	O
media	O
.	O
</s>
<s>
Some	O
volume	B-Application
managers	I-Application
also	O
allow	O
creation	O
of	O
writable	O
snapshots	B-Application
,	O
extending	O
the	O
copy-on-write	B-Language
approach	O
by	O
disassociating	O
any	O
blocks	O
modified	O
within	O
the	O
snapshot	B-Application
from	O
their	O
"	O
parent	O
"	O
blocks	O
in	O
the	O
original	O
volume	O
.	O
</s>
<s>
Such	O
a	O
scheme	O
could	O
be	O
also	O
described	O
as	O
performing	O
additional	O
copy-on-write	B-Language
operations	O
triggered	O
by	O
the	O
writes	O
to	O
snapshots	B-Application
.	O
</s>
<s>
On	O
Linux	B-Application
,	O
Logical	B-Application
Volume	I-Application
Manager	I-Application
(	O
LVM	O
)	O
allows	O
creation	O
of	O
both	O
read-only	O
and	O
read-write	O
snapshots	B-Application
.	O
</s>
<s>
Writable	O
snapshots	B-Application
were	O
introduced	O
with	O
the	O
LVM	O
version	O
2	O
(	O
LVM2	B-Application
)	O
.	O
</s>
<s>
Some	O
file	O
systems	O
,	O
such	O
as	O
WAFL	B-Application
,	O
fossil	B-Application
for	O
Plan	B-Operating_System
9	I-Operating_System
from	I-Operating_System
Bell	I-Operating_System
Labs	I-Operating_System
,	O
and	O
ODS-5	B-Application
,	O
internally	O
track	O
old	O
versions	O
of	O
files	O
and	O
make	O
snapshots	B-Application
available	O
through	O
a	O
special	O
namespace	O
.	O
</s>
<s>
Others	O
,	O
like	O
UFS2	B-Operating_System
,	O
provide	O
an	O
operating	O
system	O
API	B-General_Concept
for	O
accessing	O
file	O
histories	O
.	O
</s>
<s>
In	O
NTFS	B-Application
,	O
access	O
to	O
snapshots	B-Application
is	O
provided	O
by	O
the	O
Volume	O
Shadow-copying	O
Service	O
(	O
VSS	O
)	O
in	O
Windows	B-Application
XP	I-Application
and	O
Windows	B-Application
Server	I-Application
2003	I-Application
and	O
Shadow	O
Copy	O
in	O
Windows	B-Application
Vista	I-Application
.	O
</s>
<s>
Melio	O
FS	O
provides	O
snapshots	B-Application
via	O
the	O
same	O
VSS	O
interface	O
for	O
shared	O
storage	O
.	O
</s>
<s>
Snapshots	B-Application
have	O
also	O
been	O
available	O
in	O
the	O
NSS	B-Application
(	O
Novell	B-Application
Storage	I-Application
Services	I-Application
)	O
file	O
system	O
on	O
NetWare	B-Operating_System
since	O
version	O
4.11	O
,	O
and	O
more	O
recently	O
on	O
Linux	B-Application
platforms	O
in	O
the	O
Open	B-Application
Enterprise	I-Application
Server	I-Application
product	O
.	O
</s>
<s>
EMC	O
's	O
Isilon	O
OneFS	O
clustered	O
storage	O
platform	O
implements	O
a	O
single	O
scalable	O
file	O
system	O
that	O
supports	O
read-only	O
snapshots	B-Application
at	O
the	O
file	O
or	O
directory	O
level	O
.	O
</s>
<s>
Any	O
file	O
or	O
directory	O
within	O
the	O
file	O
system	O
can	O
be	O
snapshotted	O
and	O
the	O
system	O
will	O
implement	O
a	O
copy-on-write	B-Language
or	O
point-in-time	O
snapshot	B-Application
dynamically	O
based	O
on	O
which	O
method	O
is	O
determined	O
to	O
be	O
optimal	O
for	O
the	O
system	O
.	O
</s>
<s>
On	O
Linux	B-Application
,	O
the	O
Btrfs	B-Operating_System
and	O
OCFS2	B-Application
file	O
systems	O
support	O
creating	O
snapshots	B-Application
(	O
cloning	O
)	O
of	O
individual	O
files	O
.	O
</s>
<s>
Additionally	O
,	O
Btrfs	B-Operating_System
also	O
supports	O
the	O
creation	O
of	O
snapshots	B-Application
of	O
subvolumes	O
.	O
</s>
<s>
On	O
AIX	O
,	O
JFS2	B-Application
also	O
support	O
snapshots	B-Application
.	O
</s>
<s>
Sun	O
Microsystems	O
ZFS	B-Application
has	O
a	O
hybrid	O
implementation	O
which	O
tracks	O
read-write	O
snapshots	B-Application
at	O
the	O
block	O
level	O
,	O
but	O
makes	O
branched	O
file	O
sets	O
nameable	O
to	O
user	O
applications	O
as	O
"	O
clones	O
"	O
.	O
</s>
