<s>
A	O
versioning	B-Application
file	I-Application
system	I-Application
is	O
any	O
computer	B-Application
file	I-Application
system	I-Application
which	O
allows	O
a	O
computer	B-Operating_System
file	I-Operating_System
to	O
exist	O
in	O
several	O
versions	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
Thus	O
it	O
is	O
a	O
form	O
of	O
revision	B-Architecture
control	I-Architecture
.	O
</s>
<s>
Most	O
common	O
versioning	B-Application
file	I-Application
systems	I-Application
keep	O
a	O
number	O
of	O
old	O
copies	O
of	O
the	O
file	O
.	O
</s>
<s>
Others	O
instead	O
take	O
periodic	O
snapshots	B-Application
whose	O
contents	O
can	O
be	O
accessed	O
with	O
similar	O
semantics	O
to	O
normal	O
file	B-Application
access	I-Application
.	O
</s>
<s>
A	O
versioning	B-Application
file	I-Application
system	I-Application
is	O
similar	O
to	O
a	O
periodic	O
backup	B-Protocol
,	O
with	O
several	O
key	O
differences	O
.	O
</s>
<s>
Backups	B-Protocol
are	O
normally	O
triggered	O
on	O
a	O
timed	O
basis	O
,	O
while	O
versioning	O
occurs	O
when	O
the	O
file	O
changes	O
.	O
</s>
<s>
Backups	B-Protocol
are	O
usually	O
system-wide	O
or	O
partition-wide	O
,	O
while	O
versioning	O
occurs	O
independently	O
on	O
a	O
file-by-file	O
basis	O
.	O
</s>
<s>
Backups	B-Protocol
are	O
normally	O
written	O
to	O
separate	O
media	O
,	O
while	O
versioning	B-Application
file	I-Application
systems	I-Application
write	O
to	O
the	O
same	O
hard	O
drive	O
(	O
and	O
normally	O
the	O
same	O
folder	O
,	O
directory	O
,	O
or	O
local	O
partition	O
)	O
.	O
</s>
<s>
Versioning	B-Application
file	I-Application
systems	I-Application
provide	O
some	O
of	O
the	O
features	O
of	O
revision	B-Architecture
control	I-Architecture
systems	I-Architecture
.	O
</s>
<s>
However	O
,	O
unlike	O
most	O
revision	B-Architecture
control	I-Architecture
systems	I-Architecture
,	O
they	O
are	O
transparent	O
to	O
users	O
,	O
not	O
requiring	O
a	O
separate	O
"	O
commit	O
"	O
step	O
to	O
record	O
a	O
new	O
revision	O
.	O
</s>
<s>
Versioning	B-Application
file	I-Application
systems	I-Application
should	O
not	O
be	O
confused	O
with	O
journaling	B-Application
file	I-Application
systems	I-Application
.	O
</s>
<s>
Whereas	O
journaling	B-Application
file	I-Application
systems	I-Application
work	O
by	O
keeping	O
a	O
log	O
of	O
the	O
changes	O
made	O
to	O
a	O
file	O
before	O
committing	O
those	O
changes	O
to	O
that	O
file	B-Application
system	I-Application
(	O
and	O
overwriting	O
the	O
prior	O
version	O
)	O
,	O
a	O
versioning	B-Application
file	I-Application
system	I-Application
keeps	O
previous	O
copies	O
of	O
a	O
file	O
when	O
saving	O
new	O
changes	O
.	O
</s>
<s>
Some	O
Object	B-Application
storage	I-Application
implementations	O
offers	O
object	O
versioning	O
such	O
as	O
Amazon	B-General_Concept
S3	I-General_Concept
.	O
</s>
<s>
An	O
early	O
implementation	O
of	O
versioning	O
,	O
possibly	O
the	O
first	O
,	O
was	O
in	O
MIT	O
's	O
ITS	B-Application
.	O
</s>
<s>
In	O
ITS	B-Application
,	O
a	O
filename	O
consisted	O
of	O
two	O
six-character	O
parts	O
;	O
if	O
the	O
second	O
part	O
was	O
numeric	O
(	O
consisted	O
only	O
of	O
digits	O
)	O
,	O
it	O
was	O
treated	O
as	O
a	O
version	O
number	O
.	O
</s>
<s>
Another	O
early	O
implementation	O
of	O
versioning	O
was	O
in	O
TENEX	B-Operating_System
,	O
which	O
became	O
TOPS-20	B-Operating_System
.	O
</s>
<s>
A	O
powerful	O
example	O
of	O
a	O
file	O
versioning	B-Architecture
system	I-Architecture
is	O
built	O
into	O
the	O
RSX-11	B-Application
and	O
OpenVMS	B-Operating_System
operating	O
system	O
from	O
Digital	O
Equipment	O
Corporation	O
.	O
</s>
<s>
In	O
essence	O
,	O
whenever	O
an	O
application	O
opens	O
a	O
file	O
for	O
writing	O
,	O
the	O
file	B-Application
system	I-Application
automatically	O
creates	O
a	O
new	O
instance	O
of	O
the	O
file	O
,	O
with	O
a	O
version	O
number	O
appended	O
to	O
the	O
name	O
.	O
</s>
<s>
The	O
"	O
purge	O
"	O
DCL/CCL	B-Operating_System
command	O
can	O
be	O
used	O
at	O
any	O
time	O
to	O
manage	O
the	O
number	O
of	O
versions	O
in	O
a	O
specific	O
directory	O
.	O
</s>
<s>
VMS	B-Operating_System
systems	O
are	O
often	O
scripted	O
to	O
purge	O
user	O
directories	O
on	O
a	O
regular	O
schedule	O
;	O
this	O
is	O
sometimes	O
misconstrued	O
by	O
end-users	O
as	O
a	O
property	O
of	O
the	O
versioning	B-Architecture
system	I-Architecture
.	O
</s>
<s>
NILFS	B-Application
-	O
A	O
log-structured	O
file	B-Application
system	I-Application
supporting	O
versioning	O
of	O
the	O
entire	O
file	B-Application
system	I-Application
and	O
continuous	O
snapshotting	B-Application
.	O
</s>
<s>
Tux3	B-Application
-	O
Most	O
recent	O
change	O
was	O
in	O
2014	O
.	O
</s>
<s>
Next3	B-Application
-	O
Most	O
recent	O
update	O
was	O
in	O
2012	O
.	O
</s>
<s>
ext3cow	B-Application
-	O
Most	O
recent	O
release	O
was	O
in	O
2005	O
.	O
</s>
<s>
The	O
Lisp	O
Machine	O
File	B-Application
System	I-Application
supports	O
versioning	O
.	O
</s>
<s>
Such	O
an	O
operating	O
system	O
was	O
Symbolics	B-Application
Genera	I-Application
.	O
</s>
<s>
Starting	O
with	O
Lion	B-Application
(	O
10.7	B-Application
)	O
,	O
macOS	B-Application
has	O
a	O
feature	O
called	O
Versions	O
which	O
allows	O
Time	O
Machine-like	O
saving	O
and	O
browsing	O
of	O
past	O
versions	O
of	O
documents	O
for	O
applications	O
written	O
to	O
use	O
Versions	O
.	O
</s>
<s>
This	O
functionality	O
,	O
however	O
,	O
takes	O
place	O
at	O
the	O
application	O
layer	O
,	O
not	O
the	O
filesystem	B-Application
layer	O
;	O
Lion	B-Application
and	O
later	O
releases	O
do	O
not	O
incorporate	O
a	O
true	O
versioning	B-Application
file	I-Application
system	I-Application
.	O
</s>
<s>
HTFS	B-Application
,	O
adopted	O
as	O
the	O
primary	O
filesystem	B-Application
for	O
SCO	B-Operating_System
OpenServer	I-Operating_System
in	O
1995	O
,	O
supports	O
file	O
versioning	O
.	O
</s>
<s>
Non-current	O
versions	O
remain	O
in	O
the	O
filesystem	B-Application
namespace	O
,	O
under	O
the	O
name	O
of	O
the	O
original	O
file	O
but	O
with	O
a	O
suffix	O
attached	O
consisting	O
of	O
a	O
semicolon	O
and	O
version	O
sequence	O
number	O
.	O
</s>
<s>
The	O
environment	O
variable	O
and	O
general	O
accessibility	O
allow	O
versions	O
to	O
be	O
managed	O
with	O
the	O
usual	O
filesystem	B-Application
utilities	O
,	O
though	O
there	O
is	O
also	O
an	O
"	O
undelete	O
"	O
command	O
that	O
can	O
be	O
used	O
to	O
purge	O
and	O
restore	O
files	O
,	O
enable	O
and	O
disable	O
versioning	O
on	O
directories	O
,	O
etc	O
.	O
</s>
<s>
Subversion	B-Application
has	O
a	O
feature	O
called	O
"	O
autoversioning	O
"	O
where	O
a	O
WebDAV	B-Protocol
source	O
with	O
a	O
subversion	B-Application
backend	O
can	O
be	O
mounted	O
as	O
a	O
file	B-Application
system	I-Application
on	O
systems	O
that	O
support	O
this	O
kind	O
of	O
mount	O
(	O
Linux	O
,	O
Windows	O
and	O
others	O
do	O
)	O
and	O
saves	O
to	O
that	O
file	B-Application
system	I-Application
generate	O
new	O
revisions	O
on	O
the	O
revision	B-Architecture
control	I-Architecture
system	I-Architecture
.	O
</s>
<s>
The	O
commercial	O
Clearcase	O
configuration	O
management	O
and	O
revision	B-Architecture
control	I-Architecture
software	I-Architecture
has	O
also	O
supported	O
"	O
MVFS	O
"	O
(	O
multi	O
version	O
file	B-Application
system	I-Application
)	O
on	O
HP-UX	O
,	O
AIX	O
and	O
Windows	O
since	O
the	O
early	O
1990s	O
.	O
</s>
<s>
The	O
following	O
are	O
not	O
versioning	B-Application
filesystems	I-Application
,	O
but	O
allow	O
similar	O
functionality	O
.	O
</s>
<s>
APFS	B-Application
and	O
ZFS	B-Application
support	O
instantaneous	O
snapshots	B-Application
and	O
clones	O
.	O
</s>
<s>
Btrfs	B-Operating_System
supports	O
snapshots	B-Application
.	O
</s>
<s>
HAMMER	B-Application
in	O
DragonFlyBSD	O
has	O
the	O
ability	O
to	O
store	O
revisions	O
in	O
the	O
filesystem	B-Application
.	O
</s>
<s>
NILFS	B-Application
,	O
which	O
supports	O
snapshotting	B-Application
.	O
</s>
<s>
Plan	B-Operating_System
9	I-Operating_System
's	O
Fossil	B-Application
file	I-Application
system	I-Application
can	O
provide	O
a	O
similar	O
feature	O
,	O
taking	O
periodic	O
snapshots	B-Application
(	O
often	O
hourly	O
)	O
and	O
making	O
them	O
available	O
in	O
.	O
</s>
<s>
Fossil	B-Application
can	O
forever	O
archive	O
a	O
snapshot	B-Application
into	O
Venti	B-Application
(	O
usually	O
one	O
snapshot	B-Application
each	O
day	O
)	O
and	O
make	O
them	O
available	O
in	O
.	O
</s>
<s>
If	O
multiple	O
changes	O
are	O
made	O
to	O
a	O
file	O
during	O
the	O
interval	O
between	O
snapshots	B-Application
,	O
only	O
the	O
most	O
recent	O
will	O
be	O
recorded	O
in	O
the	O
next	O
snapshot	B-Application
.	O
</s>
<s>
Write	B-Application
Anywhere	I-Application
File	I-Application
Layout	I-Application
-	O
NetApp	O
's	O
storage	O
solutions	O
implement	O
a	O
file	B-Application
system	I-Application
called	O
WAFL	O
,	O
which	O
uses	O
snapshot	B-Application
technology	O
to	O
keep	O
different	O
versions	O
of	O
all	O
files	O
in	O
a	O
volume	O
around	O
.	O
</s>
<s>
pdumpfs	O
,	O
authored	O
by	O
Satoru	O
Takabayashi	O
,	O
is	O
a	O
simple	O
daily	O
backup	B-Protocol
system	O
similar	O
to	O
Plan	B-Operating_System
9	I-Operating_System
's	O
/n/dump	O
,	O
implemented	O
in	O
Ruby	B-Language
.	O
</s>
<s>
It	O
functions	O
as	O
a	O
snapshotting	B-Application
tool	O
,	O
which	O
makes	O
it	O
possible	O
to	O
copy	O
a	O
whole	O
directory	O
to	O
another	O
location	O
by	O
using	O
hardlinks	O
.	O
</s>
<s>
Shadow	O
Copy	O
allows	O
for	O
taking	O
manual	O
or	O
automatic	O
backup	B-Protocol
copies	I-Protocol
or	O
snapshots	B-Application
of	O
a	O
file	O
or	O
folder	O
on	O
a	O
specific	O
volume	O
at	O
a	O
specific	O
point	O
in	O
time	O
.	O
</s>
<s>
RollBack	O
Rx	O
-	O
Allows	O
snapshots	B-Application
of	O
disk	O
partitions	O
to	O
be	O
taken	O
.	O
</s>
<s>
Each	O
snapshot	B-Application
contains	O
only	O
the	O
differences	O
between	O
previous	O
snapshots	B-Application
,	O
and	O
take	O
only	O
seconds	O
to	O
create	O
.	O
</s>
<s>
GoBack	B-Application
(	O
discontinued	O
)	O
-	O
The	O
GoBack	B-Application
software	O
for	O
Windows	O
from	O
Symantec	O
enables	O
reversion	O
of	O
files	O
,	O
directories	O
or	O
disks	O
to	O
previous	O
states	O
.	O
</s>
<s>
It	O
can	O
record	O
a	O
maximum	O
of	O
8GB	O
in	O
changes	O
,	O
and	O
temporarily	O
stops	O
recording	O
each	O
change	O
in	O
the	O
event	O
of	O
high	O
I/O	B-General_Concept
activity	O
.	O
</s>
<s>
Cascade	O
File	B-Application
System	I-Application
exposes	O
a	O
Subversion	B-Application
or	O
Perforce	B-General_Concept
repository	O
via	O
a	O
file	B-Application
system	I-Application
driver	O
.	O
</s>
<s>
git	B-Application
implementation	O
documents	O
call	O
git	B-Application
a	O
"	O
content	O
addressable	O
filesystem	B-Application
with	O
a	O
VCS	O
user	O
interface	O
written	O
on	O
top	O
of	O
it.	O
"	O
</s>
<s>
There	O
's	O
also	O
a	O
3rd-party	O
FUSE	O
implementation	O
exists	O
that	O
may	O
extend	O
git	B-Application
as	O
a	O
mountable	O
,	O
read-write	O
versioning	B-Application
filesystem	I-Application
.	O
</s>
