<s>
Unionfs	B-Application
is	O
a	O
filesystem	B-Application
service	O
for	O
Linux	B-Application
,	O
FreeBSD	B-Operating_System
and	O
NetBSD	B-Device
which	O
implements	O
a	O
union	B-Application
mount	I-Application
for	O
other	O
file	B-Application
systems	I-Application
.	O
</s>
<s>
It	O
allows	O
files	O
and	O
directories	O
of	O
separate	O
file	B-Application
systems	I-Application
,	O
known	O
as	O
branches	O
,	O
to	O
be	O
transparently	O
overlaid	O
,	O
forming	O
a	O
single	O
coherent	O
file	B-Application
system	I-Application
.	O
</s>
<s>
Contents	O
of	O
directories	O
which	O
have	O
the	O
same	O
path	O
within	O
the	O
merged	O
branches	O
will	O
be	O
seen	O
together	O
in	O
a	O
single	O
merged	O
directory	O
,	O
within	O
the	O
new	O
,	O
virtual	O
filesystem	B-Application
.	O
</s>
<s>
The	O
different	O
branches	O
may	O
be	O
either	O
read-only	O
or	O
read/write	O
file	B-Application
systems	I-Application
,	O
so	O
that	O
writes	O
to	O
the	O
virtual	O
,	O
merged	O
copy	O
are	O
directed	O
to	O
a	O
specific	O
real	O
file	B-Application
system	I-Application
.	O
</s>
<s>
This	O
allows	O
a	O
file	B-Application
system	I-Application
to	O
appear	O
as	O
writable	O
,	O
but	O
without	O
actually	O
allowing	O
writes	O
to	O
change	O
the	O
file	B-Application
system	I-Application
,	O
also	O
known	O
as	O
copy-on-write	B-Language
.	O
</s>
<s>
Unionfs	B-Application
was	O
originally	O
developed	O
by	O
Professor	O
Erez	O
Zadok	O
and	O
his	O
team	O
at	O
Stony	O
Brook	O
University	O
.	O
</s>
<s>
In	O
Knoppix	O
,	O
a	O
union	O
between	O
the	O
file	B-Application
system	I-Application
on	O
the	O
CD-ROM	B-Device
or	O
DVD	O
and	O
a	O
file	B-Application
system	I-Application
contained	O
in	O
an	O
image	O
file	O
called	O
knoppix.img	O
(	O
knoppix-data.img	O
for	O
Knoppix	O
7	O
)	O
on	O
a	O
writable	O
drive	O
(	O
such	O
as	O
a	O
USB	O
memory	O
stick	O
)	O
can	O
be	O
made	O
,	O
where	O
the	O
writable	O
drive	O
has	O
priority	O
over	O
the	O
read-only	O
filesystem	B-Application
.	O
</s>
<s>
Unionfs	B-Application
can	O
also	O
be	O
used	O
to	O
create	O
a	O
single	O
common	O
template	O
for	O
a	O
number	O
of	O
file	B-Application
systems	I-Application
,	O
or	O
for	O
security	O
reasons	O
.	O
</s>
<s>
Docker	B-Language
uses	O
file	B-Application
systems	I-Application
inspired	O
by	O
Unionfs	B-Application
,	O
such	O
as	O
Aufs	B-Application
,	O
to	O
layer	O
Docker	B-Language
images	O
.	O
</s>
<s>
This	O
strategy	O
enables	O
Docker	B-Language
's	O
lightweight	O
images	O
,	O
as	O
only	O
layer	O
updates	O
need	O
to	O
be	O
propagated	O
(	O
compared	O
to	O
full	O
VMs	O
,	O
for	O
example	O
)	O
.	O
</s>
<s>
UbuntuLTSP	O
,	O
the	O
Linux	B-Application
Terminal	O
Server	O
Project	O
implementation	O
for	O
Ubuntu	O
,	O
uses	O
Unionfs	B-Application
when	O
PXE	O
booting	O
thin	O
or	O
thick	O
clients	O
.	O
</s>
<s>
Unionfs	B-Application
for	O
Linux	B-Application
has	O
two	O
versions	O
.	O
</s>
<s>
aufs	B-Application
is	O
an	O
alternative	O
version	O
of	O
unionfs	B-Application
.	O
</s>
<s>
overlayfs	B-Application
written	O
by	O
Miklos	O
Szeredi	O
has	O
been	O
used	O
in	O
OpenWRT	O
and	O
considered	O
by	O
Ubuntu	O
and	O
has	O
been	O
merged	O
into	O
the	O
mainline	O
Linux	B-Application
kernel	O
on	O
26	O
October	O
2014	O
after	O
many	O
years	O
of	O
development	O
and	O
discussion	O
for	O
version	O
3.18	O
of	O
the	O
kernel	O
.	O
</s>
<s>
unionfs-fuse	O
is	O
an	O
independent	O
project	O
,	O
implemented	O
as	O
a	O
user	B-Application
space	I-Application
filesystem	I-Application
program	O
,	O
instead	O
of	O
a	O
kernel	O
module	O
or	O
patch	O
.	O
</s>
<s>
Like	O
Unionfs	B-Application
,	O
it	O
supports	O
copy-on-write	B-Language
and	O
read-only	O
or	O
read	O
–	O
write	O
branches	O
.	O
</s>
<s>
Plan	B-Operating_System
9	I-Operating_System
from	I-Operating_System
Bell	I-Operating_System
Labs	I-Operating_System
operating	B-General_Concept
system	I-General_Concept
uses	O
union	B-Application
mounts	I-Application
extensively	O
to	O
build	O
custom	O
namespaces	O
per	O
user	O
or	O
processes	O
.	O
</s>
<s>
Union	B-Application
mounts	I-Application
have	O
also	O
been	O
available	O
in	O
BSD	B-Operating_System
since	O
at	O
least	O
1995	O
.	O
</s>
<s>
The	O
GNU	B-Application
Hurd	I-Application
has	O
an	O
implementation	O
of	O
Unionfs	B-Application
.	O
</s>
<s>
mhddfs	O
works	O
like	O
Unionfs	B-Application
but	O
permits	O
balancing	O
files	O
over	O
drives	O
with	O
the	O
most	O
free	O
space	O
available	O
.	O
</s>
<s>
It	O
is	O
implemented	O
as	O
a	O
user	B-Application
space	I-Application
filesystem	I-Application
.	O
</s>
<s>
mergerfs	O
is	O
a	O
FUSE	B-Application
based	O
union	B-Application
filesystem	I-Application
which	O
offers	O
multiple	O
policies	O
for	O
accessing	O
and	O
writing	O
files	O
as	O
well	O
as	O
other	O
advanced	O
features	O
(	O
xattrs	O
,	O
managing	O
mixed	O
RO	O
and	O
RW	O
drives	O
,	O
link	O
CoW	O
,	O
etc	O
.	O
</s>
<s>
Sun	O
Microsystems	O
introduced	O
the	O
first	O
implementation	O
of	O
a	O
stacked	O
,	O
layered	O
file	B-Application
system	I-Application
with	O
copy-on-write	B-Language
,	O
whiteouts	O
(	O
hiding	O
files	O
in	O
lower	O
layers	O
from	O
higher	O
layers	O
)	O
,	O
etc	O
.	O
</s>
<s>
as	O
the	O
Translucent	O
File	O
Service	O
in	O
SunOS	B-Operating_System
3	O
,	O
circa	O
1986	O
.	O
</s>
<s>
JailbreakMe	B-Operating_System
3.0	O
,	O
a	O
tool	O
for	O
jailbreaking	B-Application
iOS	O
devices	O
released	O
in	O
July	O
2011	O
,	O
uses	O
unionfs	B-Application
techniques	O
to	O
speed	O
up	O
the	O
installation	O
process	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
modification	O
.	O
</s>
