<s>
A	O
flash	B-Application
file	I-Application
system	I-Application
is	O
a	O
file	B-Application
system	I-Application
designed	O
for	O
storing	O
files	B-Operating_System
on	O
flash	O
memorybased	O
storage	O
devices	O
.	O
</s>
<s>
While	O
flash	B-Application
file	I-Application
systems	I-Application
are	O
closely	O
related	O
to	O
file	B-Application
systems	I-Application
in	O
general	O
,	O
they	O
are	O
optimized	O
for	O
the	O
nature	O
and	O
characteristics	O
of	O
flash	B-Device
memory	I-Device
(	O
such	O
as	O
to	O
avoid	O
write	B-Error_Name
amplification	I-Error_Name
)	O
,	O
and	O
for	O
use	O
in	O
particular	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
While	O
a	O
block	O
device	O
layer	O
can	O
emulate	O
a	O
disk	O
drive	O
so	O
that	O
a	O
general-purpose	O
file	B-Application
system	I-Application
can	O
be	O
used	O
on	O
a	O
flash-based	O
storage	O
device	O
,	O
this	O
is	O
suboptimal	O
for	O
several	O
reasons	O
:	O
</s>
<s>
Erasing	O
blocks	O
:	O
flash	B-Device
memory	I-Device
blocks	O
have	O
to	O
be	O
explicitly	O
erased	O
before	O
they	O
can	O
be	O
written	O
to	O
.	O
</s>
<s>
Random	B-General_Concept
access	I-General_Concept
:	O
general-purpose	O
file	B-Application
systems	I-Application
are	O
optimized	O
to	O
avoid	O
disk	O
seeks	O
whenever	O
possible	O
,	O
due	O
to	O
the	O
high	O
cost	O
of	O
seeking	O
.	O
</s>
<s>
Flash	B-Device
memory	I-Device
devices	O
impose	O
no	O
seek	O
latency	O
.	O
</s>
<s>
Wear	B-Application
leveling	I-Application
:	O
flash	B-Device
memory	I-Device
devices	O
tend	O
to	O
wear	O
out	O
when	O
a	O
single	O
block	O
is	O
repeatedly	O
overwritten	O
;	O
flash	B-Application
file	I-Application
systems	I-Application
are	O
designed	O
to	O
spread	O
out	O
writes	O
evenly	O
.	O
</s>
<s>
Log-structured	B-Application
file	I-Application
systems	I-Application
have	O
all	O
the	O
desirable	O
properties	O
for	O
a	O
flash	B-Application
file	I-Application
system	I-Application
.	O
</s>
<s>
Such	O
file	B-Application
systems	I-Application
include	O
JFFS2	B-Application
and	O
YAFFS	B-Application
.	O
</s>
<s>
Because	O
of	O
the	O
particular	O
characteristics	O
of	O
flash	B-Device
memory	I-Device
,	O
it	O
is	O
best	O
used	O
with	O
either	O
a	O
controller	B-Device
to	O
perform	O
wear	B-Application
leveling	I-Application
and	O
error	B-Error_Name
correction	I-Error_Name
or	O
specifically	O
designed	O
flash	B-Application
file	I-Application
systems	I-Application
,	O
which	O
spread	O
writes	O
over	O
the	O
media	O
and	O
deal	O
with	O
the	O
long	O
erase	O
times	O
of	O
NAND	O
flash	O
blocks	O
.	O
</s>
<s>
The	O
basic	O
concept	O
behind	O
flash	B-Application
file	I-Application
systems	I-Application
is	O
:	O
when	O
the	O
flash	O
store	O
is	O
to	O
be	O
updated	O
,	O
the	O
file	B-Application
system	I-Application
will	O
write	O
a	O
new	O
copy	O
of	O
the	O
changed	O
data	O
over	O
to	O
a	O
fresh	O
block	O
,	O
remap	O
the	O
file	O
pointers	O
,	O
then	O
erase	O
the	O
old	O
block	O
later	O
when	O
it	O
has	O
time	O
.	O
</s>
<s>
In	O
practice	O
,	O
flash	B-Application
file	I-Application
systems	I-Application
are	O
used	O
only	O
for	O
Memory	B-Application
Technology	I-Application
Devices	I-Application
(	O
MTDs	O
)	O
,	O
which	O
are	O
embedded	O
flash	B-Device
memories	I-Device
that	O
do	O
not	O
have	O
a	O
controller	B-Device
.	O
</s>
<s>
Removable	O
flash	B-Device
memory	I-Device
cards	I-Device
and	O
USB	O
flash	O
drives	O
have	O
built-in	O
controllers	O
to	O
manage	O
MTD	O
with	O
dedicated	O
algorithms	O
,	O
like	O
wear	B-Application
leveling	I-Application
,	O
bad	O
block	O
recovery	O
,	O
power	O
loss	O
recovery	O
,	O
garbage	O
collection	O
and	O
error	B-Error_Name
correction	I-Error_Name
,	O
so	O
use	O
of	O
a	O
flash	B-Application
file	I-Application
system	I-Application
has	O
limited	O
benefit	O
.	O
</s>
<s>
Flash-based	O
memory	O
devices	O
are	O
becoming	O
more	O
prevalent	O
as	O
the	O
number	O
of	O
mobile	O
devices	O
is	O
increasing	O
,	O
the	O
cost	O
per	O
memory	O
size	O
decreases	O
,	O
and	O
the	O
capacity	O
of	O
flash	B-Device
memory	I-Device
chips	O
increases	O
.	O
</s>
<s>
The	O
earliest	O
flash	B-Application
file	I-Application
system	I-Application
,	O
managing	O
an	O
array	O
of	O
flash	O
as	O
a	O
freely	O
writable	O
disk	O
,	O
was	O
TrueFFS	O
by	O
M-Systems	O
of	O
Israel	O
,	O
presented	O
as	O
a	O
software	O
product	O
in	O
PC-Card	B-Architecture
Expo	O
at	O
Santa	O
Clara	O
,	O
California	O
,	O
in	O
July	O
1992	O
and	O
patented	O
in	O
1993	O
.	O
</s>
<s>
One	O
of	O
the	O
earliest	O
flash	B-Application
file	I-Application
systems	I-Application
was	O
Microsoft	O
's	O
FFS2	O
,	O
for	O
use	O
with	O
MS-DOS	B-Application
,	O
released	O
in	O
autumn	O
1992	O
.	O
</s>
<s>
FFS2	O
was	O
preceded	O
by	O
an	O
earlier	O
product	O
,	O
called	O
"	O
FFS	O
"	O
,	O
which	O
however	O
fell	O
short	O
of	O
being	O
a	O
flash	B-Application
file	I-Application
system	I-Application
,	O
managing	O
a	O
flash	O
array	O
as	O
write	B-Device
once	I-Device
read	I-Device
many	I-Device
(	O
WORM	B-Device
)	O
space	O
rather	O
than	O
as	O
a	O
freely	O
writable	O
disk	O
.	O
</s>
<s>
Around	O
1994	O
,	O
the	O
PCMCIA	B-Architecture
,	O
an	O
industry	O
group	O
,	O
approved	O
the	O
Flash	O
Translation	O
Layer	O
(	O
FTL	O
)	O
specification	O
,	O
based	O
on	O
the	O
design	O
of	O
M-Systems	O
'	O
TrueFFS	O
.	O
</s>
<s>
Endorsed	O
by	O
Intel	O
,	O
FTL	O
became	O
a	O
popular	O
flash	B-Application
file	I-Application
system	I-Application
design	O
in	O
non-PCMCIA	O
media	O
as	O
well	O
.	O
</s>
<s>
JFFS	B-Application
was	O
the	O
first	O
flash-specific	O
file	B-Application
system	I-Application
for	O
Linux	B-Operating_System
,	O
but	O
it	O
was	O
quickly	O
superseded	O
by	O
JFFS2	B-Application
,	O
originally	O
developed	O
for	O
NOR	O
flash	O
.	O
</s>
<s>
Then	O
YAFFS	B-Application
was	O
released	O
in	O
2002	O
,	O
dealing	O
specifically	O
with	O
NAND	O
flash	O
,	O
and	O
JFFS2	B-Application
was	O
updated	O
to	O
support	O
NAND	O
flash	O
too	O
.	O
</s>
<s>
UBIFS	B-Application
has	O
been	O
merged	O
since	O
Linux	O
2.6.22	O
in	O
2008	O
.	O
</s>
<s>
UBIFS	B-Application
has	O
been	O
actively	O
developed	O
from	O
its	O
initial	O
merge	O
.	O
</s>
<s>
UBIFS	B-Application
has	O
hosted	O
at	O
infradead.org	O
along	O
with	O
JFFS2	B-Application
and	O
MTD	O
drivers	O
.	O
</s>
<s>
Some	O
initial	O
comparison	O
show	O
UBIFS	B-Application
with	O
compression	O
faster	O
than	O
F2FS	B-Application
.	O
</s>
<s>
LogFS	B-Application
,	O
another	O
Linux	B-Operating_System
flash-specific	O
file	B-Application
system	I-Application
,	O
is	O
being	O
developed	O
to	O
address	O
the	O
scalability	O
issues	O
of	O
JFFS2	B-Application
.	O
</s>
<s>
F2FS	B-Application
(	O
Flash-Friendly	B-Application
File	I-Application
System	I-Application
)	O
was	O
added	O
to	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
3.8	O
.	O
</s>
<s>
Instead	O
of	O
being	O
targeted	O
at	O
speaking	O
directly	O
to	O
raw	O
flash	O
devices	O
,	O
F2FS	B-Application
is	O
designed	O
to	O
be	O
used	O
on	O
flash-based	O
storage	O
devices	O
that	O
already	O
include	O
a	O
flash	O
translation	O
layer	O
,	O
such	O
as	O
SD	B-Device
cards	I-Device
.	O
</s>
<s>
Overlayfs	O
,	O
Unionfs	O
,	O
and	O
aufs	O
are	O
union	O
filesystems	B-Application
,	O
that	O
allow	O
multiple	O
filesystems	B-Application
to	O
be	O
combined	O
and	O
presented	O
to	O
the	O
user	O
as	O
a	O
single	O
tree	O
.	O
</s>
<s>
This	O
allows	O
the	O
system	O
designer	O
to	O
place	O
parts	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
that	O
are	O
nominally	O
read-only	O
on	O
different	O
media	O
to	O
the	O
normal	O
read-write	O
areas	O
.	O
</s>
<s>
OpenWrt	B-Application
is	O
usually	O
installed	O
on	O
raw	O
flash	B-Device
chips	I-Device
without	O
FTL	O
.	O
</s>
<s>
It	O
uses	O
overlayfs	O
to	O
combine	O
a	O
compressed	O
read-only	O
SquashFS	B-Application
with	O
JFFS2	B-Application
.	O
</s>
