<s>
The	O
Journaling	O
Flash	O
File	O
System	O
(	O
or	O
JFFS	B-Application
)	O
is	O
a	O
log-structured	B-Application
file	I-Application
system	I-Application
for	O
use	O
on	O
NOR	O
flash	B-Device
memory	I-Device
devices	O
on	O
the	O
Linux	B-Application
operating	I-Application
system	I-Application
.	O
</s>
<s>
It	O
has	O
been	O
superseded	O
by	O
JFFS2	B-Application
.	O
</s>
<s>
Flash	B-Device
memory	I-Device
(	O
specifically	O
NOR	O
flash	O
)	O
must	O
be	O
erased	O
prior	O
to	O
writing	O
.	O
</s>
<s>
Flash	B-Device
memory	I-Device
can	O
only	O
be	O
erased	O
a	O
limited	O
number	O
of	O
times	O
(	O
typically	O
103	O
–	O
106	O
)	O
before	O
it	O
becomes	O
worn	O
out	O
.	O
</s>
<s>
These	O
constraints	O
combine	O
to	O
produce	O
a	O
profound	O
asymmetry	O
between	O
patterns	O
of	O
read	O
and	O
write	O
access	O
to	O
flash	B-Device
memory	I-Device
.	O
</s>
<s>
In	O
contrast	O
,	O
magnetic	O
hard	O
disk	O
drives	O
offer	O
nearly	O
symmetrical	O
read	O
and	O
write	O
access	O
:	O
read	O
speed	O
and	O
write	O
speed	O
are	O
nearly	O
identical	O
(	O
as	O
both	O
are	O
constrained	O
by	O
the	O
rate	O
at	O
which	O
the	O
disk	O
spins	O
)	O
,	O
it	O
is	O
possible	O
to	O
both	O
read	O
and	O
write	O
small	O
blocks	O
or	O
sectors	O
(	O
typically	O
512	O
or	O
4096	B-Device
bytes	O
)	O
,	O
and	O
there	O
is	O
no	O
practical	O
limit	O
to	O
the	O
number	O
of	O
times	O
magnetic	O
media	O
can	O
be	O
written	O
and	O
rewritten	O
.	O
</s>
<s>
Traditional	O
file	O
systems	O
,	O
such	O
as	O
ext2	B-Application
or	O
FAT	B-Application
which	O
were	O
designed	O
for	O
use	O
on	O
magnetic	O
media	O
typically	O
update	O
their	O
data	O
structures	O
in-place	O
,	O
with	O
data	O
structures	O
like	O
inodes	O
and	O
directories	O
updated	O
on-disk	O
after	O
every	O
modification	O
.	O
</s>
<s>
This	O
concentrated	O
lack	O
of	O
wear-levelling	B-Application
makes	O
conventional	O
file	O
systems	O
unsuitable	O
for	O
read-write	O
use	O
on	O
flash	O
devices	O
.	O
</s>
<s>
JFFS	B-Application
enforces	O
wear	B-Application
levelling	I-Application
by	O
treating	O
the	O
flash	O
device	O
as	O
a	O
circular	O
log	O
.	O
</s>
<s>
When	O
this	O
runs	O
low	O
,	O
a	O
garbage	B-General_Concept
collector	I-General_Concept
copies	O
valid	O
nodes	O
from	O
the	O
head	O
to	O
the	O
tail	O
and	O
skips	O
obsolete	O
ones	O
,	O
thus	O
reclaiming	O
space	O
.	O
</s>
