<s>
An	O
open-channel	O
solid	B-Device
state	I-Device
drive	I-Device
is	O
a	O
solid-state	B-Device
drive	I-Device
which	O
does	O
not	O
have	O
a	O
firmware	O
Flash	O
Translation	O
Layer	O
implemented	O
on	O
the	O
device	O
,	O
but	O
instead	O
leaves	O
the	O
management	O
of	O
the	O
physical	O
solid-state	O
storage	O
to	O
the	O
computer	O
's	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
The	O
Linux	B-Operating_System
4.4	I-Operating_System
kernel	I-Operating_System
is	O
an	O
example	O
of	O
an	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
that	O
supports	O
open-channel	B-Device
SSDs	I-Device
which	O
follow	O
the	O
NVM	B-Application
Express	I-Application
specification	O
.	O
</s>
<s>
The	O
interface	O
used	O
by	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
access	O
open-channel	O
solid	B-Device
state	I-Device
drives	I-Device
is	O
called	O
LightNVM	O
.	O
</s>
<s>
Since	O
SSDs	B-Device
use	O
NAND	O
flash	O
memory	O
for	O
storing	O
data	O
,	O
it	O
is	O
important	O
to	O
understand	O
the	O
characteristics	O
of	O
this	O
medium	O
.	O
</s>
<s>
Because	O
of	O
these	O
constraints	O
SSD	B-Device
controllers	O
write	O
data	O
to	O
NAND	O
flash	O
memory	O
in	O
another	O
order	O
than	O
the	O
logical	O
block	O
order	O
.	O
</s>
<s>
This	O
implies	O
that	O
the	O
SSD	B-Device
controller	O
must	O
maintain	O
a	O
mapping	O
table	O
from	O
host	O
(	O
logical	O
)	O
to	O
NAND	O
(	O
physical	O
)	O
addresses	O
.	O
</s>
<s>
Open	O
Channel	O
SSDs	B-Device
provide	O
more	O
flexibility	O
with	O
regard	O
to	O
data	O
placement	O
decisions	O
,	O
overprovisioning	O
,	O
scheduling	O
,	O
garbage	O
collection	O
and	O
wear	O
leveling	O
.	O
</s>
<s>
Open-Channel	B-Device
SSDs	I-Device
can	O
,	O
however	O
,	O
not	O
be	O
considered	O
a	O
uniform	O
class	O
of	O
devices	O
,	O
as	O
critical	O
device	O
characteristics	O
such	O
as	O
minimum	O
unit	O
of	O
read	O
and	O
minimum	O
unit	O
of	O
write	O
varies	O
from	O
device	O
to	O
device	O
.	O
</s>
<s>
One	O
can	O
therefore	O
not	O
design	O
an	O
FTL	O
that	O
automatically	O
works	O
on	O
all	O
Open-Channel	B-Device
SSDs	I-Device
.	O
</s>
<s>
Traditional	O
SSDs	B-Device
maintain	O
the	O
L2P	O
table	O
in	O
DRAM	O
on	O
the	O
SSD	B-Device
and	O
use	O
their	O
own	O
CPU	O
for	O
maintaining	O
that	O
L2P	O
table	O
.	O
</s>
<s>
With	O
Open	O
Channel	O
SSDs	B-Device
the	O
L2P	O
table	O
is	O
stored	O
in	O
host	O
memory	O
and	O
the	O
host	O
CPU	O
maintains	O
that	O
table	O
.	O
</s>
<s>
While	O
the	O
Open	O
Channel	O
SSD	B-Device
approach	O
is	O
more	O
flexible	O
,	O
a	O
significant	O
amount	O
of	O
host	O
memory	O
and	O
host	O
CPU	O
cycles	O
is	O
required	O
for	O
L2P	O
management	O
.	O
</s>
<s>
With	O
an	O
average	O
write	O
size	O
of	O
4	O
KB	O
,	O
almost	O
3	O
GB	O
RAM	O
is	O
required	O
for	O
an	O
SSD	B-Device
with	O
a	O
size	O
of	O
1	O
TB	O
.	O
</s>
<s>
With	O
open-channel	B-Device
SSDs	I-Device
,	O
the	O
L2P	O
mapping	O
can	O
be	O
directly	O
integrated	O
or	O
merged	O
with	O
storage	O
management	O
in	O
file	O
systems	O
.	O
</s>
<s>
This	O
avoids	O
the	O
redundancy	O
between	O
system	O
software	O
and	O
SSD	B-Device
firmware	O
,	O
and	O
thus	O
improves	O
performance	O
and	O
endurance	O
.	O
</s>
<s>
Further	O
,	O
open-channel	B-Device
SSDs	I-Device
enables	O
more	O
flexible	O
control	O
over	O
flash	O
memory	O
.	O
</s>
<s>
The	O
internal	O
parallelism	O
is	O
exploited	O
by	O
coordinating	O
the	O
data	O
layout	O
,	O
garbage	O
collection	O
and	O
request	O
scheduling	O
of	O
both	O
system	O
software	O
and	O
SSD	B-Device
firmware	O
to	O
remove	O
the	O
conflicts	O
,	O
and	O
thus	O
improves	O
and	O
smooths	O
the	O
performance	O
.	O
</s>
