<s>
Fusion	B-Device
Drive	I-Device
is	O
a	O
type	O
of	O
hybrid	B-Device
drive	I-Device
technology	O
created	O
by	O
Apple	O
Inc	O
.	O
</s>
<s>
It	O
combines	O
a	O
hard	B-Device
disk	I-Device
drive	I-Device
with	O
a	O
NAND	O
flash	O
storage	O
(	O
solid-state	B-Device
drive	I-Device
of	O
24	O
GB	O
or	O
more	O
)	O
and	O
presents	O
it	O
as	O
a	O
single	O
Core	B-Operating_System
Storage	I-Operating_System
managed	O
logical	B-Application
volume	I-Application
with	O
the	O
space	O
of	O
both	O
drives	O
combined	O
.	O
</s>
<s>
The	O
operating	O
system	O
automatically	O
manages	O
the	O
contents	O
of	O
the	O
drive	O
so	O
the	O
most	O
frequently	O
accessed	O
files	O
are	O
stored	O
on	O
the	O
faster	O
flash	O
storage	O
,	O
while	O
infrequently	O
used	O
items	O
move	O
to	O
or	O
stay	O
on	O
the	O
hard	B-Device
drive	I-Device
.	O
</s>
<s>
For	O
example	O
,	O
if	O
spreadsheet	B-Application
software	I-Application
is	O
used	O
often	O
,	O
the	O
software	O
will	O
be	O
moved	O
to	O
the	O
flash	O
storage	O
for	O
faster	O
user	O
access	O
.	O
</s>
<s>
In	O
software	O
,	O
this	O
logical	B-Application
volume	I-Application
speeds	O
up	O
performance	O
of	O
the	O
computer	O
by	O
performing	O
both	O
caching	B-General_Concept
for	O
faster	O
writes	O
and	O
auto	B-Application
tiering	I-Application
for	O
faster	O
reads	O
.	O
</s>
<s>
The	O
Fusion	B-Device
Drive	I-Device
was	O
announced	O
as	O
part	O
of	O
an	O
Apple	O
event	O
held	O
on	O
October	O
23	O
,	O
2012	O
,	O
with	O
the	O
first	O
supporting	O
products	O
being	O
two	O
desktops	O
:	O
the	O
iMac	B-Device
and	O
Mac	B-Device
Mini	I-Device
with	O
OS	B-Operating_System
X	I-Operating_System
Mountain	I-Operating_System
Lion	I-Operating_System
released	O
in	O
late	O
2012	O
.	O
</s>
<s>
Fusion	B-Device
Drive	I-Device
remains	O
available	O
in	O
subsequent	O
models	O
of	O
these	O
computers	O
,	O
but	O
was	O
not	O
expanded	O
to	O
other	O
Apple	O
devices	O
:	O
the	O
latest	O
MacBook	O
and	O
Mac	O
Pro	O
models	O
use	O
exclusively	O
flash	O
storage	O
,	O
and	O
while	O
this	O
was	O
an	O
optional	O
upgrade	O
for	O
the	O
mid-2012	O
non-Retina	O
MacBook	B-Device
Pro	I-Device
discontinued	O
by	O
Apple	O
,	O
it	O
will	O
replace	O
the	O
standard	O
hard	B-Device
disk	I-Device
drive	I-Device
instead	O
of	O
complementing	O
it	O
in	O
the	O
fashion	O
of	O
Fusion	B-Device
Drive	I-Device
.	O
</s>
<s>
As	O
of	O
November	O
2021	O
,	O
no	O
Mac	O
offers	O
a	O
fusion	B-Device
drive	I-Device
.	O
</s>
<s>
Apple	O
's	O
Fusion	B-Device
Drive	I-Device
design	O
incorporates	O
proprietary	O
features	O
with	O
limited	O
documentation	O
.	O
</s>
<s>
It	O
has	O
been	O
reported	O
that	O
the	O
design	O
of	O
Fusion	B-Device
Drive	I-Device
has	O
been	O
influenced	O
by	O
a	O
research	O
project	O
called	O
Hystor	O
.	O
</s>
<s>
According	O
to	O
the	O
paper	O
,	O
this	O
hybrid	O
storage	O
system	O
unifies	O
a	O
high-speed	O
SSD	B-Device
and	O
a	O
large-capacity	O
hard	B-Device
drive	I-Device
with	O
several	O
design	O
considerations	O
of	O
which	O
one	O
has	O
been	O
used	O
in	O
the	O
Fusion	B-Device
Drive	I-Device
.	O
</s>
<s>
The	O
SSD	B-Device
and	O
the	O
hard	B-Device
drive	I-Device
are	O
logically	O
merged	O
into	O
a	O
single	O
block	O
device	O
managed	O
by	O
the	O
operating	O
system	O
,	O
which	O
is	O
independent	O
of	O
file	O
systems	O
and	O
requires	O
no	O
changes	O
to	O
applications	O
.	O
</s>
<s>
A	O
portion	O
of	O
SSD	B-Device
space	O
is	O
used	O
as	O
a	O
write-back	O
buffer	O
to	O
absorb	O
incoming	O
write	O
traffic	O
,	O
which	O
hides	O
perceivable	O
latencies	O
and	O
boosts	O
write	O
performance	O
.	O
</s>
<s>
More	O
frequently	O
accessed	O
data	O
is	O
stored	O
on	O
the	O
SSD	B-Device
and	O
the	O
larger	O
,	O
less	O
frequently	O
accessed	O
data	O
stored	O
on	O
the	O
HDD	O
.	O
</s>
<s>
Data	O
movement	O
is	O
based	O
on	O
access	O
patterns	O
:	O
if	O
data	O
has	O
been	O
on	O
the	O
HDD	O
and	O
suddenly	O
becomes	O
frequently	O
accessed	O
,	O
it	O
will	O
usually	O
get	O
moved	O
to	O
the	O
SSD	B-Device
by	O
the	O
program	O
controlling	O
the	O
Fusion	B-Device
Drive	I-Device
.	O
</s>
<s>
Several	O
experimental	O
studies	O
have	O
been	O
conducted	O
to	O
speculate	O
about	O
the	O
internal	O
mechanism	O
of	O
Fusion	B-Device
Drive	I-Device
.	O
</s>
<s>
Fusion	B-Device
Drive	I-Device
is	O
a	O
block-level	O
solution	O
based	O
on	O
Apple	O
's	O
Core	B-Operating_System
Storage	I-Operating_System
,	O
a	O
logical	B-Application
volume	I-Application
manager	I-Application
managing	O
multiple	O
physical	O
devices	O
.	O
</s>
<s>
The	O
capacity	O
of	O
a	O
Fusion	B-Device
Drive	I-Device
is	O
confirmed	O
to	O
be	O
the	O
sum	O
of	O
two	O
devices	O
.	O
</s>
<s>
Fusion	B-Device
Drive	I-Device
is	O
file	O
system	O
agnostic	O
and	O
effective	O
for	O
both	O
HFS	B-Application
Plus	I-Application
and	O
ZFS	B-Application
.	O
</s>
<s>
Part	O
of	O
the	O
SSD	B-Device
space	O
is	O
used	O
as	O
a	O
write	O
buffer	O
for	O
incoming	O
writes	O
.	O
</s>
<s>
A	O
small	O
spare	O
area	O
is	O
set	O
aside	O
on	O
the	O
SSD	B-Device
for	O
performance	O
consistency	O
.	O
</s>
<s>
Data	O
is	O
promoted	O
to	O
the	O
SSD	B-Device
based	O
on	O
its	O
access	O
frequency	O
.	O
</s>
<s>
The	O
frequency	O
is	O
detected	O
at	O
the	O
block	O
level	O
and	O
below	O
file	O
system	O
memory	B-General_Concept
cache	I-General_Concept
.	O
</s>
<s>
Operating	O
system	O
and	O
other	O
critical	O
documents	O
are	O
always	O
cached	O
on	O
the	O
SSD	B-Device
.	O
</s>
