<s>
The	O
NOVA	B-Application
(	O
non-volatile	B-General_Concept
memory	I-General_Concept
accelerated	O
)	O
file	B-Application
system	I-Application
is	O
an	O
open-source	B-Application
,	O
log-structured	B-Application
file	I-Application
system	I-Application
for	O
byte-addressable	O
persistent	B-Library
memory	I-Library
(	O
for	O
example	O
non-volatile	B-Device
dual	I-Device
in-line	I-Device
memory	I-Device
module	I-Device
(	O
NVDIMM	B-Device
)	O
and	O
3D	B-Device
XPoint	I-Device
DIMMs	O
)	O
for	O
Linux	B-Operating_System
.	O
</s>
<s>
NOVA	B-Application
is	O
designed	O
specifically	O
for	O
byte-addressable	O
persistent	B-Library
memories	I-Library
and	O
aims	O
to	O
provide	O
high-performance	O
,	O
atomic	O
file	O
and	O
metadata	O
operations	O
,	O
and	O
fault	O
tolerance	O
.	O
</s>
<s>
To	O
meet	O
these	O
goals	O
NOVA	B-Application
combines	O
several	O
techniques	O
found	O
in	O
other	O
file	B-Application
systems	I-Application
.	O
</s>
<s>
NOVA	B-Application
uses	O
log	B-Application
structure	I-Application
,	O
copy-on-write	B-Language
(	O
COW	O
)	O
,	O
journaling	B-Application
,	O
and	O
log-structured	O
metadata	O
updates	O
to	O
provide	O
strong	O
atomicity	B-General_Concept
guarantees	O
,	O
and	O
it	O
uses	O
a	O
combination	O
replication	O
,	O
metadata	O
checksums	B-Algorithm
,	O
and	O
RAID	O
4	O
parity	O
to	O
protect	O
data	O
and	O
metadata	O
from	O
media	O
errors	O
and	O
software	B-Error_Name
bugs	I-Error_Name
.	O
</s>
<s>
NOVA	B-Application
was	O
developed	O
at	O
the	O
University	O
of	O
California	O
,	O
San	O
Diego	O
,	O
in	O
the	O
Non-Volatile	B-General_Concept
Systems	O
Laboratory	O
of	O
the	O
Computer	O
Science	O
and	O
Engineering	O
Department	O
.	O
</s>
<s>
Patches	O
were	O
initially	O
made	O
available	O
for	O
version	O
4.12	O
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
it	O
is	O
limited	O
to	O
x86-64	B-Device
Linux	B-Operating_System
,	O
and	O
not	O
ready	O
for	O
merging	O
with	O
the	O
upstream	O
kernel	B-Operating_System
.	O
</s>
<s>
NOVA	B-Application
is	O
primarily	O
a	O
log-structured	B-Application
file	I-Application
system	I-Application
,	O
but	O
it	O
differs	O
from	O
other	O
log-structured	B-Application
file	I-Application
systems	I-Application
in	O
several	O
respects	O
.	O
</s>
<s>
First	O
,	O
rather	O
than	O
using	O
a	O
single	O
log	O
for	O
the	O
entire	O
file	B-Application
system	I-Application
,	O
each	O
inode	B-Application
has	O
its	O
own	O
,	O
dedicated	O
log	O
that	O
records	O
the	O
updates	O
to	O
the	O
inode	B-Application
.	O
</s>
<s>
This	O
allows	O
for	O
increased	O
concurrency	O
in	O
file	O
operations	O
,	O
since	O
different	O
threads	B-Operating_System
can	O
operate	O
on	O
inodes	B-Application
in	O
parallel	O
.	O
</s>
<s>
Instead	O
,	O
NOVA	B-Application
stores	O
the	O
logs	O
in	O
a	O
linked	B-Data_Structure
list	I-Data_Structure
of	O
4KB	O
memory	O
pages	O
.	O
</s>
<s>
NOVA	B-Application
uses	O
the	O
logs	O
to	O
provide	O
atomicity	B-General_Concept
for	O
operations	O
that	O
affect	O
a	O
single	O
file	O
(	O
e.g.	O
,	O
writing	O
to	O
a	O
file	O
or	O
modifying	O
its	O
metadata	O
)	O
.	O
</s>
<s>
To	O
do	O
this	O
,	O
NOVA	B-Application
writes	O
a	O
log	O
entry	O
to	O
empty	O
space	O
past	O
the	O
end	O
of	O
the	O
log	O
and	O
then	O
atomically	B-General_Concept
updates	O
the	O
inode	B-Application
's	O
pointer	O
to	O
the	O
log	O
tail	O
.	O
</s>
<s>
NOVA	B-Application
uses	O
copy-on-write	B-Language
(	O
COW	O
)	O
to	O
update	O
file	O
data	O
.	O
</s>
<s>
When	O
a	O
program	O
writes	O
data	O
to	O
a	O
file	O
,	O
NOVA	B-Application
allocates	O
some	O
unused	O
memory	O
pages	O
to	O
hold	O
the	O
data	O
and	O
writes	O
the	O
data	O
into	O
them	O
.	O
</s>
<s>
Then	O
,	O
it	O
appends	O
a	O
log	O
entry	O
to	O
the	O
inode	B-Application
's	O
log	O
that	O
points	O
to	O
the	O
new	O
pages	O
and	O
describes	O
their	O
logical	O
location	O
in	O
the	O
file	O
.	O
</s>
<s>
Some	O
file	O
operations	O
(	O
e.g.	O
,	O
moving	O
a	O
file	O
from	O
one	O
directory	O
to	O
another	O
)	O
require	O
modifying	O
multiple	O
inodes	B-Application
.	O
</s>
<s>
To	O
make	O
these	O
operations	O
atomic	O
,	O
NOVA	B-Application
uses	O
a	O
simple	O
journaling	B-Application
mechanisms	O
.	O
</s>
<s>
First	O
,	O
it	O
writes	O
the	O
new	O
log	O
entries	O
to	O
ends	O
of	O
the	O
inodes	B-Application
that	O
the	O
operation	O
will	O
affect	O
,	O
then	O
it	O
uses	O
the	O
journal	O
to	O
record	O
the	O
necessary	O
updates	O
to	O
the	O
inodes	B-Application
 '	O
log	O
tail	O
pointers	O
.	O
</s>
<s>
NOVA	B-Application
uses	O
replication	O
and	O
checksums	B-Algorithm
to	O
provide	O
protection	O
against	O
metadata	O
corruption	O
due	O
to	O
media	O
errors	O
and	O
software	B-Error_Name
bugs	I-Error_Name
.	O
</s>
<s>
Every	O
metadata	O
structure	O
(	O
e.g.	O
,	O
inodes	B-Application
,	O
superblocks	O
,	O
and	O
log	O
entries	O
)	O
contains	O
a	O
CRC32	O
checksum	B-Algorithm
that	O
allows	O
NOVA	B-Application
to	O
detect	O
if	O
structures	O
contents	O
have	O
changed	O
with	O
its	O
knowledge	O
.	O
</s>
<s>
NOVA	B-Application
also	O
stores	O
two	O
copies	O
of	O
each	O
data	O
structure	O
–	O
the	O
"	O
primary	O
"	O
and	O
the	O
"	O
replica	O
"	O
–	O
and	O
stores	O
them	O
far	O
from	O
one	O
another	O
in	O
memory	O
.	O
</s>
<s>
Whenever	O
NOVA	B-Application
accesses	O
a	O
metadata	O
structure	O
,	O
it	O
first	O
recomputes	O
the	O
checksum	B-Algorithm
on	O
both	O
the	O
primary	O
and	O
the	O
replica	O
.	O
</s>
<s>
If	O
either	O
check	O
results	O
in	O
a	O
mismatch	O
,	O
NOVA	B-Application
repairs	O
the	O
damage	O
using	O
the	O
other	O
copy	O
.	O
</s>
<s>
If	O
neither	O
checksum	B-Algorithm
matches	O
,	O
then	O
the	O
structure	O
is	O
lost	O
and	O
NOVA	B-Application
returns	O
an	O
error	O
.	O
</s>
<s>
NOVA	B-Application
uses	O
RAID	O
4	O
to	O
protect	O
file	O
data	O
.	O
</s>
<s>
It	O
divides	O
each	O
4KB	O
page	O
into	O
512-byte	O
strips	O
and	O
stores	O
a	O
parity	O
strip	O
in	O
a	O
dedicated	O
region	O
of	O
persistent	B-Library
memory	I-Library
.	O
</s>
<s>
It	O
also	O
computes	O
(	O
and	O
stores	O
a	O
replica	O
of	O
)	O
a	O
CRC32	O
checksum	B-Algorithm
for	O
the	O
eight	O
data	O
strips	O
and	O
the	O
parity	O
strip	O
.	O
</s>
<s>
When	O
NOVA	B-Application
reads	O
a	O
page	O
,	O
it	O
confirms	O
the	O
checksum	B-Algorithm
on	O
each	O
strip	O
.	O
</s>
<s>
Otherwise	O
,	O
recovery	O
fails	O
,	O
the	O
contents	O
of	O
the	O
page	O
are	O
lost	O
,	O
and	O
NOVA	B-Application
returns	O
an	O
error	O
.	O
</s>
