<s>
In	O
computing	O
,	O
data	B-General_Concept
deduplication	I-General_Concept
is	O
a	O
technique	O
for	O
eliminating	O
duplicate	O
copies	O
of	O
repeating	O
data	O
.	O
</s>
<s>
A	O
related	O
technique	O
is	O
single-instance	B-Application
(	O
data	O
)	O
storage	O
,	O
which	O
replaces	O
multiple	O
copies	O
of	O
content	O
at	O
the	O
whole-file	O
level	O
with	O
a	O
single	O
shared	O
copy	O
.	O
</s>
<s>
While	O
possible	O
to	O
combine	O
this	O
with	O
other	O
forms	O
of	O
data	O
compression	O
and	O
deduplication	O
,	O
it	O
is	O
distinct	O
from	O
newer	O
approaches	O
to	O
data	B-General_Concept
deduplication	I-General_Concept
(	O
which	O
can	O
operate	O
at	O
the	O
segment	O
or	O
sub-block	O
level	O
)	O
.	O
</s>
<s>
Deduplication	O
is	O
different	O
from	O
data	O
compression	O
algorithms	O
,	O
such	O
as	O
LZ77	B-Algorithm
and	I-Algorithm
LZ78	I-Algorithm
.	O
</s>
<s>
With	O
data	B-General_Concept
deduplication	I-General_Concept
,	O
only	O
one	O
instance	O
of	O
the	O
attachment	O
is	O
actually	O
stored	O
;	O
the	O
subsequent	O
instances	O
are	O
referenced	O
back	O
to	O
the	O
saved	O
copy	O
for	O
deduplication	O
ratio	O
of	O
roughly	O
100	O
to	O
1	O
.	O
</s>
<s>
In	O
computer	B-Application
code	I-Application
,	O
deduplication	O
is	O
done	O
by	O
,	O
for	O
example	O
,	O
storing	O
information	O
in	O
variables	O
so	O
that	O
they	O
do	O
n't	O
have	O
to	O
be	O
written	O
out	O
individually	O
but	O
can	O
be	O
changed	O
all	O
at	O
once	O
at	O
a	O
central	O
referenced	O
location	O
.	O
</s>
<s>
Examples	O
are	O
CSS	O
classes	O
and	O
named	O
references	O
in	O
MediaWiki	B-Application
.	O
</s>
<s>
Storage-based	O
data	B-General_Concept
deduplication	I-General_Concept
reduces	O
the	O
amount	O
of	O
storage	O
needed	O
for	O
a	O
given	O
set	O
of	O
files	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
data	B-Protocol
backups	I-Protocol
,	O
which	O
routinely	O
are	O
performed	O
to	O
protect	O
against	O
data	O
loss	O
,	O
most	O
data	O
in	O
a	O
given	O
backup	B-Protocol
remain	O
unchanged	O
from	O
the	O
previous	O
backup	B-Protocol
.	O
</s>
<s>
Common	O
backup	B-Protocol
systems	O
try	O
to	O
exploit	O
this	O
by	O
omitting	O
(	O
or	O
hard	B-Application
linking	I-Application
)	O
files	O
that	O
have	O
n't	O
changed	O
or	O
storing	O
differences	B-Application
between	O
files	O
.	O
</s>
<s>
Hard-linking	O
does	O
not	O
help	O
with	O
large	O
files	O
that	O
have	O
only	O
changed	O
in	O
small	O
ways	O
,	O
such	O
as	O
an	O
email	O
database	O
;	O
differences	B-Application
only	O
find	O
redundancies	O
in	O
adjacent	O
versions	O
of	O
a	O
single	O
file	O
(	O
consider	O
a	O
section	O
that	O
was	O
deleted	O
and	O
later	O
added	O
in	O
again	O
,	O
or	O
a	O
logo	O
image	O
included	O
in	O
many	O
documents	O
)	O
.	O
</s>
<s>
In-line	O
network	O
data	B-General_Concept
deduplication	I-General_Concept
is	O
used	O
to	O
reduce	O
the	O
number	O
of	O
bytes	O
that	O
must	O
be	O
transferred	O
between	O
endpoints	O
,	O
which	O
can	O
reduce	O
the	O
amount	O
of	O
bandwidth	O
required	O
.	O
</s>
<s>
See	O
WAN	B-Protocol
optimization	I-Protocol
for	O
more	O
information	O
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
if	O
a	O
given	O
virtual	O
machine	O
customizes	O
a	O
file	O
,	O
deduplication	O
will	O
not	O
change	O
the	O
files	O
on	O
the	O
other	O
virtual	O
machines	O
—	O
something	O
that	O
alternatives	O
like	O
hard	B-Application
links	I-Application
or	O
shared	O
disks	O
do	O
not	O
offer	O
.	O
</s>
<s>
The	O
benefit	O
is	O
that	O
there	O
is	O
no	O
need	O
to	O
wait	O
for	O
the	O
hash	B-Algorithm
calculations	O
and	O
lookup	O
to	O
be	O
completed	O
before	O
storing	O
the	O
data	O
,	O
thereby	O
ensuring	O
that	O
store	O
performance	O
is	O
not	O
degraded	O
.	O
</s>
<s>
Alternatively	O
,	O
deduplication	O
hash	B-Algorithm
calculations	O
can	O
be	O
done	O
in-line	O
:	O
synchronized	O
as	O
data	O
enters	O
the	O
target	O
device	O
.	O
</s>
<s>
On	O
the	O
negative	O
side	O
,	O
hash	B-Algorithm
calculations	O
may	O
be	O
computationally	O
expensive	O
,	O
thereby	O
reducing	O
the	O
storage	O
throughput	O
.	O
</s>
<s>
content-agnostic	O
data	B-General_Concept
deduplication	I-General_Concept
-	O
a	O
data	B-General_Concept
deduplication	I-General_Concept
method	O
that	O
does	O
not	O
require	O
awareness	O
of	O
specific	O
application	O
data	O
formats	O
.	O
</s>
<s>
content-aware	O
data	B-General_Concept
deduplication	I-General_Concept
-	O
a	O
data	B-General_Concept
deduplication	I-General_Concept
method	O
that	O
leverages	O
knowledge	O
of	O
specific	O
application	O
data	O
formats	O
.	O
</s>
<s>
Another	O
way	O
to	O
classify	O
data	B-General_Concept
deduplication	I-General_Concept
methods	O
is	O
according	O
to	O
where	O
they	O
occur	O
.	O
</s>
<s>
This	O
generally	O
takes	O
place	O
directly	O
within	O
a	O
file	B-Application
system	I-Application
.	O
</s>
<s>
The	O
file	B-Application
system	I-Application
will	O
periodically	O
scan	O
new	O
files	O
creating	O
hashes	B-Algorithm
and	O
compare	O
them	O
to	O
hashes	B-Algorithm
of	O
existing	O
files	O
.	O
</s>
<s>
When	O
files	O
with	O
same	O
hashes	B-Algorithm
are	O
found	O
then	O
the	O
file	O
copy	O
is	O
removed	O
and	O
the	O
new	O
file	O
points	O
to	O
the	O
old	O
file	O
.	O
</s>
<s>
Unlike	O
hard	B-Application
links	I-Application
however	O
,	O
duplicated	O
files	O
are	O
considered	O
to	O
be	O
separate	O
entities	O
and	O
if	O
one	O
of	O
the	O
duplicated	O
files	O
is	O
later	O
modified	O
,	O
then	O
using	O
a	O
system	O
called	O
copy-on-write	B-Language
a	O
copy	O
of	O
that	O
changed	O
file	O
or	O
block	O
is	O
created	O
.	O
</s>
<s>
The	O
deduplication	O
process	O
is	O
transparent	O
to	O
the	O
users	O
and	O
backup	B-Protocol
applications	O
.	O
</s>
<s>
Backing	O
up	O
a	O
deduplicated	O
file	B-Application
system	I-Application
will	O
often	O
cause	O
duplication	O
to	O
occur	O
resulting	O
in	O
the	O
backups	B-Protocol
being	O
bigger	O
than	O
the	O
source	O
data	O
.	O
</s>
<s>
This	O
leads	O
to	O
a	O
new	O
form	O
of	O
"	O
linking	O
"	O
on	O
file	B-Application
systems	I-Application
called	O
the	O
reflink	O
(	O
Linux	O
)	O
or	O
clonefile	O
(	O
MacOS	O
)	O
,	O
where	O
one	O
or	O
more	O
inodes	B-Application
(	O
file	O
information	O
entries	O
)	O
are	O
made	O
to	O
share	O
some	O
or	O
all	O
of	O
their	O
data	O
.	O
</s>
<s>
It	O
is	O
named	O
analogously	O
to	O
hard	B-Application
links	I-Application
,	O
which	O
work	O
at	O
the	O
inode	B-Application
level	O
,	O
and	O
symbolic	B-Application
links	I-Application
that	O
work	O
at	O
the	O
filename	O
level	O
.	O
</s>
<s>
The	O
individual	O
entries	O
have	O
a	O
copy-on-write	B-Language
behavior	O
that	O
is	O
non-aliasing	O
,	O
i.e.	O
</s>
<s>
Microsoft	O
's	O
ReFS	B-Application
also	O
supports	O
this	O
operation	O
.	O
</s>
<s>
A	O
second	O
example	O
would	O
be	O
backup	B-Protocol
.	O
</s>
<s>
Generally	O
this	O
will	O
be	O
a	O
backup	B-Protocol
store	O
such	O
as	O
a	O
data	O
repository	O
or	O
a	O
virtual	B-Device
tape	I-Device
library	I-Device
.	O
</s>
<s>
One	O
of	O
the	O
most	O
common	O
forms	O
of	O
data	B-General_Concept
deduplication	I-General_Concept
implementations	O
works	O
by	O
comparing	O
chunks	O
of	O
data	O
to	O
detect	O
duplicates	O
.	O
</s>
<s>
For	O
that	O
to	O
happen	O
,	O
each	O
chunk	O
of	O
data	O
is	O
assigned	O
an	O
identification	O
,	O
calculated	O
by	O
the	O
software	O
,	O
typically	O
using	O
cryptographic	B-Algorithm
hash	I-Algorithm
functions	I-Algorithm
.	O
</s>
<s>
4KB	O
block	O
size	O
in	O
WAFL	B-Application
)	O
.	O
</s>
<s>
In	O
some	O
systems	O
only	O
complete	O
files	O
are	O
compared	O
,	O
which	O
is	O
called	O
single-instance	B-Application
storage	I-Application
or	O
SIS	O
.	O
</s>
<s>
Client	O
backup	B-Protocol
deduplication	O
.	O
</s>
<s>
This	O
is	O
the	O
process	O
where	O
the	O
deduplication	O
hash	B-Algorithm
calculations	O
are	O
initially	O
created	O
on	O
the	O
source	O
(	O
client	O
)	O
machines	O
.	O
</s>
<s>
Files	O
that	O
have	O
identical	O
hashes	B-Algorithm
to	O
files	O
already	O
in	O
the	O
target	O
device	O
are	O
not	O
sent	O
,	O
the	O
target	O
device	O
just	O
creates	O
appropriate	O
internal	O
links	O
to	O
reference	O
the	O
duplicated	O
data	O
.	O
</s>
<s>
To	O
date	O
,	O
data	B-General_Concept
deduplication	I-General_Concept
has	O
predominantly	O
been	O
used	O
with	O
secondary	O
storage	O
systems	O
.	O
</s>
<s>
First	O
,	O
data	B-General_Concept
deduplication	I-General_Concept
requires	O
overhead	O
to	O
discover	O
and	O
remove	O
the	O
duplicate	O
data	O
.	O
</s>
<s>
Backup	B-Protocol
application	O
in	O
particular	O
commonly	O
generate	O
significant	O
portions	O
of	O
duplicate	O
data	O
over	O
time	O
.	O
</s>
<s>
Data	B-General_Concept
deduplication	I-General_Concept
has	O
been	O
deployed	O
successfully	O
with	O
primary	O
storage	O
in	O
some	O
cases	O
where	O
the	O
system	O
design	O
does	O
not	O
require	O
significant	O
overhead	O
,	O
or	O
impact	O
performance	O
.	O
</s>
<s>
Single-instance	B-Application
storage	I-Application
(	O
SIS	O
)	O
is	O
a	O
system	O
's	O
ability	O
to	O
take	O
multiple	O
copies	O
of	O
content	O
objects	O
and	O
replace	O
them	O
by	O
a	O
single	O
shared	O
copy	O
.	O
</s>
<s>
It	O
is	O
a	O
means	O
to	O
eliminate	O
data	B-General_Concept
duplication	I-General_Concept
and	O
to	O
increase	O
efficiency	O
.	O
</s>
<s>
SIS	O
is	O
frequently	O
implemented	O
in	O
file	B-Application
systems	I-Application
,	O
e-mail	B-Protocol
server	I-Protocol
software	O
,	O
data	B-Protocol
backup	I-Protocol
and	O
other	O
storage-related	O
computer	O
software	O
.	O
</s>
<s>
Single-instance	B-Application
storage	I-Application
is	O
a	O
simple	O
variant	O
of	O
data	B-General_Concept
deduplication	I-General_Concept
.	O
</s>
<s>
While	O
data	B-General_Concept
deduplication	I-General_Concept
may	O
work	O
at	O
a	O
segment	O
or	O
sub-block	O
level	O
,	O
single	B-Application
instance	I-Application
storage	I-Application
works	O
at	O
the	O
object	O
level	O
,	O
eliminating	O
redundant	O
copies	O
of	O
objects	O
such	O
as	O
entire	O
files	O
or	O
e-mail	O
messages	O
.	O
</s>
<s>
Single-instance	B-Application
storage	I-Application
can	O
be	O
used	O
alongside	O
(	O
or	O
layered	O
upon	O
)	O
other	O
data	B-General_Concept
duplication	I-General_Concept
or	O
data	O
compression	O
methods	O
to	O
improve	O
performance	O
in	O
exchange	O
for	O
an	O
increase	O
in	O
complexity	O
and	O
for	O
(	O
in	O
some	O
cases	O
)	O
a	O
minor	O
increase	O
in	O
storage	O
space	O
requirements	O
.	O
</s>
<s>
One	O
method	O
for	O
deduplicating	O
data	O
relies	O
on	O
the	O
use	O
of	O
cryptographic	B-Algorithm
hash	I-Algorithm
functions	I-Algorithm
to	O
identify	O
duplicate	O
segments	O
of	O
data	O
.	O
</s>
<s>
If	O
two	O
different	O
pieces	O
of	O
information	O
generate	O
the	O
same	O
hash	B-Error_Name
value	I-Error_Name
,	O
this	O
is	O
known	O
as	O
a	O
collision	B-Algorithm
.	O
</s>
<s>
The	O
probability	O
of	O
a	O
collision	B-Algorithm
depends	O
mainly	O
on	O
the	O
hash	B-Algorithm
length	O
(	O
see	O
birthday	O
attack	O
)	O
.	O
</s>
<s>
Thus	O
,	O
the	O
concern	O
arises	O
that	O
data	O
corruption	O
can	O
occur	O
if	O
a	O
hash	B-Algorithm
collision	I-Algorithm
occurs	O
,	O
and	O
additional	O
means	O
of	O
verification	O
are	O
not	O
used	O
to	O
verify	O
whether	O
there	O
is	O
a	O
difference	O
in	O
data	O
,	O
or	O
not	O
.	O
</s>
<s>
The	O
hash	B-Algorithm
functions	I-Algorithm
used	O
include	O
standards	O
such	O
as	O
SHA-1	B-Algorithm
,	O
SHA-256	B-Algorithm
and	O
others	O
.	O
</s>
<s>
The	O
computational	O
resource	O
intensity	O
of	O
the	O
process	O
can	O
be	O
a	O
drawback	O
of	O
data	B-General_Concept
deduplication	I-General_Concept
.	O
</s>
<s>
To	O
improve	O
performance	O
,	O
some	O
systems	O
utilize	O
both	O
weak	O
and	O
strong	O
hashes	B-Algorithm
.	O
</s>
<s>
Weak	O
hashes	B-Algorithm
are	O
much	O
faster	O
to	O
calculate	O
but	O
there	O
is	O
a	O
greater	O
risk	O
of	O
a	O
hash	B-Algorithm
collision	I-Algorithm
.	O
</s>
<s>
Systems	O
that	O
utilize	O
weak	O
hashes	B-Algorithm
will	O
subsequently	O
calculate	O
a	O
strong	O
hash	B-Algorithm
and	O
will	O
use	O
it	O
as	O
the	O
determining	O
factor	O
to	O
whether	O
it	O
is	O
actually	O
the	O
same	O
data	O
or	O
not	O
.	O
</s>
<s>
Note	O
that	O
the	O
system	O
overhead	O
associated	O
with	O
calculating	O
and	O
looking	O
up	O
hash	B-Error_Name
values	I-Error_Name
is	O
primarily	O
a	O
function	O
of	O
the	O
deduplication	O
workflow	O
.	O
</s>
<s>
Although	O
not	O
a	O
shortcoming	O
of	O
data	B-General_Concept
deduplication	I-General_Concept
,	O
there	O
have	O
been	O
data	O
breaches	O
when	O
insufficient	O
security	O
and	O
access	O
validation	O
procedures	O
are	O
used	O
with	O
large	O
repositories	O
of	O
deduplicated	O
data	O
.	O
</s>
<s>
In	O
some	O
systems	O
,	O
as	O
typical	O
with	O
cloud	O
storage	O
,	O
an	O
attacker	O
can	O
retrieve	O
data	O
owned	O
by	O
others	O
by	O
knowing	O
or	O
guessing	O
the	O
hash	B-Error_Name
value	I-Error_Name
of	O
the	O
desired	O
data	O
.	O
</s>
<s>
Deduplication	O
is	O
implemented	O
in	O
some	O
filesystems	B-Application
such	O
as	O
in	O
ZFS	B-Application
or	O
Write	B-Application
Anywhere	I-Application
File	I-Application
Layout	I-Application
and	O
in	O
different	O
disk	B-General_Concept
arrays	I-General_Concept
models	O
.	O
</s>
<s>
It	O
is	O
a	O
service	O
available	O
on	O
both	O
NTFS	B-Application
and	O
ReFS	B-Application
on	O
Windows	O
servers	O
.	O
</s>
