<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
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
,	O
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-instance	B-Application
storage	I-Application
works	O
at	O
the	O
whole-file	O
level	O
and	O
eliminates	O
redundant	O
copies	O
of	O
entire	O
files	O
or	O
e-mail	O
messages	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
an	O
e-mail	B-Protocol
server	I-Protocol
,	O
single-instance	B-Application
storage	I-Application
would	O
mean	O
that	O
a	O
single	O
copy	O
of	O
a	O
message	O
is	O
held	O
within	O
its	O
database	O
while	O
individual	O
mailboxes	O
access	O
the	O
content	O
through	O
a	O
reference	O
pointer	O
.	O
</s>
<s>
However	O
,	O
there	O
is	O
a	O
common	O
misconception	O
that	O
the	O
primary	O
benefit	O
of	O
single-instance	B-Application
storage	I-Application
in	O
mail	B-Protocol
servers	I-Protocol
is	O
a	O
reduction	O
in	O
disk	O
space	O
requirements	O
.	O
</s>
<s>
In	O
a	O
mail	B-Protocol
server	I-Protocol
scenario	O
disk	O
space	O
savings	O
from	O
single-instance	B-Application
storage	I-Application
are	O
transient	O
and	O
drop	O
off	O
very	O
quickly	O
over	O
time	O
.	O
</s>
<s>
When	O
used	O
in	O
conjunction	O
with	O
backup	B-Protocol
software	O
,	O
single-instance	B-Application
storage	I-Application
can	O
reduce	O
the	O
quantity	O
of	O
archive	B-Application
media	O
required	O
since	O
it	O
avoids	O
storing	O
duplicate	O
copies	O
of	O
the	O
same	O
file	O
.	O
</s>
<s>
Often	O
identical	O
files	O
are	O
installed	O
on	O
multiple	O
computers	O
,	O
for	O
example	O
operating	B-General_Concept
system	I-General_Concept
files	O
.	O
</s>
<s>
With	O
single-instance	B-Application
storage	I-Application
,	O
only	O
one	O
copy	O
of	O
a	O
file	O
is	O
written	O
to	O
the	O
backup	B-Protocol
media	O
therefore	O
reducing	O
space	O
.	O
</s>
<s>
This	O
becomes	O
more	O
important	O
when	O
the	O
storage	O
is	O
offsite	O
and	O
on	O
cloud	O
storage	O
such	O
as	O
Amazon	B-General_Concept
S3	I-General_Concept
.	O
</s>
<s>
In	O
such	O
cases	O
,	O
it	O
has	O
been	O
reported	O
that	O
deduplication	O
can	O
help	O
reduce	O
the	O
costs	O
of	O
storage	O
,	O
costs	O
of	O
bandwidth	O
and	O
backup	B-Protocol
windows	O
by	O
up	O
to	O
10:1	O
.	O
</s>
<s>
Novell	B-Operating_System
GroupWise	I-Operating_System
was	O
built	O
on	O
single-instance	B-Application
storage	I-Application
,	O
which	O
accounts	O
for	O
its	O
large	O
capacity	O
.	O
</s>
<s>
SIS	O
is	O
related	O
to	O
system	O
wide	O
file	O
duplication	O
search	O
and	O
multiple	O
file	O
instance	O
detection	O
tools	O
such	O
as	O
the	O
P2P	O
application	O
BearShare	B-Protocol
(	O
5.n	O
Versions	O
and	O
below	O
)	O
but	O
differs	O
in	O
that	O
SIS	O
reduces	O
storage	O
utilization	O
automatically	O
and	O
creates	O
and	O
retains	O
symbolic	O
linkages	O
,	O
whereas	O
Bearshare	B-Protocol
allows	O
for	O
manual	O
deletion	O
of	O
duplicates	O
and	O
associated	O
user-level	O
file	B-Application
system	I-Application
,	O
Windows	B-Protocol
Explorer	I-Protocol
type	O
of	O
icon	O
links	O
.	O
</s>
<s>
A	O
typical	O
server	O
might	O
hold	O
ten	O
or	O
more	O
unique	O
installation	O
configurations	O
(	O
perhaps	O
with	O
different	O
device	B-Application
drivers	I-Application
or	O
software	B-Application
suites	I-Application
)	O
but	O
perhaps	O
only	O
20%	O
of	O
the	O
data	O
may	O
be	O
unique	O
between	O
configurations	O
.	O
</s>
<s>
Microsoft	O
states	O
that	O
"	O
SIS	O
works	O
by	O
searching	O
a	O
hard	O
disk	O
volume	O
to	O
identify	O
duplicate	B-General_Concept
files	I-General_Concept
.	O
</s>
<s>
When	O
SIS	O
finds	O
identical	O
files	O
,	O
it	O
saves	O
one	O
copy	O
of	O
the	O
file	O
to	O
a	O
central	O
repository	O
,	O
called	O
the	O
SIS	O
Common	O
Store	O
,	O
and	O
replaces	O
other	O
copies	O
with	O
pointers	B-Application
to	O
the	O
stored	O
versions.	O
"	O
</s>
<s>
Files	O
are	O
compared	O
solely	O
by	O
their	O
hash	B-Error_Name
functions	I-Error_Name
;	O
files	O
with	O
different	O
names	O
or	O
dates	O
can	O
be	O
consolidated	O
so	O
long	O
as	O
the	O
data	O
itself	O
is	O
identical	O
.	O
</s>
<s>
Windows	B-Application
Server	I-Application
2003	I-Application
Standard	O
Edition	O
has	O
SIS	O
capabilities	O
but	O
is	O
limited	O
to	O
OEM	O
OS	O
system	O
installs	O
.	O
</s>
<s>
The	O
file-based	O
Windows	O
Imaging	O
Format	O
introduced	O
in	O
Windows	B-Application
Vista	I-Application
also	O
supported	O
single-instance	B-Application
storage	I-Application
.	O
</s>
<s>
Single-instance	B-Application
storage	I-Application
was	O
a	O
feature	O
of	O
Microsoft	B-Operating_System
Exchange	I-Operating_System
Server	I-Operating_System
since	O
version	O
4.0	O
and	O
is	O
also	O
present	O
in	O
Microsoft	O
's	O
Windows	B-Application
Home	I-Application
Server	I-Application
.	O
</s>
<s>
It	O
is	O
deduplicating	O
attachments	O
only	O
in	O
Exchange	O
2007	O
and	O
was	O
dropped	O
completely	O
in	O
Microsoft	B-Operating_System
Exchange	I-Operating_System
Server	I-Operating_System
2010	O
.	O
</s>
<s>
Microsoft	O
announced	O
Windows	O
Storage	O
Server	B-Device
2008	I-Device
(	O
WSS2008	O
)	O
with	O
Single	B-Application
Instance	I-Application
Storage	I-Application
on	O
June	O
1	O
,	O
2009	O
,	O
and	O
states	O
this	O
feature	O
is	O
not	O
available	O
on	O
Windows	B-Device
Server	I-Device
2008	I-Device
.	O
</s>
<s>
The	O
feature	O
is	O
officially	O
deprecated	O
since	O
Windows	O
Server	O
2012	O
,	O
when	O
a	O
new	O
,	O
more	O
powerful	O
chunk-based	O
data	B-General_Concept
deduplication	I-General_Concept
mechanism	O
was	O
introduced	O
.	O
</s>
