<s>
An	O
incremental	B-General_Concept
backup	I-General_Concept
is	O
one	O
in	O
which	O
successive	O
copies	O
of	O
the	O
data	O
contain	O
only	O
the	O
portion	O
that	O
has	O
changed	O
since	O
the	O
preceding	O
backup	O
copy	O
was	O
made	O
.	O
</s>
<s>
When	O
a	O
full	O
recovery	O
is	O
needed	O
,	O
the	O
restoration	O
process	O
would	O
need	O
the	O
last	O
full	O
backup	O
plus	O
all	O
the	O
incremental	B-General_Concept
backups	I-General_Concept
until	O
the	O
point	O
of	O
restoration	O
.	O
</s>
<s>
Incremental	B-General_Concept
backups	I-General_Concept
are	O
often	O
desirable	O
as	O
they	O
reduce	O
storage	O
space	O
usage	O
,	O
and	O
are	O
quicker	O
to	O
perform	O
than	O
differential	B-General_Concept
backups	I-General_Concept
.	O
</s>
<s>
The	O
most	O
basic	O
form	O
of	O
incremental	B-General_Concept
backup	I-General_Concept
consists	O
of	O
identifying	O
,	O
recording	O
and	O
thus	O
,	O
preserving	O
only	O
those	O
files	O
that	O
have	O
changed	O
since	O
the	O
last	O
backup	O
.	O
</s>
<s>
Since	O
changes	O
are	O
typically	O
low	O
,	O
incremental	B-General_Concept
backups	I-General_Concept
are	O
much	O
smaller	O
and	O
quicker	O
than	O
full	O
backups	O
.	O
</s>
<s>
The	O
use	O
of	O
rsync	B-Application
's	O
option	O
is	O
what	O
makes	O
this	O
command	O
an	O
example	O
of	O
incremental	B-General_Concept
backup	I-General_Concept
.	O
</s>
<s>
A	O
more	O
sophisticated	O
incremental	B-General_Concept
backup	I-General_Concept
scheme	O
involves	O
multiple	O
numbered	O
backup	O
levels	O
.	O
</s>
<s>
An	O
incremental	B-General_Concept
backup	I-General_Concept
of	O
the	O
changes	O
made	O
between	O
two	O
instances	O
of	O
a	O
mirror	O
can	O
be	O
forward	O
or	O
reverse	O
.	O
</s>
<s>
In	O
making	O
backups	O
using	O
reverse	O
incremental	B-General_Concept
backups	I-General_Concept
,	O
each	O
time	O
a	O
reverse	O
incremental	B-General_Concept
backup	I-General_Concept
is	O
taken	O
,	O
it	O
is	O
applied	O
(	O
in	O
reverse	O
)	O
to	O
the	O
previous	O
full	O
(	O
synthetic	O
)	O
backup	O
,	O
thus	O
the	O
current	O
full	O
(	O
synthetic	O
)	O
backup	O
is	O
always	O
a	O
backup	O
of	O
the	O
most	O
recent	O
state	O
of	O
the	O
system	O
.	O
</s>
<s>
This	O
is	O
in	O
contrast	O
to	O
forward	O
incremental	B-General_Concept
backups	I-General_Concept
where	O
the	O
current	O
full	O
backup	O
is	O
a	O
backup	O
of	O
the	O
oldest	O
version	O
of	O
the	O
system	O
,	O
and	O
to	O
get	O
a	O
backup	O
of	O
the	O
most	O
recent	O
state	O
of	O
the	O
system	O
,	O
all	O
of	O
the	O
forward	O
incremental	B-General_Concept
backups	I-General_Concept
have	O
to	O
be	O
applied	O
to	O
that	O
oldest	O
version	O
successively	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
after	O
the	O
initial	O
full	O
backup	O
,	O
each	O
successive	O
incremental	B-General_Concept
backup	I-General_Concept
applies	O
the	O
changes	O
to	O
the	O
previous	O
full	O
,	O
creating	O
a	O
new	O
synthetic	O
full	O
backup	O
every	O
time	O
,	O
while	O
maintaining	O
the	O
ability	O
to	O
revert	O
to	O
previous	O
versions	O
.	O
</s>
<s>
Reverse	O
incremental	B-General_Concept
backup	I-General_Concept
works	O
for	O
both	O
tapes	O
and	O
disks	O
,	O
but	O
in	O
practice	O
tends	O
to	O
work	O
better	O
with	O
disks	O
.	O
</s>
<s>
Companies	O
using	O
the	O
reverse	O
incremental	B-General_Concept
backup	I-General_Concept
method	O
include	O
Intronis	O
and	O
Zetta.net	B-General_Concept
.	O
</s>
<s>
After	O
an	O
initial	O
full	O
backup	O
,	O
only	O
the	O
incremental	B-General_Concept
backups	I-General_Concept
are	O
sent	O
to	O
a	O
centralized	O
backup	O
system	O
.	O
</s>
<s>
These	O
backup	O
technologies	O
are	O
similar	O
to	O
the	O
"	O
block-level	O
incremental	O
"	O
backup	O
method	O
;	O
however	O
,	O
the	O
byte	O
(	O
or	O
binary	O
)	O
incremental	B-General_Concept
backup	I-General_Concept
method	O
is	O
based	O
on	O
a	O
binary	O
variation	O
of	O
the	O
files	O
compared	O
to	O
the	O
previous	O
backup	O
:	O
while	O
the	O
block-based	O
technologies	O
work	O
with	O
heavy	O
changing	O
units	O
(	O
blocks	O
of	O
8K	O
,	O
4K	O
or	O
1K	O
)	O
,	O
the	O
byte-based	O
technologies	O
work	O
with	O
the	O
minimum	O
unit	O
,	O
saving	O
space	O
when	O
reflecting	O
a	O
change	O
on	O
a	O
file	O
.	O
</s>
<s>
Instead	O
of	O
reading	O
and	O
backing	O
up	O
data	O
directly	O
from	O
the	O
disk	O
,	O
it	O
will	O
synthesize	O
the	O
data	O
from	O
the	O
previous	O
full	O
backup	O
(	O
either	O
a	O
regular	O
full	O
backup	O
for	O
the	O
first	O
backup	O
,	O
or	O
the	O
previous	O
synthetic	O
full	O
backup	O
)	O
and	O
the	O
periodic	O
incremental	B-General_Concept
backups	I-General_Concept
.	O
</s>
<s>
As	O
only	O
the	O
incremental	B-General_Concept
backups	I-General_Concept
read	O
data	O
from	O
the	O
disk	O
,	O
these	O
are	O
the	O
only	O
files	O
that	O
need	O
to	O
be	O
transferred	O
during	O
offsite	O
replication	O
.	O
</s>
<s>
A	O
differential	B-General_Concept
backup	I-General_Concept
is	O
a	O
cumulative	O
backup	O
of	O
all	O
changes	O
made	O
since	O
the	O
last	O
full	O
or	O
normal	O
backup	O
,	O
i.e.	O
,	O
the	O
differences	O
since	O
the	O
last	O
full	O
backup	O
.	O
</s>
<s>
The	O
advantage	O
to	O
this	O
is	O
the	O
quicker	O
recovery	O
time	O
,	O
requiring	O
only	O
a	O
full	O
backup	O
and	O
the	O
last	O
differential	B-General_Concept
backup	I-General_Concept
to	O
restore	O
the	O
system	O
.	O
</s>
