<s>
The	O
inode	B-Application
pointer	I-Application
structure	I-Application
is	O
a	O
structure	O
adopted	O
by	O
the	O
inode	B-Application
of	O
a	O
file	O
in	O
the	O
Unix	B-Operating_System
File	I-Operating_System
System	I-Operating_System
(	O
UFS	O
)	O
to	O
list	O
the	O
addresses	O
of	O
a	O
file	O
's	O
data	B-General_Concept
blocks	I-General_Concept
.	O
</s>
<s>
It	O
is	O
also	O
adopted	O
by	O
many	O
related	O
file	O
systems	O
,	O
including	O
the	O
ext3	B-Application
file	O
system	O
,	O
popular	O
with	O
Linux	O
users	O
.	O
</s>
<s>
These	O
pointers	O
consist	O
of	O
(	O
assuming	O
15	O
pointers	O
in	O
the	O
inode	B-Application
)	O
:	O
</s>
<s>
The	O
structure	O
allows	O
for	O
inodes	B-Application
to	O
describe	O
very	O
large	O
files	O
in	O
file	O
systems	O
with	O
a	O
fixed	O
logical	O
block	B-General_Concept
size	I-General_Concept
.	O
</s>
<s>
Central	O
to	O
the	O
mechanism	O
is	O
that	O
blocks	O
of	O
addresses	O
(	O
also	O
called	O
indirect	B-Application
blocks	I-Application
)	O
are	O
only	O
allocated	O
as	O
needed	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
12-block	O
file	O
would	O
be	O
described	O
using	O
just	O
the	O
inode	B-Application
because	O
its	O
blocks	O
fit	O
into	O
the	O
number	O
of	O
direct	O
pointers	O
available	O
.	O
</s>
<s>
However	O
,	O
a	O
13-block	O
file	O
needs	O
an	O
indirect	B-Application
block	I-Application
to	O
contain	O
the	O
thirteenth	O
address	O
.	O
</s>
<s>
The	O
inode	B-Application
pointer	I-Application
structure	I-Application
not	O
only	O
allows	O
for	O
files	O
to	O
easily	O
be	O
allocated	O
to	O
non-contiguous	O
blocks	O
,	O
it	O
also	O
allows	O
the	O
data	O
at	O
a	O
particular	O
location	O
inside	O
a	O
file	O
to	O
be	O
easily	O
located	O
.	O
</s>
<s>
This	O
is	O
possible	O
because	O
the	O
logical	O
block	B-General_Concept
size	I-General_Concept
is	O
fixed	O
.	O
</s>
<s>
For	O
example	O
,	O
if	O
each	O
block	O
is	O
8	O
kB	O
,	O
file	O
data	O
at	O
112	O
kB	O
to	O
120	O
kB	O
would	O
be	O
pointed	O
to	O
by	O
the	O
third	O
pointer	O
of	O
the	O
first	O
indirect	B-Application
block	I-Application
(	O
assuming	O
twelve	O
direct	O
pointers	O
in	O
the	O
inode	B-Application
pointer	I-Application
structure	I-Application
)	O
.	O
</s>
<s>
Unlike	O
the	O
inodes	B-Application
,	O
which	O
are	O
fixed	O
in	O
number	O
and	O
allocated	O
in	O
a	O
special	O
part	O
of	O
the	O
file	O
system	O
,	O
the	O
indirect	B-Application
blocks	I-Application
may	O
be	O
of	O
any	O
number	O
and	O
are	O
allocated	O
in	O
the	O
same	O
part	O
of	O
the	O
file	O
system	O
as	O
data	B-General_Concept
blocks	I-General_Concept
.	O
</s>
<s>
The	O
number	O
of	O
pointers	O
in	O
the	O
indirect	B-Application
blocks	I-Application
is	O
dependent	O
on	O
the	O
block	B-General_Concept
size	I-General_Concept
and	O
size	O
of	O
block	O
pointers	O
.	O
</s>
<s>
Example	O
:	O
with	O
a	O
512-byte	O
block	B-General_Concept
size	I-General_Concept
,	O
and	O
4-byte	O
block	O
pointers	O
,	O
each	O
indirect	B-Application
block	I-Application
can	O
consist	O
of	O
128	O
(	O
512	O
/	O
4	O
)	O
pointers	O
.	O
</s>
