<s>
In	O
computing	O
,	O
an	O
extent	O
is	O
a	O
contiguous	O
area	O
of	O
storage	O
reserved	O
for	O
a	O
file	O
in	O
a	O
file	B-Application
system	I-Application
,	O
represented	O
as	O
a	O
range	O
of	O
block	O
numbers	O
,	O
or	O
tracks	B-Device
on	O
count	B-Application
key	I-Application
data	I-Application
devices	O
.	O
</s>
<s>
A	O
file	O
can	O
consist	O
of	O
zero	O
or	O
more	O
extents	B-Application
;	O
one	O
file	B-Application
fragment	I-Application
requires	O
one	O
extent	O
.	O
</s>
<s>
Also	O
,	O
extent	O
allocation	O
results	O
in	O
less	O
file	B-Application
fragmentation	I-Application
.	O
</s>
<s>
Extent-based	O
file	B-Application
systems	I-Application
can	O
also	O
eliminate	O
most	O
of	O
the	O
metadata	O
overhead	O
of	O
large	O
files	O
that	O
would	O
traditionally	O
be	O
taken	O
up	O
by	O
the	O
block-allocation	O
tree	O
.	O
</s>
<s>
In	O
order	O
to	O
resist	O
fragmentation	B-Application
,	O
several	O
extent-based	O
file	B-Application
systems	I-Application
do	O
allocate-on-flush	B-Application
.	O
</s>
<s>
Many	O
modern	O
fault-tolerant	B-General_Concept
file	B-Application
systems	I-Application
also	O
do	O
copy-on-write	B-Language
,	O
although	O
that	O
increases	O
fragmentation	B-Application
.	O
</s>
<s>
As	O
a	O
similar	O
design	O
,	O
the	O
CP/M	B-Operating_System
file	I-Operating_System
system	I-Operating_System
uses	O
extents	B-Application
as	O
well	O
,	O
but	O
those	O
do	O
not	O
correspond	O
to	O
the	O
definition	O
given	O
above	O
.	O
</s>
<s>
CP/M	B-Application
'	O
s	O
extents	B-Application
appear	O
contiguously	O
as	O
a	O
single	O
block	O
in	O
the	O
combined	O
directory/allocation	O
table	O
,	O
and	O
they	O
do	O
not	O
necessarily	O
correspond	O
to	O
a	O
contiguous	O
data	O
area	O
on	O
disk	O
.	O
</s>
<s>
IBM	O
OS/360	B-Application
and	I-Application
successors	I-Application
allocate	O
files	O
in	O
multiples	O
of	O
disk	O
tracks	B-Device
or	O
cylinders	O
.	O
</s>
<s>
Files	O
could	O
originally	O
have	O
up	O
to	O
16	O
extents	B-Application
,	O
but	O
this	O
restriction	O
has	O
since	O
been	O
lifted	O
.	O
</s>
<s>
The	O
initial	O
allocation	O
size	O
and	O
the	O
size	O
of	O
additional	O
extents	B-Application
to	O
be	O
allocated	O
if	O
required	O
are	O
specified	O
by	O
the	O
user	O
via	O
Job	B-Application
Control	I-Application
Language	I-Application
.	O
</s>
<s>
The	O
systems	O
supporting	O
filesystem	B-Application
extents	B-Application
include	O
the	O
following	O
:	O
</s>
