<s>
The	O
Independent	B-General_Concept
Reference	I-General_Concept
Model	I-General_Concept
(	O
I.R.M	O
)	O
is	O
a	O
conceptual	O
model	O
used	O
in	O
the	O
analysis	O
of	O
storage	B-General_Concept
system	I-General_Concept
:	O
disk	B-General_Concept
drives	I-General_Concept
,	O
caches	O
,	O
etc	O
.	O
</s>
<s>
A	O
"	O
trace	O
"	O
is	O
simply	O
a	O
logging	O
of	O
a	O
set	O
of	O
data	O
regarding	O
the	O
performance	O
of	O
the	O
storage	O
device	O
,	O
focusing	O
on	O
the	O
I/O	B-Device
requests	I-Device
:	O
how	O
many	O
read/write	O
operations	O
,	O
the	O
size	O
of	O
each	O
request	O
,	O
the	O
exact	O
address	O
(	O
LUN-wise	O
)	O
,	O
and	O
a	O
time-stamp	O
.	O
</s>
<s>
For	O
example	O
:	O
For	O
a	O
disk	O
drive	O
with	O
4	O
tracks	O
,	O
A	O
,	O
B	O
,	O
C	O
and	O
D	O
and	O
after	O
15	O
minutes	O
of	O
work	O
,	O
the	O
I/O	B-Device
requests	I-Device
were	O
the	O
following	O
:	O
7600	O
,	O
20	O
,	O
50	O
,	O
6000	O
for	O
A	O
,	O
B	O
,	O
C	O
and	O
D	O
respectively	O
.	O
</s>
<s>
(	O
I	O
)	O
The	O
disk	O
first	O
received	O
and	O
responded	O
to	O
all	O
1,000	O
I/O	B-Device
requests	I-Device
at	O
track	O
A	O
,	O
and	O
later	O
,	O
all	O
1,000	O
I/Os	O
of	O
track	O
B	O
.	O
</s>
<s>
That	O
is	O
,	O
we	O
denote	O
N	O
as	O
the	O
sum	O
of	O
all	O
the	O
I/O	B-Device
requests	I-Device
(	O
both	O
read	O
and	O
write	O
)	O
and	O
assign	O
to	O
each	O
track	O
the	O
probability	O
the	O
number	O
of	O
I/Os	O
,	O
which	O
came	O
from	O
it	O
,	O
divided	O
by	O
N	O
.	O
</s>
