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0
L/4
2L/4
3L/4
L
12
3 … 30
31 32 33 … 60
61 62 63 … 90
91 92 93 … 120
Channel 1
91 92
93 … 120
1 2 3 … 30
31 32 33 … 60
61 62 63 … 90
Channel 2
61 62
63 …
90
91 92 93 … 120
1 2 3 … 30
31 32 33 … 60
Channel 3
31 32
33 …
60
61 62 63 … 90
91 92 93 … 120
1 2 3 … 30
Channel 4
Data retrieved in a service round
……
b
i.j
+
kN+(N
sc
−
1)H/N
sc
b
i . j
+
kN
b
i . j
+
kN
+
H/N
sc
B
i,j,k
……
B
2,j,k
B
M,j,k
G
j,o
B
1,j,k
Disk 1
Disk 2
Disk N
Disk 3
G
1,0
G
2,0
G
3,0
G
N,0
G
1,1
G
2,1
G
3,1
G
N,1
……
G
1,2
G
2,2
G
3,2
G
N,2
G
1,3
G
2,3
G
3,3
G
N,3
Figure 20.3
Data access pattern and placement of static channels
20.4.2 Interleaving of Data Blocks
Next we consider the interleaving data placement policy for serving the static channels. Con-
sider a system with
N
120 as shown in Figure 20.3, illustrating the data
access pattern for the static channels.We observe that data blocks
b
i
,
=
2
,
N
S
=
4 and
k
=
,
b
i
,
,
b
i
,
,
b
i
,
j
j
+
30
j
+
60
j
+
90
of video
i
stored in disk 1 and data blocks
b
i
,
,
b
i
,
,
b
i
,
,
b
i
,
91
stored in disk 2 are
j
+
1
j
+
31
j
+
61
j
+
always retrieved together in the same service round, with
j
29]. Thus, by placing
these data blocks in a continuous portion of the disk surface, we can effectively eliminate the
disk seeks required in conventional round-based schedulers. However, this placement policy
only works for static channels where the transmission schedules are known and fixed. We
address data retrievals for dynamic channels using replication in the next section.
∈
[1
,
3
,
5
,...
20.4.3 First T
R
Seconds Replication
Data retrievals for dynamic channels are more random in nature and hence the interleaving data
placement policy offers no advantage. Moreover, with the WSGP policy in place, serving the
dynamic channels using the interleaving data placement policy will result in the disk constantly
seeking between outer tracks and inner tracks, further degrading disk throughput. To tackle
this problem, we note that dynamic channels have one crucial property - it only serves up to
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