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6000000
Quantum
XP 34300
5000000
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Seagate
ST 12400 N
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2000000
1000000
0
0
100000
200000
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Retrieval Block Size Q (Bytes)
Figure 3.5 Effect of retrieval block size on the disk throughput
seeks that may violate the continuity requirement in equation (3.4). Consequently, the new
request can only start receiving service in the next service round and transmission will follow
after that as shown in Figure 3.3. Therefore, the total delay from receiving the new stream
request to the instant transmission begins will on the average amount to one and a half service
round, or 1.5 t max
round ( k ).
While the buffer cost will likely reduce as memory cost goes down, the admission delay
is a key performance metric experienced by the end user and a user will only have so much
patience. Therefore, ultimately the admission delay rather than buffer cost will be the limiting
factor in stretching the service round to improve disk efficiency.
3.4 Grouped Sweeping Scheme
In the previous discussion we came to the conclusion that delay/buffer and disk throughput
work against each other and thus it appears that disk throughout can only be increased so far
as limited by the admission delay. However, this analysis was based on the SCAN/CSCAN
disk schedulers and, in particular, due to the constraint that a new request can only be served
in the next service round.
To tackle this problem, Yu et al . [1] proposed a more general disk scheduler called the
Grouped Sweeping Scheme (GSS). It shares the common principle with SCAN/CSCAN in
that retrievals are also served in a scanning fashion in fixed-duration service rounds to reduce
disk seek overheads. However, instead of serving all streams in a service round, GSS divides
active streams into groups, say, g groups. Streams within the same group are served using
SCAN/CSCAN, while the groups are scheduled for service in a round-robin manner. Thus, the
disk schedule is effectively divided into two levels of service rounds - a micro service round
that serves a group of streams, and a macro service round that comprises the micro service
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