Continuous Media Storage and Retrieval - Scalable Continuous Media Streaming Systems - page 46

Q / R

. .

. .

. . . . . .

. . .

Transmission Transmission

. . . . . .

. . . . . .

. . .

Retrieval Retrieval

one round one round

Figure 3.3 Retrieval and transmission scheduling in a media server

scanning direction

2 5 4 3 1

DiskPlatter

2 5 4 3 1

Service Order:

Figure 3.4 Using SCAN scheduler to reduce disk seek overhead

Note that the order of data retrievals within a disk service round is not fixed, and can vary
from round to round. Specifically, the service order is selected according to the scanning
direction of the disk head, which goes alternatively from the innermost track to the outermost
track in a round, and then back from the outermost track to the innermost track in the next round
(see Figure 3.4). This enables the retrievals to be performed without back-and-forth seeking
within a service round, thus reducing seeking overhead. This is known as the Circular-SCAN
(CSCAN) scheduler or elevator seeking.
Note that to use CSCANwe need to knowwhich data blocks to retrieve before the beginning
of a round. This is possible in a media server because most of the time a media stream retrieves
media data sequentially from the disk for transmission to the client, thus enabling the server
to know the future data retrievals given the media bit-rate and retrieval block size.

3.2.1 Performance Modeling
Using the disk model in equation (3.1), we can formulate the time required to complete a disk
service round using CSCAN. Assuming the disk serves k requests in a round, then the service
round length, denoted by t round ( k ), is given by

t seek +

k

Q
R disk

t i
latency

t end
seek

t round ( k ) = k α + + + (3.3)

i = 1