Continuous Media Storage and Retrieval - Scalable Continuous Media Streaming Systems - page 43

3

Continuous Media Storage
and Retrieval

Continuous media data streams generate vast amount of data. Even after compression, the
data rate is often still substantial, especially if audio and video are encoded. In terms of
storage, the amount of storage space required in a, say, hard disk-based media server can
easily be estimated. Moreover, the recent advances in hard disk technology have signifi-
cantly reduced the price-per-byte storage cost. At the time of writing the cost for 100GB
of hard disk storage is less than US$100, and hard disk models of size up to 300GB are
already widely available. Thus, although it is still an issue to be considered in the de-
sign of media servers, the storage capacity issue is becoming less of a problem in recent
years.
By contrast, the I/O capacity of hard disk, while it has improved steadily over the years,
has not progressed at the same pace as storage capacity. While we can store many media
contents in a large hard disk, it may not have sufficient I/O capacity to satisfy the streaming
demand. Therefore, the challenge of improving disk I/O efficiency is still a relevant and
important research issue.
In this chapter, we review the hard disk technology with an emphasis on the impact to
media streaming. We present ways to model the hard disk to derive performance results
useful in designing media servers. Of particular emphasis are the design and engineering
of the disk scheduler, which has significant impact on disk I/O efficiency. We analyze the
trade-offs in disk scheduler design in both single-disk and multi-disk media servers.

3.1 Structure and Model of Hard Disk
Figure 3.1 depicts the basic mechanical structure of a hard disk. There are a number of disk
platters mounted on the same spindle rotating at a constant speed, ranging from 3,600 rounds-
per-minute (rpm) to over 15,000 rpm. Each disk platter is sub-divided into many concentric
tracks, which are further sub-divided into many sectors. A sector is the smallest unit for data
storage and retrieval. To read and write to the disk, a disk head is mounted on a disk arm which