Server Storage

The hardware available for the media include CD and DVD jukeboxes plus some large magnetic discs. The hard discs are used as fast, temporary storage within the server, whereas the CDs are used for mass storage. When recording, media streams are sent from the network to the magnetic discs, and then onto CDs for permanent storage. When replaying data, the media which is stored on CDs is taken from the CD and sent to the magnetic drives for subsequent playback to the user. To manage this hardware and to present media streams back to the user, some software is needed to select CDs from the CD store and place them into the CD players of the jukebox.

A jukebox is a large collection of CD discs in a box with a few CD drives. These jukeboxes have a huge capacity, going up to Terabytes. We have seen that a video stream with a high compression ratio may generate over 1 Gb of data an hour, therefore these jukebox devices seem to be ideal for video and audio because of their large capacity, however their throughput could be a limitation. Currently a high density CD has a capacity of 650 Mb per side. Figure 9.7 shows how the discs, the network, and the server software are connected.

  

Figure 9.7: Storage Devices for Multimedia on Demand Servers

Given that an hour of compressed video can be expected to require about 240 Mb of storage per source, it is possible to evaluate the amount of storage space needed by an average conference recording. From this figure we can extrapolate to determine how much storage space such a system will require to be effective.

In terms of cost, as of December 1995, a 9 Gbyte disc drive costs about 1,500, and a fully populated jukebox with 144 platters costs about 85,000. With a 9 Gbyte disc drive one gets 6 Gbytes for 1,000, with a jukebox of 144 platters, each with 650 Mbytes per side, giving a total of 172 Gbytes, one gets 2 Gbytes for 1,000. Clearly there is a factor of 3 in the cost of owning a jukebox, and to replicate this amount of storage space on magnetic media with just 20 magentic drives would only cost 30,000. The main benefit of the optical device is that there is no need to buy extra backup devices and media.