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Speed and Delay

To fully evaluate a hardware configuration the speedy and delays of the relevant devices needs to be considered, particularly, their actual performance rather than their nominal manufacturer specifications. Currently, magnetic devices are significantly faster and have significantly lower latency than optical devices.

Magnetic drives

In the Video Conference Recorder magnetic drives are used as a front end to the CD jukebox. These magnetic drives have fast access times and high bandwidth in comparison to CD jukeboxes which have slow access times and low bandwidth but very high storage capacity. Currently high-end disc drives such as 9 Gbyte drives are cheap.


Currently a high density CD has a capacity of 650 Mb per side. As a video stream may be more than 650 Mb, the server needs a mechanism to load a stream from many CDs onto the magnetic media. When changing from one CD to another there is a delay. The server mechanism should aim to hide the CD changeover time from the user. If the changeover time is not hidden, the media streams would stop until the new CD was loaded. The magnetic discs provide a buffering mechanism whereby media data can be loaded from CDs onto the magnetic discs and played out to the user with an initial delay. This delay can be large enough to hide the changeover time.

When changing CDs, the number of CD drives within the jukebox affects the performance of the server. Consider the process of accessing some data on a CD if there are:

2 drives

When using two drives it is possible to read from a CD in one drive and to load another CD into the other drive in advance of its use. When one CD is completely read, then access to another CD is just a case of flipping to the other drive. The CD in the original drive can now be replaced with a new CD. The delay is the time it takes to swap drives.

1 drive

When using one drive, the data is read from the CD until completion. Then there will be a delay whilst the CD in the drive is removed and put back into the jukebox. A new CD has to be chosen from the jukebox and placed into the drive. The delay is relatively large.

As stated, the use of front end magnetic discs can hide some of the delay by doing large amounts of buffering.

next up previous contents
Next: Indexing Techniques Up: Server Storage Previous: Use of a Jukebox