Start Lecture #24
Peripherals are varied; indeed they vary widely in many dimensions,
e.g., cost, physical size, purpose, capacity, transfer rate,
response time, support for
random access, connectors, and
Consider just transfer rate for the moment.
The text mentions three especially important dimensions.
The diagram on the right is quite oversimplified for modern PCs; a more detailed version is below.
Devices are quite varied and their data rates vary enormously.
Show a real disk opened up and illustrate the components.
The time for a disk access has five components, of which we concentrate on the first three.
Today seek times are typically 5-10ms on
It takes longer to go all the way across the disk but it
does not take twice as long to go twice as far (the
head must accelerate, decelerate, and settle on the track).
How should we calculate the average?
Since disks have just one arm the average rotational latency is half the time of a revolution, and is thus determined by the RPM (revolutions per minute) of the disk.
Disks today spin at 5400-15,000 RPM; they used to all spin at 3600 RPM.
You might consider the other three times all overhead since it is the transfer time during which the data is actually being supplied.
The transfer rate is typically a few tens of MB per second. Given the rate, which is determined by the disk in use, the transfer time is proportional to the length of the request.
Some manufacturers quote a much higher rate, but that is for cache hits. In addition to supplying data much sooner, the electronic cache can transfer data at a higher rate than the mechanical disk.