Computer Architecture

Start Lecture #24

Chapter 8: Storage, Networks, and Other Peripherals.

Introduction

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 protocol.

Consider just transfer rate for the moment.

• Some devices like keyboards and mice have tiny data rates.
• Printers, etc have moderate data rates.
• Disks and fast networks have high data rates.
• A good graphics card and monitor has a huge data rate.

The text mentions three especially important dimensions.

• Input vs. output vs. storage.
  A keyboard is an input device (meaning the device produces data that is input by the processor).
  A printer is an output device (meaning ...).
  A disk is a storage device (meaning it can be read, reread, written, and rewritten.
• Used directly by a human (e.g. a monitor)?
• Transfer rate.

The diagram on the right is quite oversimplified for modern PCs; a more detailed version is below.

8.2: Disk Storage and Dependability

Devices are quite varied and their data rates vary enormously.

Show a real disk opened up and illustrate the components.

• Platter
• Surface
• Head
• Track
• Sector
• Cylinder
• Seek time
• Rotational latency
• Transfer time

Disk Access Time

The time for a disk access has five components, of which we concentrate on the first three.

1. Seek.
2. Rotational latency.
3. Transfer time.
4. Controller overhead.
5. Queuing delays.

Seek Time

Today seek times are typically 5-10ms on average. 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?

• Add the times for all possible seeks and divide by the number of possible seeks (of course!).
• But systems achieve average seek times much smaller than this. How?
• Locality of reference!
  So most seeks are small. The locality is further enhanced by disk scheduling algorithms.
• Caches (again)!
  Disks have (electronic, not mechanical) caches.

Rotational Latency

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.

Transfer Time

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.