Hard Disks and I/O

Polytechnic University

CS2214

11/25/03

Input/Output

Processor

Cache

Memory I/O bus

Main Memory I/O controller I/O controller I/O controller

Graphics output Network

Interrupts

Disk

I/O contd. I/O bandwidth measurements

How much data can be moved through system in a given time? E.g. supercomputer applications

How many I/O operations can we do per unit of time? E.g. National Income Tax Service

I/O Characteristics Behavior

Input (read once) Output (write only, cannot be read) Storage (can be reread and usually rewritten)

Partner Human or machine or other end?

Data Rate Peak rate at which data can be transferred between

I/O device and main memory or processor.

I/O Characteristics Contd.

Device Behavior Partner Data Rate (KB/sec)

Keyboard Input Human .01

Line Printer

Output Human 1.00

Modem Input/ Output

Machine 2.00-8.00

Magnetic Disk

Storage Machine 2,000-10,000

Common Hard Disk Parameters

Data rate - Number of bytes per second that a drive can deliver to the CPU.

Seek time - Amount of time between when the CPU requests a file and when the first byte of the file is sent to the CPU.

Capacity - Number of bytes disk can hold

Inside A Hard Disk Sealed aluminum box

with controller electronics attached to one side.

Electronics control the read/write mechanism and the motor that spins the platters.

Electronics are all contained on a small board that detaches from rest of the drive.

www.howstuffworks.com

Inside A Hard Disk Contd. The platters spin at 3,600 or

7,200 rpm when the drive is operating. These platters are manufactured to amazing tolerances and are mirror-smooth.

The arm that holds the read/write heads is controlled by the mechanism in the upper-left corner, and is able to move the heads from the hub to the edge of the drive. The arm and its movement mechanism are extremely light and fast. www.howstuffworks.com

                                                                                       

                                                         

Inside A Hard Disk Contd.

www.howstuffworks.com

Hard Disk Access Times Seek time Rotational time Transfer time Controller time

Problem What is the average time to read or write 512 byte

sector for a disk rotating at 5400 RPM? Average seek time is 12ms, transfer rate is 5MB/sec and controller overhead is 2ms. Average disk access time

= Tseek+TRotational+ TTransfer+ TController

=12ms + 5.6ms+.5KB/5MB/sec +2ms

= 12+5.6+0.1+2

= 19.7 ms.

Hard Disk Animation Animation