Hard Drive Geometry

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This material was developed with funding from the National Science Foundation under Grant # DUE 1601612

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What is Hard Drive Geometry? The typical computer hard disk drives are a very complex piece of equipment. The units are composed of two or more disks or platters that are attached to a common hub or spindle and motor assembly. These platters are coated with a ferric oxide material on which data is stored magnetically. The geometry of a hard drive is the organization of data on these platters. Geometry determines how and where data is stored on the surface of each platter, and thus the maximum storage capacity of the drive.

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Remove Platter

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Actuator Arms

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Actuator Arm & Read/Write Heads All hard drives require read/write heads. This mechanism reads and writes data to and from a hard drive by manipulating the magnetic medium that composes the surface of an associated disk platter. Because a platter has two sides, each platter will have two heads (one per side). The heads move from the outside of the platter's circumference to the inside of the platter circumference by an actuator arm.

Read/Write Heads 2 per platter

Click each item on the graphic below.

Platters

1

0

Write

0

Set the switches and then click the Write button.

Platter

Read/Write Heads “WRITING” In the simulation example above, the switches represent the binary data that needs to be written to the hard drive. As binary ones and zeros are sent to the heads, they write the data to the platter. Each side of a platter stores data.

Head 1A

Head 1B

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Read/Write Heads “READING” How does a head read data? At its most basic, the head is a piece of metal that’s wrapped in wire. As the head moves over the magnetic fields on the platter, changes in magnetism induce a current that is measured and converted into a binary value. It’s not quite that simple — there are different ways of making hard drive heads and encoding data on the magnetic surface but they all boil down to magnetize particles.

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Read

Click the Read button to start reading the data from the disk.

Track 4

Track 5

Step Inward

Track 6

Step Outward

Track 7

Track 0

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Track 1

Track 8

Path of the Read Write Heads As the platters spin, the actuator arm moves the heads of each platter across and above or below the platter surface to read and write data to the hard drive platters. The actuator uses tiny steps as it writes information to the platter. It stays long enough in one place to complete the recording of data around an entire circumference of the platter. This ring around the entire platter is called a track. The head records and reads data starting from the outside track inward. Each step inward writes to a new track.

Track 2

Track 3

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Next Cylinder

Previous Cylinder

Tracks and Cylinders All the read/write heads are attached to a fixed actuator arm. This means that all the heads will be located on the outside track simultaneously. Therefore, it makes sense to write to all the platters simultaneously. This process forms a united track across the top and bottom of all platters. The track on all platters form the outside cylinder. Track zero is on the outside and the higher number tracks are on the inside. The track zero for each platter make up cylinder zero.

Next Sector

Sectors In computer Hard Disk, a sector is a PIE SHAPED subdivision of a track on the magnetic surface of the platter. Each sector IS EQUAL LENGTH and stores a fixed amount of data, traditionally 512 bytes for hard disk drives. Newer Hard Drives use 4096-byte or more bytes per sectors.

Sectors A sector is a PIE-SHAPED subdivision of a PLATTER. Each sector stores an equal, fixed amount of data. A sector on a track is how data is stored and retrieved. Normally when data is written to the drive, a series of contiguous tracks are used and are called a cluster. Traditionally there are 512 bytes stored on each sector of an older drive and up to 4,096 bytes on new drives.

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0.5K

6

1K

4

4K

Click values to calculate the total capacity.

Sector Size

Number of Platters

2K

8

Number of Sectors

8K

To determine the total capacity for hard drive storage, identify the size of each sector, the number of sectors, the number of tracks, and the number of platters (remember, there is a top and bottom platter): Sector size × No. of sectors × No. of Tracks × No. of Platters × 2 If a hard drive has a sector size of 1,024 bytes with 2,000 sectors, 8,000 tracks, and 4 platters, what is its capacity?

16K

x 2

2

Number of Tracks

× 2

= 

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CALCULATE

ERROR Please choose Sector Size, Number of Sectors, Number of Tracks and Number of Platters before clicking Calculate

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Capacity Calculator

k 

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Cylinders

Tracks

Sectors

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Putting It All Together The platters rotate on a common axis at a speed of 5400 or 7200 rotations per minute (high- performance hard disks have higher speeds and older disks are slower). Remember that each platter has two sides. There are thousands of tracks, and current drives have billions of sectors. Click next to see examples of sector numbering.

Side 1

0 / 1 / 1 / 4

Side 0

1 / 1 / 3 / 6

2 – platters 0-1

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Sector Numbering

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4 – sides 0-1

PLATTER 0

5 – tracks per platter surface 0-4

PLATTER 1

8 – sectors per track 0-7

1 / 0 / 2 / 2

PLATTER / SIDE / TRACK/ SECTOR

0 / 0 / 0 / 0

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START

Quiz 1 0f 4 Click the correct location in the diagram above  for 0 / 0 / 2 / 3

0 / 0 / 2 / 3

Correct!

Try Again!

Take A Quiz Now take a quiz and see if you can find the correct location on the hard drive.

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0 / 1 / 3 / 4

Quiz 2 0f 4 Click the correct location in the diagram above  for 0 / 1 / 3 / 4

Quiz 3 0f 4 Click the correct location in the diagram above  for 1 / 1 / 0 / 0

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1 / 1 / 0 / 0

Quiz 4 0f 4 Click the correct location in the diagram above  for 0 / 0 / 4 / 6

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0 / 0 / 4 / 6