Hard Drive Capacity

November 10 (Day); November 10 (Night)

  • Hard Drive Capacity:
  • If you know the number of cylinders (tracks), heads (surfaces), and sectors per track that are on your hard drive, you can calculate its capacity.
  • Pretend that we have a hard drive with 855 tracks, an average of 17 sectors per track, and 4 platters.
  • First of all, we calculate the capacity of each surface:

855 tracks / X / 17 sectors / X / 512 bytes / = / 7,441,920 bytes
1 track / 1 sector
  • Now, we know that there are 7,441,920 bytes on one surface. There are 4 platters in our hard drive (which means that we have 8 surfaces on which to store data) on our hard drive, so we multiply the above total by 8.

8 surfaces / X / 7,441,920 bytes / = / 59,535,360 bytes
1 surface
  • Now, we will convert the total number of bytes into megabytes:
  • First, we convert the number of bytes into kilobytes.

59,535,360 bytes / X / 1 kilobyte / = / 58,140 kilobytes
1024 bytes
  • Finally, we convert the number of kilobytes into megabytes.

58,140 kilobytes / X / 1 megabytes / = / 56.78 megabytes
1024 kilobytes
  • Hard Drive Manufacturers Will Cheat You:
  • Hard drive manufacturers cheat people everyday by misrepresenting the size of the hard drive. Here’s how:

Computer Science Way:
1024 (210) bytes = 1 KB
1024 (210) KB = 1 MB
1024 (210) MB = 1 GB / HD Manufacturer Way:
1000 bytes = 1 KB
1000 KB = 1 MB
1000 MB = 1 GB
  • Let’s pretend that a hard drive manufacturer advertises an 80 GB hard drive.
  • Using the proper Computer Science way, you should receive 85,899,345,920 bytes on this hard drive.

80 GB / X / 1024 MB / X / 1024 KB / X / 1024 bytes / = / 85,899,345,920 bytes
1 GB / 1 MB / 1 KB
  • Using the improper Hard Drive Manufacturer way, you will only receive 80,000,000,000 bytes on this hard drive.

80 GB / X / 1000 MB / X / 1000 KB / X / 1000 bytes / = / 80,000,000,000 bytes
1 GB / 1 MB / 1 KB
  • This means that you are getting cheated out of 5,899,345,920 bytes. That’s well over 5 GB.

Difference = Computer Science way – HD Manufacturer way

= 85,899,345,920 – 80,000,000,000

= 5,899,345,920 bytes

  • Measuring Disk Access Time:
  • Here are the factors necessary to calculating the amount of time it takes to access the a single file on your hard drive:
  • Head activation time – time it takes the read/write head to start; involves no physical movement; period of time is so small that is usually ignored.
  • Seek time:
  • Average seek time – average amount of time it takes to move the read/write head from one cylinder to another cylinder a random distance away; measured in milliseconds.
  • Minimum seek time – amount of time it takes to move the read/write head from one cylinder; measured in milliseconds.
  • Rotational delay – amount of time it takes for a disk to spin around so that the desired sector is underneath read/write head.
  • Calculating Seek Time:
  • Calculating the seek time of a perfectly defragmented (in other words, files are stored in continuous chunks) disk:

Seek time = Average seek time + (Minimum Seek Time * Cylinders Read - 1)

  • Pretend that we are reading a file from our hard drive that extends across 15 cylinders and consists of 300 clusters. Its average seek time is .012 sec and its minimum seek time is .003 sec.
  • It takes the full average seek time to read the first cylinder (because you do not know how far the read/write head must travel to reach the first cylinder). Since the disk has been defragmented, the remaining 14 cylinders will be adjacent to one another.

Seek time = .012 + (.003 * (15 – 1))

= .012 + (.003 * 14)

= .012 + .042

= .054 sec

  • If a disk is perfectly fragmented (in other words, each cluster of the file is scattered around on the disk), we can never assume minimum seek time. Therefore, we must use average seek time for every cluster.

Seek time = Average seek time * # of clusters

= .012 * 300

= .36