2/24: Disk Optimization

• Roll Call

• Lecture: defragmentation, disk compression, file compression, drive arrays

Review: what we know so far.

• Formatting disks: why?

• Writing files to a disk

• Reading files from a disk

Defragmention: what is it?

• Rejoining parts of files that were written in different places on the disk drive.

2. Defragmention: why do we need it?

• In the beginning, the hard drive is empty and formatted.

• Files are written, one by one.

• Files are deleted, one by one.

3. Defrag: Hooh! What is it good for?

• New files are saved in places that the old files leave vacant.

• The read/write heads have to look all over for the data.

4. Time keeps on slippin’, slippin’, ...

• The access time is reduced as the heads move more to get the data.

• Access time is defined as “how long it takes to retrieve data.”

5. Defrag is a compulsive organizer.

• The defrag moves some clusters of data to empty places so it can put related data in its place.

• It continues until everything is sorted.

6. Defrag speeds it up.

• The defrag allows the disk drive to be accessed more efficiently, thus speeding up the access time.

Part 2: Disk compression

• Disk compression allows you to pack more onto your hard drive.

• Disk compression is NOT file compression.

Regular disk file system

Compressed disk file system

1 cluster

1: How does disk compression work?

• Software creates a file that contains all the files that are on your disk.

• This file is called a CVF (compressed volume file).

Regular disk file system

Compressed disk file system

1 cluster

#1 #2 #3 #4 #5

#1 #2 #3 #4 #5

SLACK

2. Virtual drives

• The CVF is seen by the OS as a separate drive, with its own letter & designation.

• A virtual drive.

Regular disk file system

Compressed disk file system

1 cluster

#1 #2 #3 #4 #5

#1 #2 #3 #4 #5

SLACK

Part 3: File compression

• Repeated parts of the text are saved with markers put in their places.

she sells seashells down by the seashore.

she sells seashells down by the seashore.

she_sells_seashells down by the_seashore.

File compression w/ text.

she sells seashells down by the seashore.

# sells sea#lls down by the seashore. she

#@lls@a#lls down by the@ashore. she _se

41 characters.

40 characters.

37 characters.

File compression w/ graphics.

• Graphic files use various methods of compression.

• One is called lossy compression, in which gradations in color are lost.

• It is permanent.

2/26: the wide world of modems