1. 2/24: Disk Optimization Roll Call Lecture: defragmentation, disk compression, file compression, drive arrays

2. Review: what we know so far. Formatting disks: why? Writing files to a disk Reading files from a disk

3. Defragmention: what is it? Rejoining parts of files that were written in different places on the disk drive.

4. 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.

5. 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.

6. 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.”

7. 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.

8. 6. Defrag speeds it up. The defrag allows the disk drive to be accessed more efficiently, thus speeding up the access time.

9. 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

10. 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

11. 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

12. 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.

13. 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.

14. 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.

15. 2/26: the wide world of modems