(I have this question when I right-click on C drive. On GENERAL label, at the bottom, you see "compress drive to save disk space". I understand that this can help to save space on disk if you tick it, but keep wondering whether or not any harm possibly comes to my computer, can it be affected, in any way like in terms of performance) So, I surfed the internet and here are some answers to the same question raised by someone) 
 

QUESTION:  

What does it mean to compress drive to save disk space?

I am talking about files on Drive C. I run Windows XP Home Edition. Will doing this hurt my computer in any way? And how come it takes so long to perform this action?

ANSWER: 

1) If you want to save disk space, make back-up's or get a bigger hard drive. Compression comes at a price namely decrease in speed. Is it worth it? you'll have to decide.

Your C drive is where your operating system is located. If you compress this drive then each time the computer needs a system file it will have to calol a routine to uncompress the file. Do not do this. COmpress directories not drives.

2) No problem to use this feature (NTFS system do it real/time, with no data/loss and , on a 2000MHz processor, no waste time for processing on decompressing data), but not necessary on modern computers. Hard disk is mostly used by multimedia files that cannot be compressed. Operating system files used by Windows XP can be compress using this feature, but you will got only 200-300 Mb.

3) This turns your information on the drive into smaller amounts of information by using code to represent other larger groups of code. By doing this, it allows your file to shrink in size. This is effective at archiving information that you don't access all that much.

4) It means that it'll compress the files on your computer in order to save space. It won't hurt, however, performance can suffer due to the fact that files have to be constantly decompressed and recompressed again every time they're accessed.

5) File compression is a means of using fancy mathematics to save space in a file or even on a drive. For example, rather than saving a file with the words "Can Candy Can Cans?" as straight text, a computer can encode the data differently, maybe like "Can $dy$ $ $s?" where $ is a special character that represents the letters Can. As you can see the resulting text string is much shorter. This is an incredibly simple way of representing encryption. The stuff built into windows is much, much much more complex. Suffice to say it is a magical effect in that it can reduce the amount of hard disk space you use. Unfortunately some things (like text) compress better than others (say MP3s and video).
If you compress your drive, depending on your usage habits, you should probably have anywhere between 10-50% more free space on the drive. Of course, because your data is now encoded rather than simply written as is to the drive, there can be a bit of a performance hit, expecially when dealing with large compressed files (say over 100 megabytes).
The reason it takes long because right now your hard disk isn't encoded and were you to turn compress disk drive on, it would have to encode every file on you hard disk. This takes a long time.
Generally, if you are running out of hard disk space, you should try and figure out where all that space is going to. If you have large amounts of music or video files, consider moving to DVD/CD the ones you don't use very often. If hard disk space is really a problem and you cannot clean anything else off, consider upgrading or installing a new hard disk. They are relatively cheap these days.

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