Are there any disadvantages to disk compression?

[Simfish]

So I'm currently deciding whether I should compress my Windows Vista 100 GB hard drive or not. I've compressed hard drives before, with apparently no impact on the computer at all. Which then begs the question - why aren't hard disks compressed to start out with? If they aren't compressed, there could be a reason for it - and I'd like to know one.

And is there any relation between compression and fragmentation?

 

[borat_sagdiyev]

doesnt it take longer to open the files since it has to decompress them?

 

[Simfish]

doesnt it take longer to open the files since it has to decompress them?

I haven't really noticed much of a difference. Does decompression always have to come first?

 

[borat_sagdiyev]

yes