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I am recently seeing "Super Compressed" ISO files which have claimed to compress iso files to sizes < 15 mb, where the original file size is in 4GB+. How is it possible?

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Good question, I can't really find a good source about it to answer the question –  Ivo Flipse Nov 4 '09 at 7:27
    
Compression to the order you mention is theoretically possible, depending on the data being compressed. I haven't heard of anything myself, but I'd be wary of anyone offering something too good to be true. –  user3463 Nov 4 '09 at 7:35
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I don't believe it. I see lots of results when googling for supercompressed e.g. "full Windows XP 6-in-1 Iso only 9 MB". That. is. just. not. possible! –  fretje Nov 4 '09 at 7:37
    
i believe it. i bet i can get it smaller, too. (you don't actually need the original back exactly like it was, do you?) –  quack quixote Nov 4 '09 at 7:47
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A 6-in-1 XP ISO is made smaller as a lot of files are identical for the XP variants and the TOC of the CD can be made so the file appears in several directories but the content of the file is stored only once on the disc. However 9 MB, I cannot believe it at all! –  Snark Nov 4 '09 at 8:00

1 Answer 1

up vote 5 down vote accepted

It's possible IFF the data in the ISO is compressible to that size via some arbitrary compression algorithm.

ISOs are just files that contain a filesystem; it can be highly compressible. It depends on

  • Is the filesystem mostly full or mostly empty?
  • What type of data?
  • What type of compression? (lossless? lossy?)
  • How good is the algorithm?

It is possible to compress repetitive text data very small. 4GB of Apache logfiles probably compresses pretty well. But that same advanced algorithm won't work nearly as well (if at all) on audio/video or photographic data.

It could also be smoke & mirrors. I could make a 4GB ISO file that doesn't actually contain any files, and even though it's 4GB it would compress down to mere kilobytes.

It's like the old saying about lies, damned lies and statistics. Just because someone claims their algorithm can do something doesn't mean you will actually get similar results.

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