I used to compress "Old Files", and typically it would save less than 10 MBytes.
One day I tried, and it promised 100 MBytes improvement because I had not used it for a little while, and about one percent of the files exceeded the 30 day threshold.
What it delivered was a 50 MByte degradation in the other direction, AND it took over one hour to do so.
Many of the "old files" were vital system files in which Windows File Protection takes an unhealthy interest;
and because Windows does not trust Windows (and neither do I)
it mistrusted its own compression and re-validated all these system files,
and after validation it dumped duplicate copies in system32/dllcache.
INFINITELY WORSE - several files failed validation, and W.F.P. demanded a Windows CD which was never supplied - ACER pre-installed XP on my PC.
The above disasters were unleashed when I allowed Windows to choose what to compress, and it chose to deal with only one percent of the drive. I dread to think of the results had I told it to compress the whole drive ! !
As I said, I do not trust Windows, so before "cleaning up" the PC I created a disk partition image, and I immediately recovered form disaster.
Since then I noticed that the disc partition image immediately after compressing files are a little larger than the image taken before. This is because the new compressed file takes less disk space, but still holds the original data, and in addition holds an extra overhead of some sort of (de)compression dictionary, and my archive system applies its own compression, so the end-product image is NOT affected by the size of the files it compresses, but IS controlled by the amount of data (including the extra overheads.)
Regards
Alan
