I have a bunch of dvd movies made into iso for backup and would like to compress them down as much as possible for archival. I have md5 hashes made for each file but I'm concerned that if I 7z them when I uncompress I won't get the same hash. Is there a reliable method to maintain the reliability and get good compression?
To compress, uncompress and then verify md5 would be a time consuming process if I had to do it for every file.
From personal experience, I have noticed that ISO's do not compress very well to begin with. Usually only a 10% - 12% savings, at the very most. Sometimes less. It usually isn't worth compressing ISO's at all.
And if you 7-zip the ISO's and decompress them, you *will* get the same hash, unless the archive has become corrupted. 7-zip uses CRC itself to make sure you get the original file. Of course, since they don't compress well, the ISO and the 7-zip'd versions are roughly the same file size and have an equal chance of becoming corrupted, if you think about it.
Hope that helps a little,
On another note, if these are movie DVDs, perhaps if you ripped into MPEG format you could wait for 7-zip to support multimedia compression (coming soon, I hear). Just a thought. If it is data DVDs you are compressing, your best bet is to compress the data inside the ISO itself, then make an ISO out of the 7z file.
MPEG (and its subset used in DVDs) is already compressed format.You will be unable to gain too much by ANY lossless compressor programs like RAR, ZIP, 7Z or whatever else.In ISO you generally compressing some gaps and unused space.This is not too much.And multimedia compression will be not able to compress MPEG stream too much.It usually created to compress better things like 24bpp BMPs and 16-bit WAV sounds.Those formats are NOT compressed and redundant enough but in original representation they're not very fair for usual data compressors.Multimedia compression knows how to compress such data slightly better than if they will be treated as usual "unclassified" data.However MPEG is already compressed.And BTW MPEG is LOSSY algorithm.You could reduce size of mpeg streams by increasing MPEG compression.However you will loose in quality since lossy compression like MPEG is tradeoff between quality and size :-P
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