I'm using the old ncompress to create a backup file.And what i see is amazing:Here are the commands i use:compress -c mytest.iso>mytest.iso.bak.ZThe size of mytest.iso(which actually is centos6.8-livecd.iso) changes from 1033741824(1.0G) to 88099(87K).When i use:tar -cvf mytest.iso.tar.gz mytest.isothe tar.gz file doesn't change a lot in size.Is this normal?The iso file works just fine if i uncompress it.
Can ncompress compress a file to 99.99% rate?
linux;tar;compression
Compression algorithms have varying compression ratios depending on the properties of the data they are compressing.  For instance:$ dd if=/dev/zero of=test.img bs=1m count=1024$ compress -c test.img > test.img.Z$ gzip -c test.img > test.img.gz$ wc -c test.img test.img.gz test.img.Z 1073741824 test.img    4685486 test.img.gz      84781 test.img.Z 1078512091 totalHaving a file made up of mostly repeated zeroes is probably a best-case situation for this algorithm.  Since you are getting similar compression ratios, and since your file is of such a round size (1GB), it is likely that the image is much larger than necessary and merely filled with repeated data.Of course, gzip, compress, bzip2, and others will all have differing compression ratios on a given file.  This is why many large open-source projects offer several downloads compressed by different algorithms  so that users can download the smallest file for which they have a decompression utility.