Here's one for ya. Decompress a gzip'ed package, say pacman. Now, recompress every file in that directory with `xz -1`, smile :) Result: the re-compression is faster than a tar+gz and in each case so far(kerne26, gcc, perl, pacman, kdelibs) is a little smaller as well. It doesn't approach the (much smaller) size of tar+xz but it does take less time to compress and is smaller than what we had before. I know, you're thinking why do that. I wanted to implement an idea for the A.R.M, which is basically to allow downgrade on a file-by-file basis, my theory is that most(many) of the files in 2 versions of a package (esp. when they are minor versions or pkgrel bumped) are often exactly the same. My test involved kdelibs-4.3.98-1 and kdelibs-4.4.0-3. I first decompress both, and remove the duplicates from kdelibs-4.4.0-3 which results in sizes of 59.5MB for the original kdelibs-4.3.98-1 and 35.5MB for the de-deduped kdelibs-4.4.0-3. Now recompressing with `xz -1` kdelibs-4.4.0-3 is brought down to 10.7MB. Comparing with the delta (between the original packages) which 5.3MB it's not a whole lot while allowing more flexibility(IMHO). Some more numbers for comparison, the tar+xz(level -6) for kdelibs-4.4.0-3(de-duped) was 8.4M in comparison to tar+gz which was 12.6MB the original tar'd sizes were 19.8MB for gz and 13.9MB for xz. I know I didn't test based on pkgrel or try gz(as opposed to xz -1) only nor did it very scientifically but I thought it was very interesting non-the-less. I also suspect that the similar results would be seen for a test with xdelta on each file but that's not very useful to me so I'll leave that one for another day.