I've been working with some people on the use of zsync for Gentoo mentioned previously, and we came across this oddity of tar. We were trying to work out why a given directory tree took 160MB as a .tar, but only 88MB when stored in a different format. It turns out that tar uses a block size of 512 bytes, so every file takes at least 512 bytes for the data, and 512 bytes for the metadata (filename etc). So half the tarball was empty space because... because the tar format was designed to go to 512-byte-block tapes. Given that most of tar's use is for distributing files online now, I don't want to know how much space is wasted just because the old format demanded it.

cpio seems to be more efficient - it doesn't use anything like as much padding. Somehow I don't think I will boost zsync's popularity if I advise people to use cpio :-). zip also seems to have very compact metadata.