&gt; I had a hack back in the previous millenium that improved on .tar.gz, compatibly.<p>FWIW, this is known as the &#x27;sort key trick&#x27;. It&#x27;s been in the folklore, as you say, for quite a while. I give some examples in <a href="http:&#x2F;&#x2F;www.gwern.net&#x2F;Archiving%20URLs#sort---key-compression-trick" rel="nofollow">http:&#x2F;&#x2F;www.gwern.net&#x2F;Archiving%20URLs#sort---key-compression...</a> for WWW archives.<p>&gt; Reordering a tar file doesn&#x27;t change the meaning, and IMO the big, big gap in compression research is to identify such expression transformations.<p>This hasn&#x27;t been <i>totally</i> ignored. The problem is that you need to know and be able to specify in advance a useful ordering of input files to make up a better stream or a tool (like &#x27;lrzip&#x27;, designed to look for long-range correlations) must infer it which seems hard to do without requiring multiple passes; and what&#x27;s the incentive? In a world in which RAR and ZIP and Gzip are still considered the standards and people aren&#x27;t even using xz, no one wants to use such a niche tool for anything serious.