On Fri, Aug 24, 2012 at 11:04:56AM +0200, Gaël DONVAL wrote: > Le jeudi 23 août 2012 à 20:24 +0800, lina a écrit : > > > > Sorry, here you mean, > > > > once tar -Jcf a.tar.xz a > > > > again > > tar -Jcf a.tar.xz a.tar.xz > > ? > No, I think this was a joke :)
Yes it was a joke :) but it was based on a recent article where someone expressed surprise that multiple manual passes of a compressor (I think gz) resulted in smaller file sizes. (I couldn't find a copy of the article to link to) > In most programs, there is a "depth" or "pass number" parameter that > does just this already. If you try to compress again, the overhead > induced by the container (headers and such) will ultimately increase the > file size. Most compressors work on a block-cipher model in order to support stream operation and so the compressor doesn't have a global view of the data being compressed. That's why subsequent manual passes can (sometimes) have a good effect, especially with e.g. enormous log files with a lot of repetition: local areas of the file being compressed are treated in isolation, but the resulting compressed blocks have a lot of (compressed!) repetition. In practise it's almost certainly very rarely worth bothering. -- To UNSUBSCRIBE, email to debian-user-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org Archive: http://lists.debian.org/20120824091035.GE19780@debian
