Question: More and more [`tar`](http://www.gnu.org/software/tar/manual/tar.html) archives use the [`xz`](http://en.wikipedia.org/wiki/Xz) format based on LZMA2 for compression instead of the traditional [`bzip2(bz2)`](http://en.wikipedia.org/wiki/Bz2) compression. In fact *kernel.org* made a late "*Good-bye bzip2*" [announcement, 27th Dec. 2013](https://www.kernel.org/happy-new-year-and-good-bye-bzip2.html), indicating kernel sources would from this point on be released in both tar.gz and tar.xz format - and on the main page of the [website](https://www.kernel.org/) what's directly offered is in `tar.xz`.

Are there any specific reasons explaining why this is happening and what is the relevance of [`gzip`](http://en.wikipedia.org/wiki/Gzip) in this context?

Answer: 
LZMA2 is a block compression system whereas gzip is not. This means that LZMA2 lends itself to multi-threading. Also, if corruption occurs in an archive, you can generally recover data from subsequent blocks with LZMA2 but you cannot do this with gzip. In practice, you lose the entire archive with gzip subsequent to the corrupted block. With an LZMA2 archive, you only lose the file(s) affected by the corrupted block(s). This can be important in larger archives with multiple files.
**Short answer** : xz is more efficient in terms of compression ratio. So it saves disk space and optimizes the transfer through the network.  

You can see this [Quick Benchmark](http://catchchallenger.first-world.info/wiki/Quick_Benchmark:_Gzip_vs_Bzip2_vs_LZMA_vs_XZ_vs_LZ4_vs_LZO) so as to discover the difference by practical tests.