For distributing archives over the Internet, the following things are generally a priority:
- Compression ratio (i.e., how small the compressor makes the data);
- Decompression time (CPU requirements);
- Decompression memory requirements; and
- Compatibility (how wide-spread the decompression program is)
Compression memory & CPU requirements aren't very important, because you can use a large fast machine for that, and you only have to do it once.
Compared to bzip2, xz has a better compression ratio and lower (better) decompression time. It, however, requires more memory to decompress and is somewhat less widespread.
So, both gzip and xz format archives are posted, allowing you to pick:
- Need to decompress on a machine with very limited memory (<128 MB): gzip. Given, not very likely when talking about kernel sources.
- Need to decompress minimal tools available: gzip
- Want to save download time and/or bandwidth: xz
There isn't really a realistic combination of factors that'd get you to pick bzip2. So its being phased out.
I looked at compression comparisons in a blog post. I didn't attempt to replicate the results, and I suspect some of it has changed (mostly, I expect
xz has improved, as its the newest.)
(There are some specific scenarios where a good bzip2 implementation may be preferable to xz: bzip2 can compresses a file with lots of zeros and genome DNA sequences better than xz; it still possible to recover data after the point of corruption in bzip2 files whereas this is not possible for xz; bzip2 decompression can be parallelized. However none of these are relevant for kernel distribution)
1: This depends on how you count. E.g., in archive size,
xz -3 is around
bzip -9. Then xz uses less memory to decompress. But
xz -9 uses much more than
2: F21 System Wide Change: lbzip2 as default bzip2 implementation