These numbers make it look like the difference in compression ratio between lzma and zstd are insignificant, but I’m not so sure of this.
cache.nixos.org seems to use xz at the highest level (-9).
Fedora used xz with level -2 which is significantly less effective, and therefore there was no loss switching for them.
The article for Arch Linux does not state which level was used for xz, but as they claim they only have lost 0.8%, I assume they did not use it at (-9) either.
Also the source for ubuntu doesn’t reveal the level used for xz.
I made a little benchmark with qemu, which is quite a big nar file. The xz coming from cache.nixos.org is 117M in size and the zstd compressed one (-19 / highes level) is 129M.
That is a difference of around 10%
Therefore by optimizing for the people sitting behind a fast connection, we will make the experience worse for the people who have to work with limited bandwidth. As a frequent traveler, I’m often in that position. That’s why I care.
Though, I have to admit, 10% loss sounds acceptable to gain 10X speeds elsewhere. Still, it would be nice to have actual numbers here and see what would be the difference on the compressed closure size of a whole nixos system, for example.