libarchive's xz offers single threaded xz compression which is very slow
and provides ~10-20Mbps compression speed in addition to maxing a core.
In exchange, it achieves optimal compression ratios among all our
compression methods.
Nonetheless, xz prevent the saturation of 1Gbps+ connections and slow
down significantly decompression for end users. As these connections and
faster hardware is becoming prevalent for cache servers and clients, we
offer to default to zstd.
Lix is a "compress once, decompress many times" application. To avoid
incurring a high penalty to end users very sensitive to compress ratio
(very slow Internet connections), we dampen the consequences of
switching to zstd by increasing the default zstd level to 12.
On one example, xz will compress a 4.4GB file to 632MB, zstd on 12 will
compress it to 775MB, that is a ~18 % increase over the optimal xz
compression. zstd took 18 seconds to produce this file.
Increasing to level 14 leads to a 773MB file while taking 37s.
Increasing to level 16 leads to 735MB file while taking 66s.
Finally, xz took 77s, so a 50 % reduction in time taken to compress in
exchange of an increase of 18 % of the compressed size.
This change will reduce issues encountered in #945 but is probably not
the root cause.
References:
- https://discourse.nixos.org/t/switch-cache-nixos-org-to-zstd-to-fix-slow-nixos-updates-nix-downloads/23961
Change-Id: I7beda2bf2c1fed146dcb797b8f85dc290c486ab2
Signed-off-by: Raito Bezarius <raito@lix.systems>
I think it is bad for these reasons when `tests/` contains a mix of
functional and integration tests
- Concepts is harder to understand, the documentation makes a good
unit vs functional vs integration distinction, but when the
integration tests are just two subdirs within `tests/` this is not
clear.
- Source filtering in the `flake.nix` is more complex. We need to
filter out some of the dirs from `tests/`, rather than simply pick
the dirs we want and take all of them. This is a good sign the
structure of what we are trying to do is not matching the structure
of the files.
With this change we have a clean:
```shell-session
$ git show 'HEAD:tests'
tree HEAD:tests
functional/
installer/
nixos/
```
(cherry picked from commit 68c81c737571794f7246db53fb4774e94fcf4b7e)
Raito Bezarius