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>
this is only used in one place, and only to set a nicer error message on
EndOfFile. the only caller that actually *catches* this exception should
provide an error message in that catch block rather than forcing support
for setting error message so deep into the stack. copyStorePath is never
called outside of PathSubstitutionGoal anyway, which catches everything.
Change-Id: Ifbae8706d781c388737706faf4c8a8b7917ca278
eldritch horrors