This squashfstools release finally has zstd support.

Inspired by https://discourse.nixos.org/t/install-nixos-on-512-mb-ram/1713/5. Building zstd-compressed squashfs is much faster than xz or zlib

Done a few tests over installation CD squashfs.

• command: nix-build -A config.system.build.squashfsStore -I nixos-config=modules/installer/cd-dvd/installation-cd-minimal-new-kernel.nix <nixpkgs/nixos>
• XZ (default): -comp xz -Xdict-size 100%
• zstd: -comp zstd -Xcompression-level 6
• gzip: -comp gzip -Xcompression-level 6
• zstd1: -comp zstd -Xcompression-level 1
• gzip1: -comp gzip -Xcompression-level 1

XZ:    time 4m41,017s, size 497Mb
zstd:  time 35s, size 529Mb
gzip:  time 59s, size 585Mb
zstd1: time 15s, size 609Mb
gzip1: time 31s, size 639Mb

Is 32Mb (zstd -6) too much to change default compressor?

cc @Mic92

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.