lix/corepkgs/nar.nix
Eelco Dolstra 4911a10a4e Use XZ compression in binary caches
XZ compresses significantly better than bzip2.  Here are the
compression ratios and execution times (using 4 cores in parallel) on
my /var/run/current-system (3.1 GiB):

  bzip2: total compressed size 849.56 MiB, 30.8% [2m08]
  xz -6: total compressed size 641.84 MiB, 23.4% [6m53]
  xz -7: total compressed size 621.82 MiB, 22.6% [7m19]
  xz -8: total compressed size 599.33 MiB, 21.8% [7m18]
  xz -9: total compressed size 588.18 MiB, 21.4% [7m40]

Note that compression takes much longer.  More importantly, however,
decompression is much faster:

  bzip2: 1m47.274s
  xz -6: 0m55.446s
  xz -7: 0m54.119s
  xz -8: 0m52.388s
  xz -9: 0m51.842s

The only downside to using -9 is that decompression takes a fair
amount (~65 MB) of memory.
2012-06-29 15:24:52 -04:00

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with import <nix/config.nix>;
let
builder = builtins.toFile "nar.sh"
''
export PATH=${nixBinDir}:${coreutils}
echo "packing $storePath..."
mkdir $out
dst=$out/tmp.nar.xz
set -o pipefail
nix-store --dump "$storePath" | ${xz} -9 > $dst
hash=$(nix-hash --flat --type $hashAlgo --base32 $dst)
echo -n $hash > $out/nar-compressed-hash
mv $dst $out/$hash.nar.xz
'';
in
{ storePath, hashAlgo }:
derivation {
name = "nar";
system = builtins.currentSystem;
builder = shell;
args = [ "-e" builder ];
inherit storePath hashAlgo;
# Don't build in a chroot because Nix's dependencies may not be there.
__noChroot = true;
}