From mboxrd@z Thu Jan  1 00:00:00 1970
From: =?utf-8?Q?Ludovic_Court=C3=A8s?= <ludo@gnu.org>
Subject: Compressed ISO image got bigger
Date: Mon, 13 Apr 2020 16:42:39 +0200
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To: guix-devel@gnu.org, Tobias Geerinckx-Rice <me@tobias.gr>

Hi,

Ludovic Court=C3=A8s <ludo@gnu.org> skribis:

>   https://web.fdn.fr/~lcourtes/software/guix/1.1.0rc2

>   e830c2a8de vm: Transparently compress iso9660 images.

Here=E2=80=99s a comparison of the size of the ISO images for rc1 and rc2:

--8<---------------cut here---------------start------------->8---
$ du -h release-1.0.1.13450-01d5f2/guix-system-install-1.0.1.13450-01d5f2.x=
86_64-linux.iso.xz release-1.1.0rc2/guix-system-install-1.1.0rc2.x86_64-lin=
ux.iso.xz
260M    release-1.0.1.13450-01d5f2/guix-system-install-1.0.1.13450-01d5f2.x=
86_64-linux.iso.xz
405M    release-1.1.0rc2/guix-system-install-1.1.0rc2.x86_64-linux.iso.xz
$ xz -d < release-1.0.1.13450-01d5f2/guix-system-install-1.0.1.13450-01d5f2=
.x86_64-linux.iso.xz | wc -c
1524205568
$ xz -d < release-1.1.0rc2/guix-system-install-1.1.0rc2.x86_64-linux.iso.xz=
 | wc -c
589223936
--8<---------------cut here---------------end--------------->8---

As you can see:

  =E2=80=A2 The xz-compressed ISO of rc2 is 1.6 times bigger than that of r=
c1.

  =E2=80=A2 The uncompressed ISO of rc2 is 2.6 times smaller than that of r=
c1.

It=E2=80=99s not surprising that xz has a hard time compressing an
already-compressed image, but I find it surprising that the end result
is almost twice as big.

=E2=80=9Cmake release=E2=80=9D uses the default xz compression ratio, -6.  =
I tried
different levels and different compression tools (though I don=E2=80=99t wa=
nt to
change from xz now because this would likely break documentation and
links all around) and it seems we can=E2=80=99t really do any better:

--8<---------------cut here---------------start------------->8---
560M    /tmp/t.iso
415M    /tmp/t.iso.gz
406M    /tmp/t.iso.lz-6
412M    /tmp/t.iso.xz-0
407M    /tmp/t.iso.xz-3
404M    /tmp/t.iso.xz-8
--8<---------------cut here---------------end--------------->8---

Thoughts?  Ideas?

Ludo=E2=80=99.