Compressed ISO image got bigger

[Ludovic Courtès]

Hi,

Ludovic Courtès <address@hidden> skribis:

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

>   e830c2a8de vm: Transparently compress iso9660 images.

Here’s 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.x86_64-linux.iso.xz
 release-1.1.0rc2/guix-system-install-1.1.0rc2.x86_64-linux.iso.xz
260M    
release-1.0.1.13450-01d5f2/guix-system-install-1.0.1.13450-01d5f2.x86_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:

  • The xz-compressed ISO of rc2 is 1.6 times bigger than that of rc1.

  • The uncompressed ISO of rc2 is 2.6 times smaller than that of rc1.

It’s 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.

“make release” uses the default xz compression ratio, -6.  I tried
different levels and different compression tools (though I don’t want to
change from xz now because this would likely break documentation and
links all around) and it seems we can’t 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’.