I'll test those rebased patches as well.  The automated system tests no
longer pass -- although it might not be specific to this change (I'll
try running the "installed-os" test to see if all the install tests are
broken or just this new "btrfs-root-on-subvolume-os" one.).  In case you
are not familiar with system tests, a single system test can be run
with:

make check-system TESTS="btrfs-root-on-subvolume-os"

The install tests are defined under (gnu tests install) and are *very*
expensive to run (mostly in time, but they require some disk space as
well).  Hence the hack in 0001, but I'm not sure if it brings as much
benefits as when I made it (given Ludovic keeps improving the way Guix
gets built :-)).

A fresh benchmark could be interesting if you have lots of time on your
hands.

Thank you for looking at this!

Maxim