## Observation

Testing different disk sizes I could test how the installer reacts.
http://lord.arch/tests/6558#step/installation_overview/3 shows that the
installer provides a clear error message on a disk with only 4GB that there is
not enough space for installation of the default proposal including btrfs.

On a disk with 5GB there is only a warning about "only 5% free space available
on /" in
[installation_overview](http://lord.arch/tests/6559/modules/installation_overview/steps/3)
but then one can continue further on and then fail with a not very obvious
error message not pointing to the real problem, e.g.
http://lord.arch/tests/6559#step/install_and_reboot/21
 about "Failed to cache rpm database". Only the logfile ylog, e.g. within
http://lord.arch/tests/6559/file/install_and_reboot-y2logs.tar.bz2, tell about
the real problem: out of space


## Reproducible

* Create a virtual machine with disk space of 5G
* Continue default installation

## Expected result

* The installer should be more rigorous to prevent installation in these cases
* The disk usage calculation should resemble reality better