[18.04] You need at least 8.4 GB ... This computer has only 0.0 B.

Testing the bionic daily desktop amd64 ISO for 2018-02-02 in a QEMU guest allocated 8GB via an LV, started in EFI mode and using "Try Ubuntu" mode, when Ubiquity was started it reported:

You need at least 8.4 GB disk space to install Ubuntu.

This computer has only 0.0 B.

I checked the LV was correctly connected:

cat /sys/block/sda/size
16777216

I then expanded the LV to 16GB and re-ran the exercise and got the same message.

cat /sys/block/sda/size
33554432

I did some hunting through the source code and discovered that when the prepare step calls ...

/bin/parted_devices

... nothing is returned.

I ran strace on that command.

It turns out if the device is *read-only* nothing is returned.

Ubiquity really should report something more appropriate. Currently the 0.0 message is due to the variable 'free' being initialised to 0 and not being changed because parted_devices doesn't return any data. No results should provoke a helpful GUI message, and certainly something in the debug log.