feat(live): Increase the available disk space in the Live system

[lslezak]

Problem

• If you want to install additional packages or you want to copy a manually built Rust binary then you might have trouble with not enough free disk space in the Live ISO
• This test shows that the available space in the default image is about 0.5GB:

# df -h /
Filesystem           Size  Used Avail Use% Mounted on
/dev/mapper/live-rw  2.4G  1.8G  398M  83% /
# dd bs=1M if=/dev/zero of=/take_space
dd: error writing '/take_space': No space left on device
522+0 records in
521+0 records out
546709504 bytes (547 MB, 521 MiB) copied, 0.750277 s, 729 MB/s

Solution

• Unfortunately it seems Kiwi does not support manually changing the default value
• As a workaround create a huge sparse file which in reality takes tiny fraction of it's size
• Kiwi evaluates the apparent (virtual) file sizes when computing the size of the image so it makes the image bigger
• During testing I found out that after deleting the sparse file you can actually get even some more free space
• So I created a new systemd service which deletes the created file at boot

Testing

• The build ISO image has a similar size as before, the sparse file does not increase the ISO size
• Tested manually, with the fix there is about 3.4GB free space now:

# df -h /
Filesystem           Size  Used Avail Use% Mounted on
/dev/mapper/live-rw  5.3G  1.9G  3.2G  37% /
# dd bs=1M if=/dev/zero of=/take_space
dd: error writing '/take_space': No space left on device
3522+0 records in
3521+0 records out
3692621824 bytes (3.7 GB, 3.4 GiB) copied, 3.87284 s, 953 MB/s

Notes

• The writes to the system are kept in RAM, that means to fully use the new 3.4GB disk space the machine needs more than 4GB RAM!