VMs (especially templates) use pre-allocated space. The default private storage max size is 2 GB, but it's very easy to increase as needed. If the disk is completely full, you will get an `Out of disk space` error that may crash your system because Dom0 does not have enough disk space to work. So it's good practice to regularly check disk space usage with the command `df -h` in dom0 terminal.
A system that's out of space should be able to boot, but may be unable to load a desktop manager. In this case it is possible to login to dom0 terminal with Alt + Ctrl + F2. To recover disk space it may be possible to delete files in a userVM by connecting to the userVM terminal:
If this does not work, check the size of /var/lib/qubes/qubes.xml. If it is zero, you'll need to use one of the file backup (stored in /var/lib/qubes/backup), hopefully you have the current data there. Find the most recent one and place in /var/lib/qubes/qubes.xml instead of the empty file.
In any case you'll need some disk space to start the VM. Check `df -h` output if you have some. If not, some hints how to free some disk space:
1. Clean yum cache:
~~~
sudo yum clean all
~~~
1. Delete .img files of a less important VM, that can be found in
/var/lib/qubes/appvms/. Then, when the system is working again, cleanup the rest with:
~~~
qvm-remove <VMname>
~~~
With this method you lose the data of one VM, but it'll work more reliably.
1. Decrease filesystem safety margin (5% by default):
~~~
sudo tune2fs -m 4 /dev/mapper/vg_dom0-lv_root
~~~
1. Remove some unneeded files in dom0 home (if you have any, most likely not).
