Version: (using KDE 4.4.2) OS: Linux Installed from: Ubuntu Packages My root partition is 6GB in size which is easily enough for the OS, applications and multiple versions of the kernel, yet when I do a Software Update enough data can be cached in /var/cache/apt/archives/ that the system will fail part way through and I will be unable to login because KDE is unable to write to the /tmp directory and I will have to boot off a live CD and remove data manually and then tell apt to do a fix. A simple way around this would be to use the output of the df command to get the amount of free space and only cache that much minus some guard amount so that, even if all of it were needed to perform the update KDE would still be able to function.
Assuming that software updates are done via KPackageKit in Ubuntu. If not, please reassign accordingly. Regarding the impossible login with full disk, see bug 81653.
This is something for the PackageKit backends to handle, as the space requirements may differ from package system to package system. The APTcc PackageKit backend supports this, for example, and KPackageKit shows a "not enough space" error when the backend tells it to show one.