What is the worst case scenario of a hard shutdown with ubuntu (11.10)? (in other words, turning PSU off)
- data loss
- os instability
To my knowledge, all of the above. When Ubuntu closes without properly shutting down, no filesystems are unmounted, no open handles are closed and any I/O operations will be stopped mid-process. However, Ubuntu is designed to be fairly robust when it comes to unexpected hardware changes. This can lead to data corruption on any of the mounted filsystems (including /) which can lead to system instability, but this is highly unlikely unless you are running
Of course, this isn't going to happen very often at all, but it should at least be avoided as much as possible. Also, after a hard shutdown where you suspect data may have been vulnerable, try booting it up into a Memtest and into the Recovery mode to try and clean it up before booting properly again.