Developing with the newest of the new packages is always a bit tricky. Every now and then they break in interesting ways. Sometimes they corrupt the system so much that downgrading becomes impossible. Extreme circumstances may corrupt the system’s package database and so on. Traditionally fixing this has meant reinstalling the entire system, which is unpleasant and time consuming. Fortunately there is now a better way: snapshotting the system with btrfs.
The following guide assumes that you are running btrfs as your root file system. Newest quantal can boot off of btrfs root, but there may be issues, so please read the documentation in the wiki.
The basic concept in snapshotting your system is called a subvolume. It is kind of like a subpartition inside the main btrfs partition. By default Ubuntu’s installer creates a btrfs root partition with two subvolumes called @ and @home. The first one of these is mounted as root and the latter as the home directory.
Suppose you are going to do something really risky, and want to preserve your system. First you mount the raw btrfs partition somewhere:
sudo mkdir /mnt/root sudo mount /dev/sda1 /mnt/root cd /mnt/root
Here /dev/sda1 is your root partition. You can mount it like this even though the subvolumes are already mounted. If you do an ls, you see two subdirectories, @ and @home. Snapshotting is simple:
sudo btrfs subvolume snapshot @ @snapshot-XXXX
This takes maybe on second and when the command returns the system is secured. You are now free to trash your system in whatever way you want, though you might want to unmount /mnt/root so you don’t accidentally destroy your snapshots.
Restoring the snapshot is just as simple. Mount /mnt/root again and do:
sudo mv @ @broken sudo subvolume snapshot @snapshot-XXXX @
If you are sure you don’t need @snapshot-XXX any more, you can just rename it @. You can do this even if you are booted in the system, i.e. are using @ as your current system root fs.
Reboot your machine and your system has been restored to the state it was when running the snapshot command. As an added bonus your home directory does not rollback, but retains all changes made during the trashing, which is what you want most of the time. If you want to rollback home as well, just snapshot it at the same time as the root directory.
You can get rid of useless and broken snapshots with this command:
sudo btrfs subvolume delete @useless-snapshot
You can’t remove subvolumes with rm -r, even if run with sudo.