Complete Filesystem Checkpointing?

polymath69 asks: "Living on the edge of Debian unstable means that updates sometimes break stuff, occasionally to an extent that is difficult to recover from. This got me thinking about treating the entire set of mounted filesystems as a transactional database. Mark state, try something which might be dangerous, test, and approve (commit) or panic (rollback). Obviously some filesystem support would be required, but with ext3 and reiserfs available, maybe the potential is already there. And such a system would need lots of disk space, but these days that's a demand easily granted. There's lots out there on process-level checkpointing, and even some stuff about system-level checkpointing, but all I've found on that was in the context of saving and restoring processes for a system freeze and restore. But I couldn't find anything on Google or SourceForge about doing this sort of temporary branching in the filesystem. Is this idea feasible? Is anyone working on it?"