The bigger the damage, the cruder the recovery technique used. No recovery data used. Used when all other techniques fail or were not used. Sequences of actions on files are recorded. Separate file is maintained to keep track of changes, periodically merged with the main file.
Files containing previous values form a consistent backup version.
In cases of failure, comparison between the versions can be used to find a consistent version. Nothing is updated in place, with the original only being deleted after operation is complete. Combinations of two techniques can be used to offer similar protection against different kinds of failures. All newly created of modified files not previously dumped are copied to tapes once per hour. High reliability, but very high overhead.