The idea is:
The latest backup (timestamped) always contains a full copy of the current source state.
Any previous backups are stored as deltas: files that were deleted or modified compared to the next (newer) version.
There are no version numbers — just timestamps. New versions can be inserted naturally.
Each time you back up:
1. Compare the current source with the latest backup.
2. For files that changed or were deleted: move them into a new delta folder (timestamped).
3. For new/changed files: copy them into the latest snapshot folder (only as needed).
4. Optionally rotate old deltas to keep history manageable.
This means:
The latest backup is always a usable full snapshot (fast restore).
Previous versions can be reconstructed by applying reverse deltas.
If the source is intact, the system self-heals: corrupted backups are replaced on the next run.
Only one full copy is needed, like a versioned rsync mirror.
As time goes by, losing old versions is low-impact.
It's user friendly since the latest backup can be browsed through with regular file explorers.
Example:
Initial backup:
latest/ a.txt # "Hello" b.txt # "World"
Next day, a.txt is changed and b.txt is deleted:
latest/ a.txt # "Hi" backup-2024-06-27T14:00:00/ a.txt # "Hello" b.txt # "World"
The newest version is always in latest/, and previous versions can be reconstructed by applying the deltas in reverse.
I'm curious: has this been done before under another name? Are there edge cases I’m overlooking that make it impractical in real-world tools?
Would love your thoughts.
There are also other tools which have implemented reverse incremental backup or backup with reverse deduplication which store the most recent backup in contiguous form and fragment the older backups.
Yes, the deduplicated approach is superior, if you can accept requiring dedicated software to read the data or can rely on a file system that supports it (like Unix with hard links).
I'm looking for a cross-platform solution that is simple and can restore files without any app (in case I didn't maintain my app for the next twenty years).
I'm curious if the software you were working on used proprietary format, was relying on Linux, or used some other method of duplication.
According to chat GPT the forward delta approach is common because it can be implemented purely append only, whereas reverse deltas require the last snapshot to be mutable. This doesn't work well for backup tapes.
Do you also think that the forward delta approach is a mere historical artifact?
Although perhaps backup tapes are still widely used, I have no idea, I am not in this field. If so the reverse delta approach would not work in industrial settings.
This is more-or-less how most cloud backups work. You copy your “premium” SSD to something like a shingled spinning rust (SMR) that behaves almost like tape for writes but like a disk for reads. Then monthly this is compacted and/or archived to tape.
[1] For some values of nobody.