Basically, you have to create a snapshot of the volume and backup the snapshot. each write to the original volume saves the old file to a secondary filesystem. At the beginning, the snapshot contains links to the original files. whenever a write occurs, to the source volume, the system should copy the old file to the secondary filesystem, change the link in the snapshot from the original to the copy in the secondary filesystem.
The backup is done on the snapshot.
As you understand, this is not a trivial implementation and there are other issues regarding snapshots and backups.
But I guess this doesn't solve you the problem, right? I don't know of product that runs on linux and does this.
GPFault
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