[ThomasWaldmann]

deduplicating backup tools use a symmetric approach: all archives needing some chunk (piece of data) reference it via its chunkid (the keyed hash over the chunk plaintext content).

so you can delete ANY backup archive without influencing any other backup archive. a chunk will be only deleted if nothing is referencing it any more.

also, each backup is logically a FULL backup (it has ALL files, references ALL content data). it is just made in a clever way, avoiding to re-transfer data that already is present in the backup repository, thus it FEELS like incremental (considering speed, amount of CPU and I/O used).

OTOH, full/incremental style backup tools build a chain of incremental archives depending on the previous incremental and the full backup, which gets more fragile the longer the chain gets.

because of that and also because you might want to delete older backups at some time, you are forced to create new full backups regularly (causing lots of CPU and I/O load).