Decremental backup: hard-disk space consideration

Hello list members

Dar has been on my mind literally for years and now the time has finally 
come to plunge into this project. During that long period the amount of 
data that I want to back-up has grown to about one terabyte. It consists 
of pretty static data storage of local machines.

I already have a working backup mechanism using Borgbackup which does 
data deduplication and can be pretty much left alone. I want to add Dar 
as independent solution for a long-term mirror of the data storage. My 
strategy is to gradually fill up the hard-disk as long as there is 
space. Full hard-disks are archived and replaced by new bigger ones.

After reading through most of the documentation my current idea of 
backup strategy is the following. I wonder if it is at all feasible and 
would welcome experienced opinions.

= Back-up strategy =

In general, I want to use delta signatures in combination with Par2.

1.) Create monthly incremental back-ups at the first of every month. If 
the back-up plan had started on January 1st, 2020, the monthly Dar pool 
e.g. in December 2020 would consist of
a full back-up 202001
with incremental back-ups 202001, 202002, ..., 202012

2.) Create daily incremental back-ups based on the latest (incremental) 
monthly back-up. So the daily Dar pool would consist, e.g. for December 
2020, of
incremental back-ups 20201202, 20201203, ..., 20201231
all based on 202012.

For the previous months there would be
incremental back-ups 20201102, 20201103, ..., 20201130
all based on 202011
and so on.

= Restoration =

In order to restore the state of a file from December 25th, 2020, I 
would have to first re-build the incremental back-up of the specific 
month by going back from the initial full back-up. Then the process 
would consist of re-building the incremental daily back-ups from the 
monthly one.

I suspect that these two restoration chains can be combined into a 
single one. The selection of archives can easily be coded.

= Rationale =

Using all incremental backups with delta signatures keeps hard-disk 
space requirements reasonably low.

Restoring a file after a five year period would need the combination of 
at most about 90 archives, about 150 over a ten year period. These 
numbers would be about 20 times higher if using daily back-ups only.

= Questions =

Obviously, I have not tested that idea and cannot tell if it is feasible 
and efficient. Any comments?

Thanks in advance!

Sincerely,
St. Müller


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