Multi-level incremental back-ups (was: Decremental backup: hard-disk space consideration)

Am 03.02.21 um 19:07 schrieb Denis Corbin:
> let me reformulate your backup strategy to be sure to understand it
> properly:
> 
> - - you start by a full backup let's call it the level-0 backup
> - - each first day of the month you create an incremental backup based
> on the first level-0 backup. Let's call these level-1 backups
> - - each day of the month except the first, you create an incremental
> backup based on the level-1 backup of the first day of the month,
> let's call these level-2 backups
> 
> is it correct?
> 
> You don't mention the retention period of backups, but as the backup of
> each month (level-1) do only depend on the full backup (level-0), you
> can keep all of them down to only the latest level-1 backup (and still
> the level-0 backup of course).
> As level-2 backups depend on the level-1 done the first of the month,
> you can keep all of them in a month, down the lastest level-2, plus
> the latest level-1 and the latest level-0
> 
> Thus, retention is thus very flexible in the strategy you expect to use.

Thanks for taking the time for your elaborate answer! My question had 
been almost as thoroughly worked out -- up to the title, that is. Sorry 
for any confusion this may have created. I had originally been thinking 
about using decremental backuups but their incompatibility with delta 
signatures made all related questions become irrelevant. Nevertheless, 
my main concern is hard-disk space, indeed.

The pointer to dar_manager is very much appreciated as I have not 
studied it yet but definitively will.

If I understand your description correctly, then the level-1 back-ups 
would contain multiple identical copies of files because they only 
depend on the level-0 back-up but not upon each other. A file that was 
created immediately after the level-0-back-up would be contained in each 
one of them, wouldn't it?

Maybe I got the terminology wrong. I thought of incremental back-ups to 
be moving one level up at each stage as opposed to differential ones 
that always depend on a full back-up.

Using your terminology, the monthly back-ups should be level-0 for the 
original one, level-1 for the first incremental one, level-2 for the 
second etc. Within one year there would be up to level-12, within five 
years up to level-60.

The daily back-ups would be like branches to these monthly ones. Thus 
there would be level-0.1 up to level-0.31 for the first month, each 
incremental back-ups, depending on each other. For the second month 
there would be level-1.1 up to level-1.31 incremental back-ups and so on.

The first monthly level-1 incremental back-up would basically wrap the 
preceding 31 daily ones (level-0.1  up level-0.31) into one single 
incremental back-up. This way, a single file change would be included 
twice into the pool, once in a daily and once in a monthly incremental 
back-up.

Since I envision almost "perpetual" or at least very long-term retention 
of the back-ups, I could delete the daily back-ups after a while and 
retain only the monthly ones. Thereby, file changes would not balloon 
the pool as much anymore. After some epoch, the pool can be set aside 
and a new one can be started with the same back-up strategy.

It must be obvious that I do not have any hands-on experience so I am 
very thankful for any comments!

Cheers!
Stefan Müller




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