[Borgbackup] Disk usage of repository

[Thomas Waldmann]

> We are using borg for some years now, and we really like it !

You're welcome!

> But on a server backed up using borg 1.1.17, the repository has grown
> to 6 TB, as reported by borg info:
>                         Original size      Compressed size    Deduplicated size
> All archives:               85.24 TB             85.02 TB              6.29 TB
> 
> Because I need to retrieve some space, I'm trying to figure out which
> archive is using space.

Due to deduplication, that is not as easy as it sounds.

> I ran "borg info -P ''" to have a look at "Deduplicated size" of each archive.
> 
> But something is confusing me: if I make the "deduplicated size" sum
> of all archives, the result is less than 800GB... I don't think this
> make sense, and it does not help me to clean some space :/

It makes sense: the amount it shows as deduped size of an archive is the 
repo space allocated by chunks ONLY referenced by that archive. So, if 
you just delete this single archive and keep all others, it will free 
that amount.

If you always backup the same system, a lot of files that do seldomly 
change will be contained in many (N) archives, so if you delete a single 
one of them, there will be still N-1 archives referencing the same 
chunks (and thus, that space will only get freed after you delete the 
LAST archive referencing these chunks).

So "summing up" these values is an invalid way to calculate something 
interesting.