[Borgbackup] Very large delta's when pruning a static backup

[F Wessels]

Hi all,

best wishes for 2022.

I have a backup of a not changing source directory. The files within are
static aka not written to. This directory remains in the backup script so
it get's processed daily and therefore also pruned. This all works as
expected however the directory containing the repo has very large delta's
(many bytes are changed daily) therefore I need to replicate very much data
offsite while all that's hapening is pruning old data.

The source directory contains about 40 large files some 500GB in total.
borg create ended with a deduplicated size of 2.65kB
That's to be expected because everything is static (for months)

borg prune ends with -24.60GB deduplicated size.
But in the end I needed to replicate 32.764GB to my offsite mirror location.

My expectation was that since no new data was written to the source files,
no extra data was added to the repo. That behaviour seems to happen.
However I also expected that when pruning this repo data would be deleted
and that I only would be removing data in my offsite location, not
transfering so much.

I'm still using borgbackup 1.1.11 and upgrading involves red tape.

Could anybody please comment on this?

Thank you for your interest and time.

Kind regards,

Stor Keen
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