[Duplicity-talk] Set base for delta

[S3]

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Usually in backup systems in addition to doing an incremental backup
from the latest backup, you can also do relative to an older one.
Consider the length of backup chain needed to restore.
For the full backup, the chain has length 1.
Then the first delta makes it length 2 (full + delta 1).
The next delta is relative to the previous delta,
so 3 (full + delta 1 + delta 2).

Looking at duplicity currently, it appears that deltas
are always from the previous backup.  Is that correct?
Are there any options to change this?

Specifically, the data I want to back up is large
relative to my slow Internet connection.
So, I would prefer doing full backups infrequently,
and do relative to the length 2 chain, instead of length 1.
Something like:
Full backup every 3 months
Level 2 delta every 2 weeks
Ordinary incremental daily

This would also make cleaning up old deltas easier,
since the level 2 delta would make everything between
it and the full backup unnecessary.
(Note that this configuration vs. without level 2 deltas
would only have chains of length 15 instead of 90.)

- -- 
echo 'scale=2735;sqrt(57)' | BC_LINE_LENGTH=391 bc |
tail -n 1 | tr 0-9 'ertiSSngl '
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