Re: [Duplicity-talk] Set base for delta

[S3]

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>> Looking at duplicity currently, it appears that deltas
>> are always from the previous backup.  Is that correct?

> yes

>> Are there any options to change this?

> unfortunately no

>> 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

> what is the difference between a (level 2) delta and an incremental exactly? 

I will give a more explicit example.
Let each letter be a backup.
Say we do a full backup every 6th backup.
So, duplicity by default would do this:
1 2 3 4 5 6 Level
A B C D E F
G H I J K L
M N O P Q R

So, to restore at R, I would need: M N O P Q R
but not any before that.
The level of the backup is the length of the chain
needed to restore it, so A, G, and M are level 1.
B, H, and N are level 2, and F, L, and R are level 6.
Now, consider if I am able to make a level 2 backup
in addition to restarting at a full backup.

So, say every 10 is a full, and every level 4 is followed by level 2.
1 2 3 4 Level
A B C D
  E F G
  H I J
K L M N
  O P Q
  R S T

Under the previous scheme, J would be level 10.
Here, B is the delta from A to B.
But, E, instead of being the delta from D to E
is the delta from A to E.
Similarly, H is a delta from A, not G.
So, to restore J, instead of needing A-J,
you only need A H I J.
So, the maximum length is 4 instead of 10.

So, to answer your question, B, E, H, L, O, and R
are level 2 backups.  The significant part is that
it goes back to level 2 instead of level 1 (full).
Level 1 = full
Level 2 = delta from full
Level 3 = delta from delta from full
etc.

This is handy if you have a large bunch of rarely
changing files, so they will all be in the full backup,
but then you have a bunch of commonly changing files
but are only worried about their latest values, and
not all of their older values.
Most data sets have commonly update parts
and rarely updated parts.

I hope that is clearer.

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