Re: [rdiff-backup-users] snap-shots every 10 diffs...

[Simon Hobson]

address@hidden wrote:

> So, if I understand this correctly...
>
> The "snapshot" referred to by Andrew is a "snapshot" of all the
> rdiffs?
>
> So, rdiff-backup creates a "roll-up" of all the rdiffs every 10
> rdiffs. (I'm assuming it's a "roll-up" since there's no facility to
> merge the rdiffs.)

Not quite. The rdiffs allow you to get from the latest version 
(always stored in full) to a previous version. All that needs to 
happen to store an intermediate snapshot is to save the current 
version (which is the whole file at the time of the last backup) and 
write a new file to be the new current version.

Eg, over time, the repository for a single file would contain (as I 
understand it) :

1 C

2 ra C

3 ra rb C

...

9 ra rb rc rd re rf rg rh C

10 ra rb rc rd re rf rg rh ri C

11 ra rb rc rd re rf rg rh ri j C

12 ra rb rc rd re rf rg rh ri j rk C

Where 'C' is the current version of the file, a, b, c, ... are 
previous versions, and ra, rb, rc, ... are the rdiffs required to 
create a, b,c, ... from the next version. Ie, at line 3, to get 'a' 
you would start with C, apply rb to get b, then apply ra to get a.

Going to step 11, C is simply renamed as j, and a complete new 
version of C is written. There are no diffs required to get from C in 
step 9 to j in step 10.

From step 11 onwards, you can get to any version a..i by starting 
with j and applying the diffs back - you don't need any later file.

> I ask this because this issue of "snap-shots" has caused me and
> others some confusion. I think this is especially acute from the
> terms used in the discussion. (i.e. snapshot: which I think most of
> us would think of as a mirror of the source file at some point in time.)

Took me a while as well - so few "sensible" words, so many meanings :-(

--
Simon Hobson

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