On Fri, Dec 23, 2011 at 6:03 PM,  <hansbkk AT gmail DOT com> wrote:
> On Sat, Dec 24, 2011 at 5:35 AM, Arnold Krille <arnold AT arnoldarts DOT de> 
> wrote:
>> Well, actually the comparison is done against the last backup of a lower 
>> level.
>
> Actually actually <g> from my understanding their isn't any difference
> at all in BackupPC's filesystem between the two if it hasn't been
> modified. In fact you can't even say that one instance of the file is
> "in" the last full backup as opposed to "in" the incremental sets. The
> only think that is "in" those sets is the hardlink, which even at the
> underlying OS level, are identical structures, with no distinction to
> one being the "master" as you would have say with symlinks.
>
> Now in the case of a file having been modified since the last full, of
> course then the two are different, and of course it only makes sense
> to compare to the newest one, since in BPCs storage model there isn't
> any benefit to distinguish between "incremental" vs "differential"
> sets.
>
> I seem to recall this may be an issue however with non-rsync
> transports, since I never use them I don't know.
>
>
> Confirmation of the above would be appreciated; I think not
> understanding these issues is a source of confusion for newcomers to
> BPC used to thinking in terms defined by traditional backup software
> regimes.

The distinction is between the contents of the file and the directory
entries pointing to it.   The contents of hardlinked files are all the
same, but rsync doesn't know anything about the hashed filenames for
the pool links.   It strictly follows the directory tree established
by the last full run (by default).   The concept of incremental vs.
differential sort-of relates to the 'incremental level" setting that
permits the comparison to merge in previous incrementals back to the
last full, finding the latest version of each file .   That involves a
trade-off of more server side work traversing multiple directory trees
vs. likely transferring less changed data.

-- 
   Les Mikesell
    lesmikesell AT gmail DOT com

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