Re: convert old full + differentials into new full

On Fri, 16 Apr 2021 20:15:28 +0200
Denis Corbin <[email protected]> wrote:

> On 16/04/2021 19:36, imschmeg wrote:
> > Is there a way to convert an old full backup (at time t0) plus a
> > set of progressing differential backups off of that full backup (at
> > times t1...tn) into a new full backup (at time tn) by only working
> > with the backups themselves (not the original sources) and without
> > restoring anything?  
> 
> yes, this is possible, see FAQ for details:
> http://dar.linux.free.fr/doc/FAQ.html#full-from-diff

Thanks for that link, Denis!

However, the link suggests that I need to make the new full backup at
t1 by using the old one at t0 plus a differential at t1, and then
proceed with that a single incremental time step at a time.  In other
words, I can't move in one single operation from a full backup at time
t0 plus as sequence of incrementals to a full backup at time tn.  Is
that correct?

> 
> 
> > 
> > I'd like to do this as a background task ocassionally, so that
> > newer differential backups, after this background task has
> > completed, can use the newer full backup as reference.   
> 
> if you just want to base a "differential" backup on the previous
> "differential" backup (which method is also known as incremental
> backup) no need to rebuild the full backup, just give to -A option
> the previous differential/incremental backup.

That's what I am already doing.  Perhaps I am confusing the terminology
here, and should really be talking about incremental backups instead of
differential backups. What I have is a full backup at t0, then an
incremental at t1 using t0 as reference, then another incremental at t2
using the incremental at t1 as reference, etc..

At some point, though, I have many incrementals and am in danger of
needing to process them all in order to resore the most recent image of
the sources.  This is why I want to migrate the full backup forward.

Although, perhaps I should abandon that scheme for one with
differentials at t1..tn that are all based on the same full at t0, as
then I can use the full-from-diff mechanism in a single step to make a
full backup at tn.  Or mix them, so that t1..tn-1 are incremental, but
tn is differential (directly referencing the full at t0), so that I can
create a new full backup using full-from-diff in a single step at tn but
not t2..tn-1.

> 
> hint: have a look at dar_manager to ease restoration of a particular
> file from a possibly huge set of full, differential and incremental
> backups
> 
> > This way I can
> > avoid having to do a new full backup from the sources,  
> 
> Doing that will probably be slower than doing a new full backup, as
> you will have to uncompress, eventually uncipher the old full backup
> the old differential backup and manipulate than just reading the data
> from filesystem... then in both cases dar will compress and
> eventually cipher the resulting full backup.... I let you test and
> report the result of your experimentation here, if you like ;^)

I am not using dar's compression or encryption.  Or, rather, the sources
are encrypted on a per-file basis prior to dar seeing them, as the file
system I'm backing up provides this per-file encryption.  Hence, it is
useless for dar to attempt compression or encryption.

Also, I am willing to spend more time on this conversion as it can be
done without interfering with the user of the original sources,
provided it doesn't consume too much additional storage on its own.