Re: [Dar-support] Convert Differential Backup to Full?

[Denis C. <dar...@fr...>]

On 27/12/2021 13:25, Daniel Bedrenko wrote:
> Hello,

Hello,

> 
> I'd be grateful for some help with differential backups.
> 
> The way I backup my music library is an initial full backup then
> differential backups every year that are rather small because I only
> add a small amount of files to my library over time.
> 
> However this year I found that the differential backup turned out to be
> 95% of the size of the full backup because I edited the metadata on 95%
> my music files so they were correctly regarded as changed by dar. So
> the backups I have now are:
> 
> 1st backup (full) = 300GB
> 2nd backup (differential) = 5GB
> 3rd backup (differential) = 5GB
> 4th backup (differential) = 5GB
> 5th backup (differential) = 290GB
> 
> I want to avoid essentially storing my library twice. Is there a way to:
> 
> 1. Delete backups 1 to 4, AND
> 2. Turn the 5th backup into a "full" one, and backup the remaining 25GB
> of my music library that hasn't changed since the first backup.

First, you can also probably solve your problem while performing the 5th
backup using the --comparison-field option to restrict inode change that
trigger files to be resaved.

To answer your question now: once a backup has been done, dar never
modifies it again, but it can derive backups from others in many
different ways. In that way, you can "turn" a differential backup into a
full backup with the help of the full backup of reference using the
"full-from-diff" target defined in the default  ${prefix}/darrc
(/usr/local/darrc or /etc/darrc depending on the parameters used at
installation time): From a differential backup and the full backup it
has been based on, you get a new backup that is the content of both
backup as if a full backup had been made at the time the differential
backup was made (dar does not read the filesystem under backup for that,
just the full and differential backups).

Here in your case, this would lead turning the 2nd backup into a full
one (then you could remove the 1st one, after having tested the sanity
of this new backup), then the 3rd backup into a full one (then be able
to remove the 2nd one), then the 4th backup, then the 5th... but this
would lead you to manipulating a lot of data while simply doing a full
backup as 6th backup would just bring you to the same outcome, with much
less I/O and computations.


> 
> Essentially, to avoid doing a fresh full 315GB backup, when I already
> have 95% of the final product with the 290GB backup.
> 
> PS: Thank you for this amazing tool! Been doing backups with dar since
> 2018!

Thanks you :)

> 
> King regards,
> 
> Daniel Bedrenko.
> 
> 

Cheers,
Denis