Re: [rdiff-backup-users] incremental or differential backup

[Dominic]

Michael Biebl wrote:
> Hi,
>
> I'm new to this list, so if this question has already been answered
> please bare with me and point me to the relevant discussion.
>
> I was wondering, if rdiff-backup stores the snapshots differentially
> or incrementally
>   
incrementally
> Say, I create a snapshot every day.
> If I want to restore a file from 15 days ago, do I need all 15
> snapshots to restore the state (incremental) or only the current
> up-to-date state + 1 rdiff (differential)?
>   
you need them all, but rdiff-backup handles it all 'under the hood'
> If rdiff-backup only allows incremental backups, I see the following problems:
> 1.) If a file changes a little every day (big mysql db), then
> restoring the file (from say 100 days ago) will probably take a lot of
> processing time, space and memory.
>   
the extra overhead in space for many incremental backups vs. one 
differential backup is not great, but yes I guess it will take more time 
and memory
> 2.) If one of the rdiffs goes corrupt (e.g. via a bad sector), all my
> older backups are broken.
>   
hmmm, true I think, you should use raid or (better IMHO) a secondary 
backup (use rsync). There is also the --verify-at-time option in 
rdiff-backup which I confess I have not used (oops!) - this should allow 
you (I think/hope) to confirm that backups at any given point in the 
past are not corrupt. Good idea to run this before doing the secondary 
backup I guess? [Question for expert: if rdiff-backup --verify-at-time 
1Y succeeds does this also mean that all backups within the last year 
are uncorrupted too? I see that it reports success even if there are no 
backups that old in the archive.]
> 3.) I can't throw away rdiffs, say I want to create daily snapshots,
> keep 30 of them, then keep monthly snapshots, and so on (basically
> what tools like rsnaphost provide).
>   
The logic of rdiff-backup is that you don't need to try to keep just 
monthly snapshots, you can keep your daily snapshots forever. It is true 
that recovering a file from a very long time ago, and which has changed 
a lot in the meantime, might take a while, but it would be a very rare 
event. Recovering more recent files which would be a more common event 
would be much faster, and of course the most recent backup (which is 
probably what is wanted in 95% of cases) is stored in the clear.

Others may have some better informed comments...

Dominic