Hi,

Tino Schwarze wrote on 2009-01-21 10:48:50 +0100 [Re: [BackupPC-users] 
Designing a BackupPC install over a WAN -?minimising Full backups]:
> On Wed, Jan 21, 2009 at 02:08:31PM +0900, Peter Wright wrote:
[
> > Adam Goryachev, Wed, Jan 21 2009, 14:01:19 +1100:
> > > David Crisp, Wed, Jan 21 2009, 12:31:35 +1100:
]
> 
> > > > Idealy I would love the system to be able to sit in place and make
> > > > regular incrementals of the data with the most minimal of network
> > > > traffic requried to backup the data.

this is a frequently asked question. Minimal bandwidth usage with rsync is
achieved by alternating full and incremental backups, or using full backups
only. Full backups cost you in terms of disk and CPU usage on both server
and client (read "disk usage" as "disk wear" too), as well as backup time.
Using multi-level incremental backups can achieve the same bandwidth savings,
but at increasing costs for building the backup view (meaning you can probably
take it to level 3 or 4 but not level 365).

You will need to decide for yourself to which degree bandwidth costs outweigh
other costs. If you have little change in your backup data, you will prefer
doing more incrementals in between full backups, if you have much change, you
will do full backups more often.

> > > An incremental backup will transfer changed data from the previous
> > > full or incremental of a lower level.
> > 
> > Can backuppc be configured to transfer changed data from the previous
> > incremental of the *same* level, or would this require a patch?

This would simply be incorrect. If you backup relative to a level 1 backup,
you get a level 2 backup, whatever you call it. BackupPC presents an identical
view to you through web interface and restore functionality, meaning you don't
have to restore (level 0 + level 1 + level 2) - the level 2 backup *appears
like* a full backup. Patching BackupPC to call (and store) your level 2
backup as a level 1 gains you something in terms of costs for constructing the
view, but not in terms of exactness (which is very good for rsync in any case,
though). There does not seem to be much point in doing that.

> In default configuration (IIRC), your backups with weekly full would
> look like this:
> 
> - level-0 = full
> - level-1 incr
> - level-2 incr
> - level-3 incr
> - level-1 incr (based on full)
> - level-2 incr
> - level-3 incr

No, the default configuration is

        $Conf {FullPeriod} = 6.97;
        $Conf {IncrPeriod} = 0.97;
        $Conf {IncrLevels} = [1];

meaning "do 1 full backup per week and daily level 1 incrementals".
For what you are describing, you would need to change IncrLevels to [1, 2, 3].

One fact to remember is that while incremental backups are based on the
previous incremental of lower level, full backups are based on the previous
backup of *any* level. That is safe to do, because the whole file tree and all
file contents are checked anyway, and it is reasonable to assume that the
previous backup will be closer to the current state than the last full backup.
An incremental backup, in contrast, does not check file contents that appear
unchanged, so errors are possible (if unlikely) - with increasing level you
potentially get more errors.

Regards,
Holger

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