Hi,
dan wrote on 2008-09-03 14:17:00 -0600 [Re: [BackupPC-users] Backup through
slow line?]:
> On Wed, Sep 3, 2008 at 5:27 AM, Rob Owens <rob.owens AT biochemfluidics DOT
> com>wrote:
>
> > Christian Völker wrote:
> > > [...] again the question what is then the difference between a full
> > > rsync backup and an incremental one? [...]
> > >
> > With rsync, the difference between a full and incremental has more to do
> > with processor and disk activity than with bandwidth. Both use about
> > the same bandwidth. If you ask me, it doesn't make sense to call them
> > "full" and "incremental", but I guess those terms are holdovers from the
> > days when BackupPC didn't offer rsync as a transport.
>
> full or incremental really discribes how they are stored on backuppc. a
> full will hang around longer than an incremental. As far as the actual
> transfer, the only real difference is that a 'full' doesnt skipp files that
> have the same mtime where an incremental skips those files. This causes a
> full to hit the CPU on both sides more but it doesnt use very much more
> bandwidth as the only added data transfer is the checksum of those files.
yes, there are many different ways to look at this. I'll add one.
I keep repeating this, so you might all be bored, but still:
1.) Full backups make an exact image of your data set, as defined by your
configuration (meaning the image does not include things you are
deliberately excluding).
2.) Incremental backups are a trade-off, cutting down resource usage at the
price of exactness.
Both of these points are true for rsync just as much as tar or smb. rsync is
smarter than tar/smb, so
a) rsync full backups are only minimally more expensive than incrementals in
terms of bandwidth. Still, every file needs to be completely read from disk
on both sides, so there is a good reason to offer an "incremental" mode as
a speedup.
b) rsync incremental backups are *far less* likely to *not* get a precise
image of your data set. For practical purposes, the chance are probably
negligible.
When you factor pooling into the equasion, this means that the difference
between rsync full and incremental backups is smaller than for tar/smb
backups, which is yet far smaller than for tape backups for instance. Or, the
other way around, for tape backups the difference is obvious, for tar/smb you
can still see it clearly, while for rsync backups you have to look through a
microscope. But it's still the same difference, and it's still there.
I suppose it is for this reason that BackupPC applies the same rules for
backup dependency and requirement of regular full backups to rsync as to the
other transfer methods.
Regards,
Holger
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