Hi,
Peter Carlsson wrote on 2013-03-09 11:10:59 +0100 [Re: [BackupPC-users]
Archiving incremental backups?]:
> [...]
> I regulary do an archive to a USB HDD that I store offsite. This is a
> manual step since I have to bring the HDD, do the archiving, and then
> store the HDD offsite again.
>
> To know for sure that files that are modified in between these regular
> archives, I want to make tar archives of the incremental backups and
> move them on a daily basis offsite over the Internet.
>
> Why I only want to do this for the incremental backuped files is to
> reduce the amount of data over the Internet.
in theory, you could use rsync to transfer incremental changes over the
internet. This could even be substantially less traffic than an incremental
tar, presuming you have changing files that can efficiently be transferred
with rsync (like growing log files or large files where only small portions
change from day to day). In the worst case (only new files, no changed files)
there should not be much difference, except that rsync requires more
computational power, and that rsync (with the right options) will track
deletions. rsync also has built-in capability to limit transfer bandwidth.
The thing is, you would need to keep an image of your target file system on
both ends, i.e. unpack the tar file on your USB HDD and have it accessible over
the internet, and have a local copy on or near your BackupPC server. You could
use the original file system (the one you back up in the first place) instead
of a copy, presuming your concern is not to mirror backup state (your file
system will probably have changed since the last backup), and the file system
can handle the additional load of the rsync run.
One thing I would like to note, though, is that you really want at least two
independent offsite copies, so you will not be left without one if things
break while you are replacing the old copy with a new one. This is especially
true with tar archives. An rsync run will generally not destroy much (maybe
one file) if it fails prematurely, though it will leave you with a state of
your offsite copy that probably never existed on the original (which is easily
fixed if you can just restart rsync). But you should note that requiring the
offsite copy to be online (at least during the incremental update) makes it
somewhat vulnerable, so having an additional *offline* offsite backup would
be a good idea.
> But what I want to achieve is, at least in my
> opinion, better than to only have the full manual archives that at
> best will be done one or two times a month.
With rsync, in my opinion, you can almost get away without the full manual
archives. The same note applies here as in the frequently asked question,
"with rsync, do I need full backups at all?". Ideally, you would turn on the
rsync option "--ignore-times" regularly (e.g. once a month) to catch any
(rare) changes rsync might have missed (but that's just a detail).
I don't know if what I described seems possible in your scenario. We should
figure that out first before going into too much detail.
> The most important thing is that it is simple and automatic, otherwise
> it will never be done.
It should be possible to make this automatic (except for exchanging or syncing
the offsite drives once in a while, but that's much like your monthly manual
archives now). It doesn't seem overly complex, but that depends somewhat on
your setup. How were you planning to get the incrementals over the internet?
Have you done size estimates to see if it is feasible?
Jeffrey hinted at the possibility to tar up the pc/host/num directory for an
incremental backup. While that is certainly possible, it leaves you with the
problem that you would need to unmangle names and interpret attrib files.
That's not too difficult, but it would require some coding. The thing is, how
(and under what circumstances) would you ever *use* the offsite backup? In the
event of a catastrophe? Bring in the disk, restore the full backup, restore
all incrementals? I'd be in favour of having a working file system image (or
better, two identical ones - one as a backup remaining offsite) which you can
just plug in and use, if things need to go really fast, or copy over without
needing to think much (i.e. without much that can go wrong).
Another thing to keep in mind are backup levels. Normally, your incrementals
will tend to be relative to the last full, meaning they will grow from day to
day (because day 2 repeats the changes from day 1 and so on). You *can* fix
that (at the cost of more complexity at backup time), but you probably don't
want to bother with this approach anyway :).
There's, of course, the third possibility of just setting up an offsite
BackupPC server that makes independent backups of the target host. You'd
want to have a VPN for that, but my guess is that incremental tars would be
sent over one, too :-).
Regards,
Holger
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