[Box Backup] Backup the backup

[Chris Wilson]

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Hi Simon,

On Wed, 1 Nov 2006, Simon Lundstr=F6m wrote:

> From man rsync:
> -u, --update                update only (don't overwrite newer files)
>
> I thought that was going to work, but maybe not?

I suspect that you will get a mixture of newer and older files on the=20
server, as soon as the client switches in the middle of an rsync process,=
=20
which is guaranteed to happen eventually.

I also suspect that you may have problems with synchronisation of=20
directories between the two servers, but I can't put my finger on an exact=
=20
failure mode right now. Go ahead and try it, but be extremely careful,=20
that would be my advice. Also, run bbstoreaccounts check on each account=20
regularly on both sides and watch out for errors.

> Maybe unison http://en.wikipedia.org/wiki/Unison_%28file_synchronizer%29=
=20
> is the way to go?

Maybe, but I found it's unsuitable for automatic synchronisation due to=20
its tendency to develop conflicts in practice. For example, if you updated=
=20
a file and a directory on one side, and the directory (but not the file)=20
on the other, you would get a conflict on the directory but not on the=20
file. But you can't just upload the file without overriding one version of=
=20
the directory, otherwise the result is inconsistent (doesn't agree with=20
either side) and may be invalid.

> Because many of these companies are small and don't have a lot of=20
> bandwidth since they only surf and check their mail they can't only use=
=20
> remote backup but needs local backup. (5-10 ppl transmitting 2-10GB over=
=20
> 2Mbit can be come quite slow.

But they're only syncing changes and only when they're on the road, so I=20
don't think it would be so bad. Besides you would probably use exactly the=
=20
same amount of bandwidth in the other direction (upload) to update the=20
secondary server, and upload bandwidth is more scarce than download=20
bandwidth. I suppose it depends on whether the clients are more usually on=
=20
the road or in the office.

> I will only use lazy mode, i.e. backups will not be automatic. Besides=20
> not all clients can have servers or ports opened in their firewall.

You can get around the latter restriction by tunneling to your server=20
through the firewall.

Cheers, Chris.
--=20
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  / __/ / ,__(_)_  | Chris Wilson <0000 at qwirx.com> - Cambs UK |
/ (_/ ,\/ _/ /_ \ | Security/C/C++/Java/Perl/SQL/HTML Developer |
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