Re: Share a key between multiple systems

[Colin Percival <cperciva@tarsnap.com>]

On 03/26/13 12:37, Philipp Riegger wrote:
> I am thinking about using one tarsnap key for multiple systems to
> improve the effectiveness of the deduplication. There are multiple
> possibilities how I could approach that:
> 
> 1) Share the cache between the systems. I'd probably have to make sure
> that no 2 backups are running at the same time and that no 2 instances
> of tarsnap are using the cache at the same time, is this correct?

Correct.  Tarsnap won't let you create two archives at once, in fact -- if
you try then one of them will error out.

> 2) Keep a seperate cache on every system. A very naive approach would be
> to run fsck before every backup. But that takes a lot of time and wastes
> some bandwidth, I think.

Running fsck before every backup would dramatically increase your bandwidth
costs.  Not recommended.

> Is this kind of setup recommended? What is the best way to implement it?

Given your suggestion of sharing the cache, am I right in thinking that
you're backing up multiple systems which are close together (geographically
or networking-wise)?  Can you have one system which runs Tarsnap and reads
data from the others (via NFS / SMB / etc)?

> Would a tarsnap command line option be possible, that does what I need
> from the fsck, just faster and using less traffic?

Hmm... theoretically writes could be done with a "mini-fsck", but you'd
need to do a full fsck before you could delete any archives.

Still, this is an interesting possibility... I'll have to think about this.

-- 
Colin Percival
Security Officer Emeritus, FreeBSD | The power to serve
Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid