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Re: Share a key between multiple systems



On 26.03.2013 22:00, Colin Percival wrote:
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.

Sounds reasonable.

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.

That's what I thought. Is there any options to display bandwidth (and cost) for operations other than a backup?

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)?

That would be possible, yes. But it would be another way of accessing my files, so I'd rather have the cache on a share and make sure, no 2 systems do the backup at teh same time.

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.

Hmm, ok. That would be ok, I think. Since storage cost is much cheaper than bandwidth cost, there is no harm in keeping backups a little longer.

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

I'm looking forward to your thoughts. ;-)

Philipp