[tahoe-dev] Thinking about building a P2P backup system

[Shawn Willden]

On Friday 09 January 2009 08:15:56 am Shawn Willden wrote:
> But if the remote store is encrypted, the delta can't be applied.  All you
> can do is store it as a forward delta.  To get reverse deltas, you have to
> upload a full copy of the new version, plus the delta.

I omitted one other factor here.  If rsync deltas are not reversible, then you 
actually need a local copy of version n-1 to be able to generate the reverse 
delta from n to n-1.  This means you have to either have a full local backup 
or you have to retrieve the last version from the distributed store.

1.  Determine that a file really has changed and needs to be backed up.
2.  Retrieve the last full version from your local backup or the distributed 
store.
3.  Compute the reverse delta.
4.  Upload the new full version and delta to the distributed store.
5.  If you're keeping a local backup, copy the new full version there as well.
6.  Delete the now-obsolete full version from the distributed store.

So there's another bandwidth/storage tradeoff.

I really hope rsync deltas are reversible or can be made so.  I'm off to check 
on that.

	Shawn.