Find similarity between local and remote file before transferring

Dave Dykstra dwd at
Tue Nov 27 03:32:44 EST 2001

On Fri, Nov 23, 2001 at 03:22:02PM +0100, Niels Andersen wrote:
> For two different reasons, I've been thinking about how to find out how much
> data there would have to be transferred to update the local file,or how
> similar the local and remote files are.
> Reason one:
> I'm working on a frontend for rsync. I think it would be a nice feature if
> it could check how much of the existing file there would be reused, if
> updating from the remote file.
> Reason two:
> There's a lot of filesharing systems out there, each with it's own
> advantages. But you can extremely rarely resume with another downloading
> mechanism. So, if you download a 650 mb iso from somewhere, and there's an
> error, you would have to start all over. Or maybe you just need the last to
> megs, but theres a 20 hour queue.
> If it was possible, I'd suggest hosts of such systems, to also share the
> files with rsync. The rsync daemon should then check for similarity, and if
> there's more than eg. 10% difference, it should deny, and tell the user to
> use the primary download system, with queues, ratio, or whatever.

Rsync currently doesn't do that.  Would it suit your purposes if 'rsync -n'
reported the statistics of what it would transfer if -n were not used
rather that what it reports now (which appears to be the number of bytes it
actually transfers rather than what it would transfer without -n)?  I think
that's been asked for but nobody ever made a patch.

- Dave Dykstra

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