Find similarity between local and remote file before transferring

Niels Andersen niels at
Sat Nov 24 01:22:02 EST 2001

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.

/Niels Andersen

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