Non-determinism

David Bolen db3l at fitlinxx.com
Wed Apr 17 10:48:02 EST 2002


Berend Tober [btober at computer.org] writes:

> That was my point about comparing rsync to sending the entire file 
> using say, ftp or cp. That is, one might think that sending the 
> entire file via ftp or cp will produce a exact file copy, however the 
> actual transmission of the data takes the form of electrical signals 
> on a wire that must be detected at the receiving end. The detection 
> process must have some probablilty of false alarm/missed detection 
> characteristic and so there must be some estimate of the probability 
> of ftp and cp failing to produce a reliable copy. So while the 
> software algorithm of ftp and cp are deterministic, there must be 
> some quantifiable probablity of failure non-the-less. The difference 
> with rsync is that not only are the same effects of data corruption 
> at work as with ftp and cp, but the algorithm itself introduces non-
> determinism.

Except of course that rsync uses its own final checksum to balance out
its risk of incorrectly deciding a block is the same.  If the final
full-file checksum doesn't match, then rsync automatically restarts
the transfer (using a slightly different seed, I believe).

Thus, it's fairly accurate to compare rsync to performing an ftp or cp
and then doing a full checksum on the file, so one could argue it's
actually more reliable than a straight ftp/cp without the checksum.

-- David

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