[nSLUG] Auto-replication of files to web server

Vlado Keselj vlado at cs.dal.ca
Mon Oct 30 11:03:17 AST 2006


On Fri, 20 Oct 2006, Jason Kenney wrote:

> Hi,
> 
> On 10/19/06, Jeff Warnica <jeff at coherentnetworksolutions.com> wrote:
> > On Thu, 2006-10-19 at 20:55 -0300, Miles Thompson wrote:
> >
> > Option C here is not FTP, but rsync (+ssh); rsync will send only the
> > changes of the given tree (possibly only changes to given files, as
> > well?). rsync is available for Windows through Cygwin.
> 
> Yeah, rsync only sends the changes within a file, so it can be
> extremely fast. Exactly how it knows it what is different without
> being able to compare the two files directly is beyond me. The
> technique was the subject of the author's PhD (at one time available
> on his website), and years ago I started reading through to try to see
> but I gave up. Some kind of magic involving localized hashes and
> streams is the only answer I've had.

Hi,

While I agree that rsync is an excellent piece of software, it is not so 
tricky to explain its basic idea:  It works by reading chunks of file on 
one machine and calculating its hash value, i.e., its checksum (MD5, SHA1, 
or something else) and sending it to another machine, which does the same 
process on the second file to be compared.  If the checksums match, it 
means that there is not need to send it over the line, if the do not, then 
is sent.

--Vlado


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