rsync - what can it do?

[Steve Bonds]

On Wed, 5 Feb 2003, bob parker bob_parker-at-dodo.com.au |Rsync List| wrote:

> Maybe I'm barking completely up the wrong tree but here goes.
> 
> I've downloaded a 700meg iso by ftp using a steam powered dial up
> connection. It took a week and naturally many resumes.
> 
> Murphy's Law did not take a holiday of course so the md5sum of the
> downloaded iso does not match the md5sum of the one at the mirror.

If you are downloading RedHat ISOs, make sure you're getting the correct
version.  There were two versions of 7.3 out there with different MD5SUM
values.  Some mirrors had one, others had the other.  Really confused me
for a while.  ;-)

> Can rsync be used to selectively fetch the part or parts of the iso
> that are in error assuming that the mirror is also an rsync mirror?

Yes.  I've done this for the same thing under quite similar circumstances.  
Rsync does this automagically just by asking it to copy the file.

> How can I tell?

When the rsync is finished, you can tell since the MD5SUM will be what it
should be.  ;-)  When done the "speedup factor" shows you how much faster
it went than if it had to send the whole file.  If this factor is 1.0 you
just copied the whole thing again.  The "--stats" option also gives you
some interesting info on the transfer.  Using "--progress" will show you
the effective transfer rate.  If you're only validating that the file is
the same so far, this transfer rate will be quite high.  (Much higher than
your modem bandwidth.)

The hard part is finding an anonymous rsync server with the exact same
ISO.  Be sure to grab the (preferably GPG-signed) MD5SUM from that server
BEFORE you start and verify it against the one from your original
server.  Otherwise you'll just be downloading the whole thing again since
it's a different file.

This goes without saying, but be sure to save a copy of the original
downloaded file just in case something goes wrong with the rsync.

  -- Steve