rsync - May 2006 - Problem with rsync-2.6.8 and -c (checksum)

Hi

I have a problem with rsync and the -c option. I think understand what 
it does, but I'm not sure, it does it correctly. But then I guess, 
perhaps I don't understand what it does... :-)

I use "rsync  version 2.6.8  protocol version 29"



I use the following command:

/opt/rsync/bin/rsync -av 
--link-dest=/home/user/backup/old
            /home/user/source 
            /home/user/backup/new

This works ok.

If I touch a file in the source directory, I get a copy in the 
'backup/new' directory, all the other files there are hard linked to 
'backup/old'. This is still ok and how I expected rsync to work (as the 
one file changed it's timestamp).


If added '-c' to the already existing '-a'. Now it looks as
follows:

/opt/rsync/bin/rsync -cav
--link-dest=/home/user/backup/old
            /home/user/source 
            /home/user/backup/new


I expected, that rsync doesn't care about the timestamp anymore. It 
should calc the checksum of all the files in the source directory, 
compares them with the files in the '--link-dest' directory and just 
copies the files with a different checksum. But there is no change in 
the behavior of rsync. I get the same result with or without the '-c' 
option.

My guess: --checksum does not work on --link-dest. Is this by design?


Any help on this (and is it clear, what I have in mind)?

Thanks,
Andreas

On Mon 22 May 2006, Andreas Fehr wrote:
> 
> If added '-c' to the already existing '-a'. Now it looks as
follows:
> 
> /opt/rsync/bin/rsync -cav
> --link-dest=/home/user/backup/old
>             /home/user/source 
>             /home/user/backup/new
> 
> 
> I expected, that rsync doesn't care about the timestamp anymore. It 
> should calc the checksum of all the files in the source directory, 
> compares them with the files in the '--link-dest' directory and
just
> copies the files with a different checksum. But there is no change in 
> the behavior of rsync. I get the same result with or without the
'-c'
> option.
> 
> My guess: --checksum does not work on --link-dest. Is this by design?
It does, but not the way you expect it.

The -a also implies -t, so you're telling rsync to preserve timestamps.
The only way it can do that is by creating a new file, as the
--link-dest tree has a different timestamp.

The --checksum will only _increase_ the number of newly created files,
if anything; if you change a file in the source, but do not change its
modification time (e.g. by resetting that after the change) or any other
metadata such as size or ownership, then without --checksum rsync will
be "fooled" into thinking it's the same as the version in the
--link-dest tree, and will link to that. With --checksum rsync will see
that it is in fact different, and will create a new file instead of
linking.


Paul Slootman