rsync - Jun 2003 - Rsync: --link-dest when target and compare_dir both have file

On Mon, Jun 02, 2003 at 03:32:04PM -0700, Michael Rubel
wrote:
> Hi J.W.,
> 
> Please pardon this intrusion on your private address; the rsync list seems
> to be down (no messages since yesterday), so I'm forwarding this
directly
> to you.  If you'd rather wait until the list restarts, that's fine
too.
I'm CCing the list.
> 
> Best regards,
> Mike
> 
> ---------- Forwarded message ----------
> Date: Mon, 2 Jun 2003 09:56:50 -0700 (PDT)
> From: Michael Rubel <mrubel@galcit.caltech.edu>
> To: rsync@lists.samba.org
> Subject: --link-dest when target and compare_dir both have file
> 
> Hi J.W. et al,
> 
> Kevin Everets was kind enough to inform me about some strange behavior in
> his backup script, which seems to be the result of --link-dest behaving 
> unexpectedly in the case where target/ is already populated with older 
> versions of the same file.
> 
> Here's the situation:
> 
> We want to do:
> $ rsync -a --link-dest=../backup.1 source/ backup.0/
> 
> There is a file present under all three directories.  Suppose that it the
> version in backup.1/ is identical to the version in source/, and that
> backup.0/ contains an older version.
> 
> In this case, I would expect:
> 1. the version in backup.0 should be unlinked
> 2. a new hard link should be created in backup.0/ to the copy in backup.1/
> 
> In fact, rsync (at least as of 2.5.6) seems to copy the full file:
--link-dest is built on --compare-dest and behaves like
--compare-dest as indicated in the manpage.

Observe the description of --compare-dest (ALLCAPS added for
emphasis):
              This  option instructs rsync to use DIR on the des?
              tination machine as an additional directory to com?
              pare destination files against when doing transfers
              IF THE FILES ARE MISSING IN THE DESTINATION DIREC?
              TORY.

In other words, the files in the --link-dest location will
only be used if there is no existing file in the
destination.  The best way to use --link-dest is to have an
empty destination.

For a link-dest based rotating backup that reuses directory
names the best bet is to do a "rm -r $dest; mkdir $dest" or
to "rsync -a --link-dest=../backup.1 $source temp; mv temp
backup.0"

-- 
________________________________________________________________
	J.W. Schultz            Pegasystems Technologies
	email address:		jw@pegasys.ws

		Remember Cernan and Schmitt

Everyone: apologies for hitting the list twice; when my first message did 
not go through I thought I'd used the wrong From: address.

J.W.:

Thanks very much, this clears it up nicely:
> --link-dest is built on --compare-dest and behaves like
> --compare-dest as indicated in the manpage.
> 
> Observe the description of --compare-dest (ALLCAPS added for
> emphasis):
>               This  option instructs rsync to use DIR on the des?
>               tination machine as an additional directory to com?
>               pare destination files against when doing transfers
>               IF THE FILES ARE MISSING IN THE DESTINATION DIREC?
>               TORY.
> 
> In other words, the files in the --link-dest location will
> only be used if there is no existing file in the
> destination.  The best way to use --link-dest is to have an
> empty destination.
> 
> For a link-dest based rotating backup that reuses directory
> names the best bet is to do a "rm -r $dest; mkdir $dest" or
> to "rsync -a --link-dest=../backup.1 $source temp; mv temp
> backup.0"
I'll make a note on the snapshot page and respond to the person who 
forwarded it to me.

Mike