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

[jw schultz]

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 at galcit.caltech.edu>
> To: rsync at 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 at pegasys.ws

		Remember Cernan and Schmitt