I have two questions about using rsync with the --link-dest=[file] flag.

Question #1 :
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Suppose I create a "backup" of directory a/ to a0/, using

%> rsync -av a/ a0/

Now I'd like to do an "incremental backup" of a/ to a second directory a1/. The command

%> rsync -av --link-dest=../a0/ a/ a1/

does the job, and creates hardlinks in a1/ to files that are unchanged between a1/ and a0/.

My question is, Why does rsync require the syntax "--link-dest=../a0/" and not just "--link-dest=a0/"? (When I use the latter syntax, I don't get hardlinks between unchanged files, only copies).

Question #2
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Suppose I want to do the same thing as above, but this time, directories a0/ and a1/ are located on file system accessible only by ssh.

To create a0/ originally, I use :

%> rsync -av a/ me@remote.site:a0/

Now to do an incremental backup, I tried the following :

%> rsync -av --link-dest=me@remote.site:a0/ a/ me@remote.site:a1/

but this doesn't work. Directory a1/ is created, but with copies, rather than hardlinks, to unchanged files.

I've tried other variations on the above (adding complete or relative paths) but it seems that '--link-dest' doesn't work in this context.

What I have done instead is :

%> rsync -av a/ me@remote.site:a_tmp/

followed by

%> ssh me@remote.site rsync -av --link-dest=../a0/ a_tmp/ a1/

This works, and creates hard links between unchanged files in a0/ and a1/.

My questions is, Is this the only way to do incremental backups across unmounted files systems? Or am I missing something? Any suggestions would be very useful.

My local version of rsync is :

rsync version 2.6.9 protocol version 29

And remote version is :

rsync version 2.6.2 protocol version 28

In the man page under --copy-dest
there is a reminder:

"If DIR is a relative path, it is relative to the destination directory."

That explains the .. needed. After all, without the full path, the DIR must
be relative to "something".