Chris Shoemaker wrote:
Indeed, what you describe seems to have been the design motivation. I
can share what my desired application is: I want to create a mirror of a
public server onto my local machine which physically disconnected from the
Internet, and keep it current. So, I intend to first rsync update my own copy
which _is_ networked while creating the batch set. Then I can sneakernet the
batch set to the unnetworked machine and use rsync --read-batch to update it. This keeps the batch sets smallish even though the mirror is largish.
This was something I looked into a couple of years ago. Back then I even posted an email to the list (http://lists.samba.org/archive/rsync/2002-August/003433.html) and got no feedback, which led me to conclude that people were not doing any of this at the time. To restate the obvious, the batch mode thing is really just a glorified diff/patch operation. The problem I have with it is that AFAICT it's a very fragile one, since a simple change of one file on either sender or receiver after the batch has been created will invalidate the use of the batch mode. Contrast this with diff/patch, which has builtin measures to account for fuzzy matches and therefore makes it a much more robust tool.
In the end my motivation for using the rsync-via-sneakernet approach disappeared when I convinced myself that the whole operation would have been far too unreliable, at least for our application where files are updated all the time and there is never really a "freeze" of a release against which a batch file can be created. I won't go as far as saying that the feature is useless, but just caution people that they need to understand the assumptions that this use of rsync is based upon. Also, I would suggest checking out other diff/patch tools such as rdiff-backup or xdelta.
BTW, there is a work-around. If you don't mind duplicating the mirror
twice, one solution is to do a regular (no --write-batch) rsync update of one
copy of the mirror, and then do the --write-batch during a local to local
rsync update of another copy of the mirror. Actually, this has some real
advantages if your network connection is unreliable.
This is really the only circumstance under which I would even consider using batch mode. There should also be safeguards built into the batch mode operation to guarantee that the source files to which the batch is applied are in the state we expect them to be. I wouldn't otherwise want rsync to touch my files.
Thanks for your input.
Likewise. Good luck...
-- Alberto
******************************************************************** Alberto Accomazzi aaccomazzi(at)cfa harvard edu NASA Astrophysics Data System ads.harvard.edu Harvard-Smithsonian Center for Astrophysics www.cfa.harvard.edu 60 Garden St, MS 31, Cambridge, MA 02138, USA ********************************************************************
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