rsync in-place (was Re: rsync 1tb+ each day)

[Bennett Todd]

2003-02-04T14:29:48 Kenny Gorman:
> Is it possible to tell rsync to update the blocks of the target file 
> 'in-place' without creating the temp file (the 'dot file')?  I can 
> guarantee that no other operations are being performed on the file at 
> the same time.  The docs don't seem to indicate such an option.

No, it's not possible, and making it possible would require a deep
and fundamental redesign and re-implementation of rsync; the result
wouldn't resemble the current program much.

Here's a sketch of the heart of the rsync algorithm (for finer
details, see the tech report available from[1]).

Let's call the two endpoints the sender (who has the newer version
of the file), and the receiver (who wants to update its older local
copy to match that on the sender).

The receiver computes checksums on each block of the destination
file, and streams them to the sender.

The sender finds all instances of any of those blocks in the source
file. Then the sender transmits instructions to the receiver,
describing how to build a spiffy new copy of the newer source file,
using a mixture of actual chunks of new contents, and blocks taken
from the older version of the file. The receiver follows these
instructions, copying blocks as needed from the old version and
combining them with the new bits to construct the new file. It's
then moved into place.

This algorithm by nature expects that the old version of the
destination file is used as a source for taking blocks, in building
the new version. Adjusting this algorithm to work in-place is
non-trivial.

-Bennett

[1] <URL:http://samba.anu.edu.au/rsync/tech_report/>
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