Johan Ehnberg wrote:
  

OK. I can see now why this is true. But it seems like one could
rewrite the backuppc rsync protocol to check the pool for a file with
same checksum  before syncing. This could give some real speedup on
long files. This would be possible at least for the cpool where the
rsync checksums (and full file checksums) are stored at the end of
each file.
      

Now this would be quite the feature - and it fits perfecty with the idea 
of smart pooling that BackupPC has. The effects are rather interesting:

- Different incremental levels won't be needed to preserve bandwidth
- Full backups will indirectly use earlier incrementals as reference

Definite whishlist item.
    

But you'll have to read through millions of files and the common case of 
a growing logfile isn't going to find a match anyway.  The only way this 
could work is if the remote rsync could send a starting hash matching 
the one used to construct the pool filenames - and then you still have 
to deal with the odds of collisions