Write only changed blocks to disk using rsync client only

bjquinn bjquinn at seidal.com
Mon Nov 24 15:10:11 GMT 2008

I understand that if you want to only transfer changed blocks across the
network that the remote machine must be running rsync server and the local
machine must be running rsync client.  What if I don't care what gets
transferred over the network, I just care what gets written to disk (i.e.
only changed blocks)?  Is there an option to do that in rsync without
needing an rsync server on the other end?

I have an OpenSolaris server that I'm using for backups.  It's running ZFS,
so if only part of a file changes, I can take a snapshot and be able to have
access to both copies of the file but only store the differences.  However,
if the entire file gets copied back down to disk, ZFS isn't smart enough to
realize that the new blocks copied to disk are the same as the ones that
just got overwritten - so it marks all blocks of the file as changed and
doesn't provide any space-savings.  If rsync had a mode that made sure only
to write to disk the blocks that changed, this would get me around this
problem.  Also, since many of the computers I'll be backing up are Windows
machines, I don't want to hog their CPU, and I don't care how much data gets
transferred over the network, I'd rather avoid installing the rsync server
on those machines.
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