rsync compression (-z) and timestamp

[Matt McCutchen]

On Tue, 2009-01-27 at 13:21 -0700, lewis butler wrote:
> Does rsync necessarily know if a transfer is actually local or not?

Rsync considers a transfer local if and only if the source and
destination are both given as local filesystem paths, so that it forks
into sender and receiver processes connected by a local pipe over which
there is no incentive to reduce traffic.  If the source or destination
path points to a network filesystem, the OS will send extra traffic to
carry out the system calls rsync makes, but rsync doesn't know about
this and can't do anything about it anyway.

When the source or destination is given using one of rsync's remote
access methods (remote shell or daemon), rsync assumes that traffic
reduction is desirable and enables the delta-transfer algorithm by
default.  If the other rsync process turns out to be on the same machine
or the network is just really fast, traffic reduction may again be
pointless, but rsync won't know this, so the user has to pass
--whole-file.

-- 
Matt