[Solved] diff vs rsync

Question

I have two folders, on different machines, with the same content. One of the machines has access to both folders, through an sshfs mount. The filesystem is the same on both machines: ext4. Running diff -ur a/ b/ yields no output: i.e. the folders' content is the same. However, running "rsync --progress -avz a/ b/"  causes the ALL the content to start being copied (in this example, b/ is the sshfs mount point). Moreover, the folders are about 1 Gb large, and both contain a lot of files. If I interrupt rync's copying, and resume afterwords, the copying start ***at the point where it had been interrupted***. So I guess my question is... what is causing rsync to behave in this way? (not the way in which it resumes where it left of; rather, why does it think that files that are in fact equal (according to diff at least) are different?
Last edited by gauthma (2013-06-18 00:39:11)

Answer 1

From the rsync man page, under OPTIONS:
-c, --checksum
        This  changes the way rsync checks if the files have been changed and are   
        in need of a transfer.  Without this option, rsync uses a  "quick  check"
        that  (by  default) checks if each file’s size and time of last modifica‐
        tion match between the sender and receiver.  This option changes this  to
        compare  a 128-bit checksum for each file that has a matching size.
Edit: Seems the -c option could mean lots of disk thrashing because both copies have to be read in full to compute the checksums.
Last edited by thisoldman (2013-06-17 23:25:00)