Rsync keeps writing

Question

I just recently updated my macbook pro to Mountain Lion. I use rsync to make backups of certain directories on my macbook pro my timecapsule wireless router with build-in harddisk.

rsync -avzm --delete --exclude-from '/some_dir/exclude_list.txt' \
  /Volumes/Data/some_other_dir /Volumes/TimeCapsule/some_other_dir/ \
  2>> ~/tmp/sync_error.txt

Before Mountain Lion everything worked fine. Rsync only copied new or changed files. Now, after the upgrade, rsync wants to copy all files every time, over and over again. Even if they did not change or were not deleted on the remote host (timecapsule). I don't know why (I do not encounter any errors during the rsync).

Any ideas?

Answer 1

This is a tentative answer, it would explain the symptoms but I have no idea whether the cause, and consequently the solution, is the issue here.

Let's say that the source filesystem stores file times with a microsecond precision, and the destination filesystem stores file times with only a precision of 1 second. Then the modification time of the destination files will not be exactly equal to the mtime of the source files. Hence rsync will synchronize the files again each time.

It's possible that something has changed with respect to time precision. I would have expected old files (dating from before the upgrade) not to be copied again, though. Maybe there was a change in how rsync dealt with this issue.

Try passing the --modify-window=1 to rsync. This allows times to differ by up to a second, and rsync will still treat them as equal.

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Another possibility is that rsync is trying to copy some new extended attribute, and failing because the backup device doesn't support that attribute. However I would expect it not to copy the files again in that case, only their attributes.