+dit(bf(--compare-dest=DIR)) This option instructs rsync to use DIR as an
+additional directory to compare destination files against when doing
+transfers. This is useful for doing transfers to a new destination while
+leaving existing files intact, and then doing a flash-cutover when all
+files have been successfully transferred (for example by moving directories
+around and removing the old directory, although this requires also doing
+the transfer with -I to avoid skipping files that haven't changed). This
+option increases the usefulness of --partial because partially transferred
+files will remain in the new temporary destination until they have a chance
+to be completed. If DIR is a relative path, it is relative to the
+destination directory.
+