Using the --backup parameter enables the delta-transfer algorithm, so the use of the -u parameter is not necessary.
How can we understand
--backup
implies the delta-transfer algorithm?-u, --update
skips files that are newer on the receiver. The delta-transfer algorithm is to actually transfer a file, but not to make a decision whether this file will actually be transferred or not. So how does enabling delta-transfer algorithm imply-u
?
Note that I found neither of the two are mentioned in the manpage of rsync, but I might miss something.
Thanks.