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From http://www.jveweb.net/en/archives/2011/02/using-rsync-and-cron-to-automate-incremental-backups.html#jveweb_en_027_06

Using the --backup parameter enables the delta-transfer algorithm, so the use of the -u parameter is not necessary.

  1. How can we understand --backup implies the delta-transfer algorithm?

  2. -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.

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Using the --backup parameter enables the delta-transfer algorithm, so the use of the -u parameter is not necessary.

Wrong! https://xkcd.com/386/ (someone is wrong on the Internet).

How can we understand --backup implies the delta-transfer algorithm?

It doesn't. The delta-transfer algorithm is the default.

So how does enabling delta-transfer algorithm imply -u?

It doesn't.

  • Thanks. (1). Since the link contains inaccuracy, is the link correct that the files that are backed up by --backup are exactly the destination files to be overwritten or to be deleted? Does --backup record the source files that don't exist on the destination and are to be transferred? (2). When --backup backs up some destination file, does it backup the entire of the destination file or the "difference" between the preexisting destination file and the source file? In other words, does --backup copy files in a similar way to the delta-transfer algorithm? – Tim Jan 28 '17 at 8:53

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