Using rsync and 20M files is a good enough example: - to check the date of each file on UNIX you must run "stat" once per file (unless you have an external log

Using rsync and 20M files is a good enough example: - to check the date of each file on UNIX you must run "stat" once per file (unless you have an external log that says what to skip) so that's very slow. - to backup a big file is "--append-verify" or something like this and one streaming read per file.