Rsync options –update and –append-verify

[hancooper]

I am struggling to understand exactly what the rsync options --update and --append-verify do.

Doing info rsync gives

-u, --update
         This forces rsync to skip any files which exist on the destina‐
         tion and have a modified time that is  newer  than  the  source
         file.  (If an existing destination file has a modification time
         equal to the source file’s, it will be updated if the sizes are
         different.)

--append-verify
        This works just like the --append option, but the existing data
        on  the  receiving  side  is included in the full-file checksum
        verification step, which will cause a file to be resent if  the
        final verification step fails (rsync uses a normal, non-append‐
        ing --inplace transfer for the resend).

I am using rsync to transfer directories recursively. There are times where I have to stop the rsync transfer, and resume the transfer a few hours or days later, without affecting the already transferred files at destination.

I also got some files that return errors by rsync, such as

rsync: read errors mapping "/media/hc1/a1-chaos/amvib/IACL-2017-07-19T00:00:00-2017-07-19T23:59:59.mseed": Input/output error (5)

I would like to retry the transfer of these files, at a later time too, without affecting the files that had been transferred successfully.