from rsync manual:
--compare-dest=DIR
This option instructs rsync to use DIR on the destination machine as an
additional hierarchy to compare destination files against doing transfers
(if the files are missing in the destination directory). If a file is found
in DIR that is identical to the sender's file, the file will NOT be
transferred to the destination directory. This is useful for creating
a sparse backup of just files that have changed from an earlier backup.
--copy-dest=DIR
This option behaves like --compare-dest, but rsync will also copy unchanged
files found in DIR to the destination directory using a local copy.
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.
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