Modern rsync (err, 3.0+, so maybe not that modern any longer) can scan the directory tree concurrently with sending the files. (I'm not sure how it's done, pre

Modern rsync (err, 3.0+, so maybe not that modern any longer) can scan the directory tree concurrently with sending the files. (I'm not sure how it's done, presumably interleaving readdir() calls with non-blocking sendfile, allowing page cache prefetching as well as the actual sending to be done in parallel by the kernel.)