I've been using this extensively recently. I was setting up remote virtual machines that boot a live ISO containing all the software for the machine. Sometimes I need to change a small config file, which would lead to generating a new 1.7GiB ISO, but 99.9% of that ISO is identical to the previous one. So I used rsync. Blew my mind when after a day of working on these images, uploading 1.7GiB ISO after 1.7GiB ISO, wireguard showed that I had only sent 600MiBs.
Fun surprise, rsync uses file size and modified time first to see if the files are identical. I build these ISOs with nix. Nix sets the time to Jan 1st 1970 for reproducible builds, and I suspect the ISOs are padded out to the next sector. So rsync was not noticing the new ISO images when I made small changes to config files until I added the --checksum flag.
In the past I downloaded daily diffs from iso which were only few MB. I then applied this diff to my iso from yesterday. Forgot the name of this tool though. I did this on my machine, if parent wants to update in a remote machine I'm not sure it works the same way.
Borg is available for download as a standalone binary, easily dropped onto any Linux system even with very limited privs. And in the repos of every distro easily installed and kept up-to-date.
By avoiding that one step and using rsync instead, you're resigning yourself to "send 600MiBs" over the network for every tiny config change. Not a good trade-off.
Fun surprise, rsync uses file size and modified time first to see if the files are identical. I build these ISOs with nix. Nix sets the time to Jan 1st 1970 for reproducible builds, and I suspect the ISOs are padded out to the next sector. So rsync was not noticing the new ISO images when I made small changes to config files until I added the --checksum flag.