I can't speak about how debian's system of mirrors might affect zsync,
but for linuxcnc's images I've seen substantial savings with zsync,
typically 80-85% reduced data transfer when downloading the next .iso
image.
One thing I just discovered is that zsync apparently can't handle https:
URLs! :-/
could not read control file from URL
http://example.com/html/lively/debian-live-9.1.0-amd64-xfce.iso.zsync
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=459706
In my local experiment, I got the following statistics when downloading
debian-live-9.1.0-amd64-xfce.iso via zsync when I was able to use
debian-live-9.0.0-amd64-xfce.iso as a source image (zsync -i):
used 1676443648 local, fetched 286171331
so just 15% of the full image size had to be transferred. I don't know
if this includes the size of the .zsync metadata, but that's just about
3.7MiB. (0.2%?)
Jeff
