I have an image in my head where a link is too slow to realize the real-time transfer of files, but fast enough to catch up every day. What I'd like to see is a master <-> master setup where when I write a file to Server A, the metadata will transfer to Server B immediately and the file will transfer at idle or immediately when Server B's client tries to read the file before Server A has sent it.

It seems that there are many filesystems which can perform well over fast links, but I don't know of any that do well with a big bottle neck and a few hours of latency.