We have two offices, one in London and one in Shanghai, each needing to be able to access the same set of files. This means we need a solid, speedy means of synchronising a set of folders between servers at either office. They're likely to be Windows servers, but we could look at Linux boxes if the software side makes more sense on *nix.
We've considered Rsync, Unison, Gluster, and a few other options, but none of them seem capable of reliably keeping the servers in sync between such distant office locations. Each office is on DSL connectivity over the open internet, so encryption is also a factor.
Does anyone have any hints for getting the servers synchronising in as close to real time as possible, without dying constantly?