Way to make distant servers to appear to have the same data ?

[David Timms]

What is the most effective, most robust way to allow servers that are 
quite distant, and on slow networks "appear" to have the same content ?

In my example, the content is read/write at 4 sites. Hopefully the 
system should make caching possible for files that were originally at 
another site. If a file were not already cached, then it would get 
loaded across the slow network.

redhat global file system appears to be designed to do this:
http://linux.sys-con.com/read/166309_2.htm
but then talks about storage area network or LAN connections rather than 
  slow wan links.

http://www.drbd.org/ raid across machines?

I did see a few other projects designed to solve this sort of problem, 
but I am having trouble finding them now {search hints ?}

http://www.coda.cs.cmu.edu/ljpaper/lj.html

Has anybody used / appraised coda ?

David Timms.