NBD and shared local devices?

Hello.

I've seen you are using NBD instead of NFS for accessing local devices.
What are the main reasons for using NBD? I've read it is faster and it is 
possible to put any file system on it. But if someone has mounted NBD rw it 
must be assured that no one else will have it mounted. In my case different 
users working at the same time on the same client (every user has his own 
xsession). What about the accessiability of local devices in this case?

Christian
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