The end of Dovecot Director?

[dovecot at ptld.com]

> the problem that prevents most load balancers from handling the backend imap/pop traffic is that the load balancer needs to be aware of the context of each connection. which all boils down to the index files
> (only a single dovecot server can access a set of index files concurrently, else the indexes will get corrupted)


As someone else asked on this thread, what prevents two clients, both being directed to the same server, from fighting over index files?
Wouldn't file locks over NFS prevent this problem? And if so, doesn't that also prevent two dovecot installations from fighting over index files?

What is a way to test your system to know if dovecot is using the default fcntl file locks over NFS4 and they actually work?
Or is it better/safer to use dotlock on NFS4 without director?