clustered (file) system on multiple host

[Martin P. Hellwig]

Reading the list about vkernel, clustering and file system I've began to 
wonder.

How on earth can you make a cluster without paying for the extreme 
overhead for locking everything?

Is it a question of locking more specific or is there some 'magical' 
technology that helps you out of that. Logical speaking (and this more a 
problem of my knowledge) there always must be some overhead since all 
machines must know at the same time what the state of an object is, thus 
creating 'much'(?) traffic for synchronization. So I guess that there 
must be some tricks to avoid too much traffic.

Or do I just get the concept of clustering wrong (I see it as 
high-availability combined with load-balancing)?

--
mph