Honest question: how does GlusterFS not meet these goals? I know that it's easy to install, and can easily run on three computers. And I believe it can scale to thousands.
Actually GlusterFS is close to my needs, however the "easy to install" and actually bad performance on many small files is a road blocker for us. However it runs well, even on just 3 servers.
Gluster provides files storage but not block storage.