Best practices for mailbox network file storage?

Is there a current best practice for a shared filesystem backend for mailboxes (in my case, all virtual accounts)?  NFS 3/4, GFS2, GlusterFS, other?  In the case of the latter ones, is the use of Director necessary?  Any specific anti-recommendations?  Also, if it matters, I *think* I want to use BackBlaze racks for storage.  Also, I've seen recommendations for the filesystem underneath like XFS or ext4 or whatever.  Is there a best practice?

Thanks, Drew