Exanet - single FS per cluster
As an Exanet customer I can say their systems have 1 file system per cluster. You can have multiple volumes in that file system but there's no requirement for such. The downside is there is only 1 file system per cluster, no storage tiering - yet.
As the article implies most of the performance comes from the spindles, most NAS systems are "fast enough".
Exanet had another customer that did informal testing last year showing 3PAR back end storage providing double the per-spindle (NFS) IOPS as their own tier 2 storage. As a customer of both companies I've been hounding them off and on to do a joint SFS benchmark to show what the combined system can do(since they pimp each other's gear quite heavily around the country). I'm confident that Exanet can achieve the same(or better) high end IOPS numbers with roughly 300 drives on a 3PAR vs 500+ on the tier 2 stuff. Time will tell though. The better architecture, caching, and controllers on the 3PAR just destroys that tier 2 crap. (Exanet will tell you this themselves as well).
I certainly like the flexibility of being able to share the same spindles on the NAS with Exanet as I do on the SAN with 3PAR, having volumes striped across every spindle instead of having volumes composed of physical spindles and having to dedicate them to a particular server/cluster. Which is one(among tons) of reasons why I liked Exanet(hardware agnostic for the most part, fast too) and 3PAR(fastest block storage in the west, and east, and north, and south).
Too many folks take back end storage for granted it seems, most people think it's all about the head unit. The back end is at least as important if not more so than the front end. BlueArc finally admitted that to us last year after months of talks with them(used to be a BlueArc customer), they said that their tier 2 stuff(same gear many companies re-sell or OEM) just isn't suitable for performance intensive and really high availability. They go as far as completely disabling the write cache on their tier 2 storage systems because it's _that_ inefficient.