[Beowulf] NFS shared file system

[Ted Sariyski]

Hi,
We had three 30 nodes discless clusters (CM5) used for CFD calculations.
Each cluster had its own head node and its own cheap, in-house build
RAID exported over GB NFS. Recently we combined the existing clusters
into one and the first problem we have is with the mass storage,
occasionally it cannot handle the IO load. My question is if I buy a
commercial NAS what are the chances that after that I'll need to replace
GB with Mirinet (e.g.)? 

I do have the option to break the cluster back into three smaller
clusters but from what I read in this newsgroups my understanding is
that 90 nodes is a small cluster and I didn't expect scalability
problems at this level. If a commercial storage optimized for IO is a
solution what is the price I'm facing? Any recomendations?
 
I'll highly appreciate any comments. 
Thanks in advance, Ted