At what workload is a parallel (network) file system required?

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http://serverfault.com – Any hard numbers, or rules-of-thumgs, for what workload (e.g. in terms of simultaneously active users running average HPC center jobs) on a computer cluster, that would make a parallel network shared file system such as Lustre, GPFS, Ibrix or Panasas, a requirement? ...that is, when is SAN storage not enough anymore, and you need some kind of parallel NAS? (HowTos)