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 an NFS shared SAN storage not enough anymore, and you need some kind of parallel NAS?
EDIT: Added "NFS shared" before "SAN".
First off, there is no "average" in HPC that I'm aware of - I've had the pleasure of working in three HPC environments, and all 3 had wildly different dynamics (in classical bottleneck-modeling terms one was disk bound, one was CPU bound, and one was memory-constrained).
This leads directly to my answer to your question: You need "some kind of parallel NAS" when you are disk bound to the point that it impacts performance. You will know this is the case when you can no longer keep all of your CPUs saturated (and you still have free RAM, or some nodes are completely idle waiting on the disk).
If your current storage system has performance monitoring features you can watch your statistics and plan to take action when you get to say 66% or 75% of the maximum performance values, but there is no hard and fast rule that says "X nodes, Y tasks - now you need to deploy GPFS."
You are comparing apples and oranges. A SAN provides logical volumes - not filesystems. If you've got more than one system trying to mount the same logical volume as writeable (i.e. even with access time enabled) then you need a filesystem which supports concurrent access.
Typically the term NAS is applied to a system which provides file sharing services such as NFS or SMB which makes using a cluster filesystem redundant (only the controller accesses the filesystem directly). However the term is sometimes applied to the iSCSI protocol - in which case the device is behaving like a SAN.