A reference architecture says parallel file system, so the shortlist is parallel file systems. Nobody checks whether the requirement justifies it, and the operational cost arrives later.
What each is genuinely for
Scale-out NAS presents NFS or SMB from a cluster of nodes. Clients are ordinary clients with no special software. Each client talks to one node at a time, so single-client bandwidth is bounded by what one node can serve, while aggregate bandwidth across many clients scales with the cluster.
Parallel file system requires a specific client that stripes each file across many servers. A single client can pull from the whole cluster at once.
That difference — single-client bandwidth — is the actual decision criterion, and it is rarely the one discussed.
The test
Does any single client need more bandwidth than one storage node can deliver, against data that other clients also need?
If yes, you need a parallel file system and nothing else will do.
If no, you probably want NAS, and choosing otherwise means acquiring a kernel client dependency, an upgrade coordination problem, and a specialist skill requirement in exchange for capability you will not use.
Running the numbers on a typical enterprise AI cluster
A dozen accelerated nodes doing fine-tuning and inference on datasets in the tens of terabytes. Per-node ingest requirement in the low single-digit gigabytes per second. A modern NAS node serves that comfortably, and a cluster of them serves twelve of them.
This configuration does not need a parallel file system. It gets one because the reference architecture was written for a training cluster an order of magnitude larger.
Now a training cluster with a hundred nodes, each needing many gigabytes per second against a shared dataset, with a multi-terabyte synchronised checkpoint every half hour. That needs one, clearly, and the operational cost is justified because there is no alternative.
The middle ground worth knowing
Several modern systems blur this. They present NFS but implement a multi-path client-side driver that gets parallel-like behaviour from standard-looking mounts. Others offer both protocols over the same back end.
If your requirement sits near the boundary, these are worth serious evaluation, because they let you start with standard clients and add the specialised path only where measurement says you need it.
The costs nobody puts in the comparison
Parallel file systems bring: client software lifecycle tied to your compute nodes, coordinated upgrades, per-target monitoring you must build, a skill requirement, and a support relationship you will use.
NAS brings: a protocol every operating system speaks, snapshots and replication that work the way your existing team expects, backup integration that already exists, and a ceiling on single-client bandwidth.
Put both lists in the comparison document. A total cost model that counts hardware and licences but not the operating model is not a comparison, and this is the category where that omission is most expensive.
The version I would like to see more often
“We evaluated a parallel file system and concluded our per-client bandwidth requirement does not justify it. We are deploying scale-out NAS and will revisit if single-job bandwidth becomes the constraint.”
That is a defensible engineering position. It is also, in my experience, very rarely written down, because it reads as choosing the less impressive thing.
Choose the less impressive thing when the requirement says so. You will be running it for five years.