docs(pnfs): distributed file storage concept, pNFS performance, deployment architectures, prerequisites

[schmidt-scaled]

Adds the architecture view of pNFS (shared ReadWriteMany file storage) on top of the pNFS docs from #238.

Changes

• Concepts → Distributed File Storage (architecture/concepts/distributed-file-storage.md, new): how pNFS separates metadata (metadata server) from data (direct NVMe/TCP per client), with two new diagrams: the data path and the open → LAYOUTGET → direct I/O → LAYOUTCOMMIT sequence. Also what a shared volume inherits from a logical volume. The existing detailed page concepts/pnfs.md stays and now links to it.
• Architecture → Performance and QoS: new section "Shared File Storage at Block Performance (pNFS)": no file server in the data path, throughput scales with clients, the same SPDK latency path, QoS unchanged, and the limits (metadata-bound workloads, fallback without the client driver).
• Deployment Architectures: new section "Shared File Storage (pNFS)": the metadata server per storage cluster in each architecture and topology, DR not covering pNFS volumes yet, placement and network notes, plus a decision-table row.
• Software Requirements: new "pNFS Requirements":
  • the KVM node;
  • privileged pods in the operator namespace;
  • the client kernel (CONFIG_PNFS_BLOCK with NVMe support and nvme_get_unique_id; RHEL 9.8 validated, 9.5 lacks it);
  • no host NFS packages;
  • TCP 2049 to a Service ClusterIP;
  • NVMe/TCP;
  • spec.pnfs.mds, the StorageClass and the state disk.

Sources

• operator operator/docs/designs/design-pnfs-rwx.md (P0-8 ClusterIP, P0-9 nvme_get_unique_id, P0-10 CONFIG_PNFS_BLOCK) and design-pnfs-mds-vm.md (KVM, guest kernel);
• csi-driver/internal/mds/netsetup (TCP 2049, /dev/net/tun), csi-driver/internal/csi/node/kvm_capability.go (label), operator/api/v1alpha2/simplyblockdriver_types.go;
• the existing docs from Initial pNFS / ReadWriteMany documentation #238.

Upcoming, not documented as available

• striped pNFS volumes (design-pnfs-striped.md, Draft);
• creating a pNFS volume from a snapshot or volume;
• DR protection of pNFS volumes;
• the extended pNFS features in spdk#77 (open).

Quality gate

• No errors in the changed pages. The only remaining error is reference/api/index.md (openapi.json), which is already on main.
• Remaining warnings in the changed pages: list items that start upper case, and pre-existing heading-level and description warnings.

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[github-actions]

🤖: Deployment available as https://docstest.simplyblock.io/docs-pnfs-architecture