Garth Goodson et al. have authored an introduction to standardizing storage clusters using the pNFS (parallel NFS) protocol as part of the NFSv4.1 specification to bridge the gap between current NFS protocols (versions 2, 3, and 4) and parallel cluster file system interfaces. While current NFS protocols force clients to access all files on a given file-system volume from a single server node (which can become a bottleneck for scalable performance), pNFS as a standardized extension to NFSv4 provides clients with scalable end-to-end performance and the flexibility to interoperate with a variety of clustered storage service architectures.

The authors explain that the pNFS protocol enables clients to directly access file data spread over multiple storage servers in parallel. As a result, each client can leverage the full aggregate bandwidth of a clustered storage service at the granularity of an individual file. A standard protocol also improves manageability of storage client software and allows for interoperability across heterogeneous storage nodes. Finally, the pNFS protocol is backward-compatible with the base NFSv4 protocol, which allows interoperability between old and new clients and servers.

Using the pNFS protocol, clients gather metadata, called layouts, about how files are distributed across data servers. These layouts are maintained internally by the pNFS server. Once the client understands the file’s layout, it is able to directly access the data servers in parallel. Unlike NFSv4, whereby a client accesses data via the NFS protocol from a single NFS server, a pNFS client communicates with the data servers using a variety of storage access protocols, including NFSv4 and iSCSI/Fibre Channel using the SCSI block command set or the new SCSI object command set. The pNFS specification allows for the addition of new layout distributions and storage access protocols while also providing significant flexibility in the implementation of the back-end storage system.

The authors review several aspects of pNFS standardization, including:

• Design principles and general architecture (familiar semantics; simplicity; flexibility)
• Layout types
• Protocol operations
• File-sharing semantics
• Access control and security
• Caching layouts
• System architecture considerations
• Client/server implementation

The authors conclude that the pNFS protocol introduces a major shift to the NFS standard. It provides a standard interface for clients to leverage the aggregate I/O capacity of these systems more effectively by enabling them to issue I/Os in parallel to multiple servers. Although the separation of metadata and data increases client complexity, it also provides flexibility to storage architecture implementations. Although parallel file systems fill a niche market today, pNFS has the opportunity to standardize this interface and potentially commoditize parallel file systems.

More Information

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