Short for parallel Network File System, pNFS is a protocol extension of NFS 4.1, the first major performance upgrade to the widely deployed NFS in more than a decade. pNFS provides parallel I/O to file systems accessible over NFS to eliminate performance bottlenecks. With pNFS clients can directly access storage devices in parallel. metadata processing. pNFS moves the metadata server out of the data transfer path, boosting performance by allowing multiple disk drives to serve up data in parallel. pNFS let's storage administrators do things, such as stripe a single file across multiple NFS servers (essentially the same as RAID0), which boosts performance by allowing multiple disk drives to serve up data in parallel. pNFS takes this concept and extends it to multiple storage devices connected to the NFS client over a network.
See also "NFS Enters a Parallel Universe" on EnterpriseStorageForum.com