Accessing Data with NFSv4

MapR lets you mount the cluster via NFS so that your applications can read and write data directly. MapR allows direct file modification and multiple concurrent reads and writes via POSIX semantics. With an NFS-mounted cluster, you can read and write data directly with standard tools, applications, and scripts. For example, you could run a MapReduce application that outputs to a CSV file, then import the CSV file directly into SQL via NFS.

MapR uses NFS Ganesha for supporting NFSv4 features. NFS Ganesha is an Open Source userspace implementation of the NFS server. The MapR NFSv4, running as a userspace process, registers callbacks with NFS Ganesha through the File System Abstraction Layer (FSAL), which is a shared library ( NFS Ganesha loads and uses this library whenever MapR Filesystem is exported/mounted. The FSAL, in turn, uses FileClient ( library) to connect to the cluster.

The following diagram illustrates how the MapR processes read and write operations to the MapR cluster using NFSv4. When the user enters a command (such as ls), the NFS client submits the request over TCP to the MapR NFSv4 server. The NFS server uses MapR FileClient to perform the requested operation on the cluster and returns the response to the NFS client over TCP.

MapR exports each cluster as the directory /mapr/<cluster name> (for example, /mapr/ If you create a mount point with the local path /mapr, then Hadoop FS paths and NFS paths to the cluster will be the same. This makes it easy to work on the same files via NFS and Hadoop. In a multi-cluster setting, the clusters share a single namespace, and you can see them all by mounting the top-level /mapr directory.

For NFSv4, MapR also requires alias or pseudo-path, which when specified masks the mount path from the NFS client. MapR’s NFSv4 server provides a pseudo-file system where only the exported volumes are visible. This is especially useful in scenarios where one or more volumes in the hierarchy should be hidden and not be visible. For more information, see NFSv4 RFC.

Unsupported NFSv4 Features

MapR does not currently support the following NFSv4.0 features:

  • pNFS
  • Delegations
  • Mandatory locking
  • Lock upgrades and downgrades
  • Deny share
  • Access control lists (ACL)
  • Namespaces
  • Persistent reply cache
  • Data retention