What is NFS in Linux..?

A Network File System (NFS) allows remote hosts to mount file systems over a network and interact with those file systems as though they are mounted locally. This enables system administrators to consolidate resources onto centralized servers on the network.

This chapter focuses on fundamental NFS concepts and supplemental information.

9.1. How It Works

Currently, there are three versions of NFS. NFS version 2 (NFSv2) is older and widely supported. NFS version 3 (NFSv3) supports safe asynchronous writes and is more robust at error handling than NFSv2; it also supports 64-bit file sizes and offsets, allowing clients to access more than 2Gb of file data.

NFS version 4 (NFSv4) works through firewalls and on the Internet, no longer requires an rpcbind service, supports ACLs, and utilizes stateful operations. Red Hat Enterprise Linux 6 supports NFSv2, NFSv3, and NFSv4 clients. When mounting a file system via NFS, Red Hat Enterprise Linux uses NFSv4 by default, if the server supports it.

All versions of NFS can use Transmission Control Protocol (TCP) running over an IP network, with NFSv4 requiring it. NFSv2 and NFSv3 can use the User Datagram Protocol (UDP) running over an IP network to provide a stateless network connection between the client and server.

When using NFSv2 or NFSv3 with UDP, the stateless UDP connection (under normal conditions) has less protocol overhead than TCP. This can translate into better performance on very clean, non-congested networks. However, because UDP is stateless, if the server goes down unexpectedly, UDP clients continue to saturate the network with requests for the server. In addition, when a frame is lost with UDP, the entire RPC request must be retransmitted; with TCP, only the lost frame needs to be resent. For these reasons, TCP is the preferred protocol when connecting to an NFS server.

The mounting and locking protocols have been incorporated into the NFSv4 protocol. The server also listens on the well-known TCP port 2049. As such, NFSv4 does not need to interact with rpcbind ^[3], lockd, and rpc.statd daemons. The rpc.mountd daemon is still required on the NFS server to set up the exports, but is not involved in any over-the-wire operations.

Note

TCP is the default transport protocol for NFS version 2 and 3 under Red Hat Enterprise Linux. UDP can be used for compatibility purposes as needed, but is not recommended for wide usage. NFSv4 requires TCP.

All the RPC/NFS daemons have a '-p' command line option that can set the port, making firewall configuration easier.

After TCP wrappers grant access to the client, the NFS server refers to the /etc/exports configuration file to determine whether the client is allowed to access any exported file systems. Once verified, all file and directory operations are available to the user.

Important

In order for NFS to work with a default installation of Red Hat Enterprise Linux with a firewall enabled, configure IPTables with the default TCP port 2049. Without proper IPTables configuration, NFS will not function properly.

The NFS initialization script and rpc.nfsd process now allow binding to any specified port during system start up. However, this can be error-prone if the port is unavailable, or if it conflicts with another daemon.

9.1.1. Required Services

Red Hat Enterprise Linux uses a combination of kernel-level support and daemon processes to provide NFS file sharing. All NFS versions rely on Remote Procedure Calls (RPC) between clients and servers. RPC services under Red Hat Enterprise Linux 6 are controlled by the rpcbind service. To share or mount NFS file systems, the following services work together depending on which version of NFS is implemented:

nfs

service nfs start starts the NFS server and the appropriate RPC processes to service requests for shared NFS file systems.

nfslock

service nfslock start activates a mandatory service that starts the appropriate RPC processes allowing NFS clients to lock files on the server.

rpcbind

rpcbind accepts port reservations from local RPC services. These ports are then made available (or advertised) so the corresponding remote RPC services can access them. rpcbind responds to requests for RPC services and sets up connections to the requested RPC service. This is not used with NFSv4.

The following RPC processes facilitate NFS services:

rpc.mountd

This process is used by an NFS server to process MOUNT requests from NFSv2 and NFSv3 clients. It checks that the requested NFS share is currently exported by the NFS server, and that the client is allowed to access it. If the mount request is allowed, the rpc.mountd server replies with a Success status and provides the File-Handle for this NFS share back to the NFS client.

rpc.nfsd

rpc.nfsd allows explicit NFS versions and protocols the server advertises to be defined. It works with the Linux kernel to meet the dynamic demands of NFS clients, such as providing server threads each time an NFS client connects. This process corresponds to the nfs service.

lockd

lockd is a kernel thread which runs on both clients and servers. It implements the Network Lock Manager (NLM) protocol, which allows NFSv2 and NFSv3 clients to lock files on the server. It is started automatically whenever the NFS server is run and whenever an NFS file system is mounted.

rpc.statd

This process implements the Network Status Monitor (NSM) RPC protocol, which notifies NFS clients when an NFS server is restarted without being gracefully brought down. rpc.statd is started automatically by the nfslock service, and does not require user configuration. This is not used with NFSv4.

rpc.rquotad

This process provides user quota information for remote users. rpc.rquotad is started automatically by the nfs service and does not require user configuration.

rpc.idmapd

rpc.idmapd provides NFSv4 client and server upcalls, which map between on-the-wire NFSv4 names (which are strings in the form of user@domain) and local UIDs and GIDs. For idmapd to function with NFSv4, the /etc/idmapd.conf file must be configured. This service is required for use with NFSv4, although not when all hosts share the same DNS domain name.

9.2. pNFS

Support for Parallel NFS (pNFS) as part of the NFS v4.1 standard is available as of Red Hat Enterprise Linux 6.4. The pNFS architecture improves the scalability of NFS, with possible improvements to performance. That is, when a server implements pNFS as well, a client is able to access data through multiple servers concurrently. It supports three storage protocols or layouts: files, objects, and blocks.

To enable this functionality, use one of the following mount options on mounts from a pNFS-enabled server:

-o minorversion=1

or

-o v4.1

After the server is pNFS-enabled, the nfs_layout_nfsv41_files kernel is automatically loaded on the first mount. Use the following command to verify the module was loaded:

$ lsmod | grep nfs_layout_nfsv41_files

Another way to verify a successful NFSv4.1 mount is with the mount command. The mount entry in the output should contain minorversion=1.

Important

The protocol allows for three possible pNFS layout types: files, objects, and blocks. However the Red Hat Enterprise Linux 6.4 client only supports the files layout type, so will use pNFS only when the server also supports the files layout type.

For more information on pNFS.