33.1 NFS Concepts | HP Certified: HP-UX System Administration

When an NFS server makes one or more of its files and directories shareable, this process is called exporting. The exported files or directories are placed in the /etc/exports file. Only the exported files and directories can be accessed by an NFS client. The rules that govern communication between a client and server are as follows .

A system can act as an NFS client and server at the same time.
A server can export a file, a directory, or a complete file system.
A server can export only local file systems. File systems mounted from other servers can't be re-exported.
If a directory is exported, all of the directory tree under this directory are automatically exported.
A client can mount the exported directories or a subdirectory in the directory tree under the exported directory.
User and group IDs must match on the client and server machines for proper permissions.

Remote Mount Process

The process of mounting a remote file system is complete when the following steps are completed on the NFS server and client.

The NFS server processes are started on the server machine. This is done by running the /sbin/init.d/nfs. core and /sbin/init.d/nfs.server scripts at boot time.
The shared directories are located in the /etc/exports file and are exported using the exportfs command. This command is executed at boot time by the nfs.server script. However, if you make any change to the /etc/exports file, you must use this command to re-export shared directories.
NFS client processes are running on the client machine. These are started with the help of the /sbin/init.d/nfs.core and /sbin/init.d/nfs.client scripts. Both of these scripts are invoked at run level 2.
The remote file system names and mount points are present in the /etc/fstab file.

Remote file systems can also be mounted at any time using the mount command. However, to have a remote file system mount automatically each time you reboot the system, it must be present in the /etc/fstab file.

NFS and RPC

Remote Procedure Calls (RPCs) allow procedures on the server machine to be called by programs on a client machine. All NFS communication between client and server takes place through RPCs. Since RPC (and NFS) is supported on a number of operating systems and platforms, a common data format must be used between client and server for passing data to RPCs. RPCs are made through program numbers. These program numbers are listed in the /etc/rpc file, which is displayed here.

 ## # pragma VERSIONID "@(#)rpc:    11R2-4" #       file of rpc program name to number mappings ## rpcbind         100000  portmap sunrpc rpcbind rstatd          100001  rstat rup perfmeter rusersd         100002  rusers nfs             100003  nfsprog ypserv          100004  ypprog mountd          100005  mount showmount ypbind          100007 walld           100008  rwall shutdown yppasswdd       100009  yppasswd etherstatd      100010  etherstat rquotad         100011  rquotaprog quota rquota sprayd          100012  spray selection_svc   100015  selnsvc # pcnfsd          150001  pcnfs # # NEW SERVICES ADDED AT 6.5 # rexd            100017  rex llockmgr        100020 nlockmgr        100021 status          100024 # # SUN SUPPORTS THE FOLLOWING THAT HP DOES NOT @ release 6.5 # 3270_mapper     100013 rje_mapper      100014 database_svc    100016 alis            100018 sched           100019 x25.inr         100022 statmon         100023 bootparam       100026 ypupdated       100028  ypupdate keyserv         100029  keyserver tfsd            100037 nsed            100038 nsemntd         100039 ypxfrd          100069 nisd            100300  rpc.nisd nispasswd       100303  rpc.nispasswdd nis_cachemgr    100301 nisd_resolv     100302  rpc.nisd_resolv automountd      100099 ttdbserver      100083 cmsd            100068  dtcalendar  #

Each line in this file starts with the RPC server name followed by a program number. Aliases can be listed after a program number. A line that starts with the # character is a comment.

The portmap and rpcbind Daemons

Contact between any RPC client and server starts with the help of the portmap or rpcbind daemon. Up to version 10.20, the portmap daemon was used, which was replaced by rpcbind in the later versions. The process of establishing a connection between client and server is carried out through the following steps. When an RPC server starts, it registers its program number with portmap and the TCP or UDP ports to which it is listening.

The client contacts portmap using port number 111 on the server machine requesting the port number used by the RPC server. The client sends the RPC program number of the server program as defined in the /etc/rpc file.
The portmap returns the port number used by a server using that program number.
The client connects to the server at the port number returned by rpcbind .

The rpcbind daemon facilitates dynamic binding of program numbers to ports.

NFS Versions

Starting with HP-UX version 10.30, NFS version 3 has been used by default. Advantages of version 3 over NFS version 2 are:

NFS version 3 supports 64-bit file offset, whereas version 2 is limited to 32 bits. A file offset of 32 bits can address a maximum file size of 2 GBytes. NFS version 3 supports a maximum file size of 128 GBytes.
Version 3 supports asynchronous I/O using kernel buffer cache management that provides data safety.
File handle size up to 64 bits can be used.
Cache management is improved.

A detailed discussion of the features supported by different NFS versions is beyond the scope of this book.

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