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NAME    [Toc]    [Back]

       nfs, nfs_intro - Network File System (NFS) information

DESCRIPTION    [Toc]    [Back]

       NFS  is  a  facility  for sharing files in a heterogeneous
       environment of processors,  operating  systems,  and  networks.
   Sharing is accomplished by mounting a remote file
       system or directory on a local system and then reading  or
       writing the files as though they were local.

       Sharing file systems has the following advantages: Removes
       the need to copy files across the network from one  system
       to  another Provides easier access to remote files Reduces
       the risk of having out-of-date copies of the same file  on
       different systems

   The NFS Environment    [Toc]    [Back]
       NFS  is  based on the client-server model, in which client
       systems  request  resources  from  other  systems   called
       servers.  A server is any host system or process that provides
 a network service. A client is any  host  system  or
       process that uses services from a server.

       A  single  host, or server, can provide more than one service.
  Servers are passive; they do not call clients, they
       wait for clients to call them.

       A one-to-one  correspondence between servers, clients, and
       systems does not always exist.  A system that  acts  as  a
       server  can  also  act as a client.  A server that exports
       file systems and directories can also  mount  remote  file
       systems  and  directories  exported by other systems, thus
       becoming a client.  To export a file system  or  directory
       is  to make it available for NFS clients to mount remotely
       on their local systems.

       The /etc/exports file defines the  NFS  file  systems  and
       directories  that  can  be  exported.  The following table
       summarizes the distinctions between client and server systems
 for NFS mounts.
       Client                          Server
       Requests a remote mount         Responds to the mount request
       Reads the /etc/fstab file       Reads the /etc/exports file
       Checks if the server is known   Checks if the client is known

       The  client always initiates the remote mount.  The server
       completes the bindings, subject  to  access-control  rules
       specific  to  NFS.   Because  most  network administration
       problems occur at bind time, you should know how a  client
       binds  to  a  server  and  what access control policy each
       server uses.  For more information on access control,  see
       the  NFS  Service  section and the Network Administration:
       Services manual.

       You mount a remote file system by using either  the  mount
       command or an automatic mounting daemon, such as Automount
       or AutoFS.  In addition, you can use the WebNFS  protocol,
       which bypasses mount and gives direct access to the WebNFS
       served file system.

       An NFS client selects a  specific  server  from  which  to
       mount a file system or directory.  A client can mount file
       systems and directories from several different servers.

   Network Transport Protocols    [Toc]    [Back]
       NFS can use either the Transmission Control Protocol (TCP)
       or  User Datagram Protocol (UDP) protocol.  The TCP protocol
 is best for congested wide area  networks  because  of
       its  superior  retransmit features and congestion control.
       The UDP protocol is best in networks where congestion  and
       lost messages are not a problem because its lower overhead
       might yield higher throughput.   The  client  selects  the
       transport protocol in the mount command.

   The NFS Service    [Toc]    [Back]
       Once  a remote file system or directory is mounted, it can
       be used as a local file system by client programs.   Typically,
 a client mounts one or more remote file systems and
       directories at start up time, if lines similar to the following
  are  in  the  /etc/fstab  file.  The mount program
       reads these lines when the system starts  up:  titan:/usr2
       /usr2  nfs rw,bg 0 0 venus:/usr/man /usr/man nfs rw,bg 0 0
       The first line in the example shows that the /usr2  directory
 at system titan is mounted at local mount point /usr2
       when you boot the local system.  The  rest  of  this  line
       describes  the  mount.   See fstab(4) for a description of
       the file format.

       NFS servers control access to their resources by  limiting
       named  file  systems  and directories to a specific set of
       clients with an  entry  in  the  /etc/exports  file.   The
       /etc/exports  file  allows  you  to  limit  access  to NFS
       clients but not to individual remote users.  By default in
       Tru64  UNIX,  mounts  can  be limited to client superusers

       The following four programs  implement  the  NFS  service:
       portmap mountd nfsiod nfsd

       A  client's  mount  request  is  transmitted to the remote
       server's mountd daemon after obtaining  its  address  from
       portmap.   A  port mapper is a Remote Procedure Call (RPC)
       daemon that maps RPC program numbers of  network  services
       to their UDP or TCP protocol port numbers.

       The  mountd  daemon  checks  the  access permission of the
       client and returns a pointer to the file system or  directory.
   After  the mount is completed, access to files and
       directories at and below that mount point go  through  the
       pointer  to  the  server's  NFS daemon (nfsd) using remote
       procedure calls.  Some file access requests  (write-behind
       and  read-ahead)  are  handled  by  the  block  I/O daemon
       (nfsiod) on the client.

   PC-NFS    [Toc]    [Back]
       The PC-NFS facility provides the benefits of NFS  to  personal
  computers on your network.  PC-NFS enables personal
       computers to share resources and exchange files, and  like
       NFS,  is  based  on  the  client-server model.  The client
       software runs on the personal computer;  the server  software
 resides in the /usr/sbin directory.

       For  information  on  the  PC-NFS  client, see your PC-NFS
       client documentation.

       For instructions on making the PC-NFS server available  to
       clients, refer to Network Administration: Services.

   The NFS Locking Service    [Toc]    [Back]
       The  NFS  Locking Service (rpc.lockd) allows you to create
       advisory locks on files and  file  regions  on  local  and
       remotely  mounted  file  systems.   Advisory locks are not

       To make use of the NFS Locking Service, a programmer needs
       to  understand  how  to  use  the fcntl system call or the
       lockf subroutine.  See fcntl(2) and lockf(3) for  programming

       File locking is a way to manage access to shared files.  A
       process takes the following steps when locking a  file  or
       region  of a file: Determines if the file or region within
       the file is locked Applies a lock if the file or region is
       not locked Makes the changes to the file or region Unlocks
       the file or region

       The NFS Locking Service coordinates the dispersal of locks
       to  remote  file systems.  It communicates with the kernel
       and status monitor (rpc.statd) of  the  local  system,  as
       well as with the other lock daemons on the network.

       The NFS Locking Service uses a stateless approach to failure
 recovery.  The fundamental element of this approach is
       that  the  status  monitor  detects both client and server
       recoveries.  This approach is passive.   When  the  client
       status monitor detects that a failed server has reinitialized
 (recovered), it notifies the local locking daemon  of
       the failure.  The lock daemon then activates the appropriate
 recovery mechanism.

       If the NFS server fails and the  NFS  Locking  Service  is
       enabled,  all the locks managed by the server's local processes
 are lost.  However, when the server  recovers,  the
       lock  manager  daemons  on the client systems send reclaim
       requests for the  NFS  locks.   The  server  lock  manager
       reestablishes  the  previously  acquired  locks associated
       with the  reclaim  requests,  provided  the  requests  are
       received  within the grace period built into the NFS Locking

       During the grace period, the server  lock  manager  honors
       only  reclaim  requests.   Once  the grace period expires,
       reclaim requests are no longer valid, and the server  lock
       manager  accepts  only  new  requests.   At this time, the
       server lock manager cannot reestablish old locks.  A  user
       process  whose  lock  could  not  be  reclaimed  is sent a
       SIGLOST signal.  The client lock manager  can  create  new
       locks  by using the interface primitives in the fcntl system
 call or the lockf subroutine.

       If a client fails while it is using the NFS  Locking  Service,
  then  when  the client recovers, the status monitor
       daemon notifies the appropriate servers.  The server  lock
       manager  then releases the locks.  The client applications
       can then issue new lock requests as part of their recovery

   Automatic Mounting Daemons    [Toc]    [Back]
       NFS  clients can optionally run an automatic mounting daemon,
 such as  Automount  or  AutoFS.   These  daemons  are
       client    interfaces    that    perform    remote   mounts
       automatically, and only when they are needed.  If you need
       to  access  a  remotely  mounted  file  or  directory, the
       selected daemon mounts it, keeping it mounted as  long  as
       you need it.

       The  automount  and  autofsd daemons are told which remote
       file systems to mount from a database file called  a  map.
       A map lists the remote file systems that the selected daemon
 monitors, their locations, and any mount options.  The
       maps can be shared through NIS.

       There  are  three  types  of  maps:  Master - Although not
       required, the master map helps to organize mounts.   If  a
       master  map exists, it is read first when the automount or
       autofsmount commands are invoked, acting as a  pointer  to
       other maps.

              Both  commands look for an NIS map called auto.master
 when invoked.  They can also be  instructed  to
              use  a  local master map.  Direct - Specifies a key
              that is the pathname of  the  mount  point,  and  a
              location,  which is the location of the file system
              or directory in which it resides, specified in this
              format:  server:pathname.  For direct maps, a local
              mount point is specified as an  absolute  pathname,
              such  as  /doclib.   Indirect  - Like a direct map,
              specifies the pathname of the mount point  and  the
              location  of  the  file system on which it resides.
              For indirect maps, a local mount point is specified
              as  a  simple  pathname, such as docsrc because the
              whole map is associated with a directory.

       For more information these daemons, see automount(8), autofsd(8),  and  autofsmount(8).   Or,  refer to the Network
       Administration: Services manual.


       Commands: autofsmount(8), automount(8), nfsstat(8)

       Daemons:  autofsd(8),  automount(8),  mountd(8),  nfsd(8),
       nfsiod(8), rpc.lockd(8), rpc.statd(8)

       Files: advfs(4), cdfs(4), fstab(4)

       Technical Overview, Network Administration: Services delim

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