nfs

NFS(5)                        File Formats Manual                       NFS(5)



NAME
       nfs - fstab format and options for the nfs file systems

SYNOPSIS
       /etc/fstab

DESCRIPTION
       NFS is an Internet Standard protocol created by Sun Microsystems in
       1984. NFS was developed to allow file sharing between systems residing
       on a local area network.  Depending on kernel configuration, the Linux
       NFS client may support NFS versions 2, 3, 4.0, 4.1, or 4.2.

       The mount(8) command attaches a file system to the system's name space
       hierarchy at a given mount point.  The /etc/fstab file describes how
       mount(8) should assemble a system's file name hierarchy from various
       independent file systems (including file systems exported by NFS
       servers).  Each line in the /etc/fstab file describes a single file
       system, its mount point, and a set of default mount options for that
       mount point.

       For NFS file system mounts, a line in the /etc/fstab file specifies the
       server name, the path name of the exported server directory to mount,
       the local directory that is the mount point, the type of file system
       that is being mounted, and a list of mount options that control the way
       the filesystem is mounted and how the NFS client behaves when accessing
       files on this mount point.  The fifth and sixth fields on each line are
       not used by NFS, thus conventionally each contain the digit zero. For
       example:

               server:path   /mountpoint   fstype   option,option,...   0 0

       The server's hostname and export pathname are separated by a colon,
       while the mount options are separated by commas. The remaining fields
       are separated by blanks or tabs.

       The server's hostname can be an unqualified hostname, a fully qualified
       domain name, a dotted quad IPv4 address, or an IPv6 address enclosed in
       square brackets.  Link-local and site-local IPv6 addresses must be
       accompanied by an interface identifier.  See ipv6(7) for details on
       specifying raw IPv6 addresses.

       The fstype field contains "nfs".  Use of the "nfs4" fstype in
       /etc/fstab is deprecated.

MOUNT OPTIONS
       Refer to mount(8) for a description of generic mount options available
       for all file systems. If you do not need to specify any mount options,
       use the generic option defaults in /etc/fstab.

   Options supported by all versions
       These options are valid to use with any NFS version.

       nfsvers=n      The NFS protocol version number used to contact the
                      server's NFS service.  If the server does not support
                      the requested version, the mount request fails.  If this
                      option is not specified, the client tries version 4.2
                      first, then negotiates down until it finds a version
                      supported by the server.

       vers=n         This option is an alternative to the nfsvers option.  It
                      is included for compatibility with other operating
                      systems

       soft / hard    Determines the recovery behavior of the NFS client after
                      an NFS request times out.  If neither option is
                      specified (or if the hard option is specified), NFS
                      requests are retried indefinitely.  If the soft option
                      is specified, then the NFS client fails an NFS request
                      after retrans retransmissions have been sent, causing
                      the NFS client to return an error to the calling
                      application.

                      NB: A so-called "soft" timeout can cause silent data
                      corruption in certain cases. As such, use the soft
                      option only when client responsiveness is more important
                      than data integrity.  Using NFS over TCP or increasing
                      the value of the retrans option may mitigate some of the
                      risks of using the soft option.

       intr / nointr  This option is provided for backward compatibility.  It
                      is ignored after kernel 2.6.25.

       timeo=n        The time in deciseconds (tenths of a second) the NFS
                      client waits for a response before it retries an NFS
                      request.

                      For NFS over TCP the default timeo value is 600 (60
                      seconds).  The NFS client performs linear backoff: After
                      each retransmission the timeout is increased by timeo up
                      to the maximum of 600 seconds.

                      However, for NFS over UDP, the client uses an adaptive
                      algorithm to estimate an appropriate timeout value for
                      frequently used request types (such as READ and WRITE
                      requests), but uses the timeo setting for infrequently
                      used request types (such as FSINFO requests).  If the
                      timeo option is not specified, infrequently used request
                      types are retried after 1.1 seconds.  After each
                      retransmission, the NFS client doubles the timeout for
                      that request, up to a maximum timeout length of 60
                      seconds.

       retrans=n      The number of times the NFS client retries a request
                      before it attempts further recovery action. If the
                      retrans option is not specified, the NFS client tries
                      each UDP request three times and each TCP request twice.

                      The NFS client generates a "server not responding"
                      message after retrans retries, then attempts further
                      recovery (depending on whether the hard mount option is
                      in effect).

       rsize=n        The maximum number of bytes in each network READ request
                      that the NFS client can receive when reading data from a
                      file on an NFS server.  The actual data payload size of
                      each NFS READ request is equal to or smaller than the
                      rsize setting. The largest read payload supported by the
                      Linux NFS client is 1,048,576 bytes (one megabyte).

                      The rsize value is a positive integral multiple of 1024.
                      Specified rsize values lower than 1024 are replaced with
                      4096; values larger than 1048576 are replaced with
                      1048576. If a specified value is within the supported
                      range but not a multiple of 1024, it is rounded down to
                      the nearest multiple of 1024.

                      If an rsize value is not specified, or if the specified
                      rsize value is larger than the maximum that either
                      client or server can support, the client and server
                      negotiate the largest rsize value that they can both
                      support.

                      The rsize mount option as specified on the mount(8)
                      command line appears in the /etc/mtab file. However, the
                      effective rsize value negotiated by the client and
                      server is reported in the /proc/mounts file.

       wsize=n        The maximum number of bytes per network WRITE request
                      that the NFS client can send when writing data to a file
                      on an NFS server. The actual data payload size of each
                      NFS WRITE request is equal to or smaller than the wsize
                      setting. The largest write payload supported by the
                      Linux NFS client is 1,048,576 bytes (one megabyte).

                      Similar to rsize , the wsize value is a positive
                      integral multiple of 1024.  Specified wsize values lower
                      than 1024 are replaced with 4096; values larger than
                      1048576 are replaced with 1048576. If a specified value
                      is within the supported range but not a multiple of
                      1024, it is rounded down to the nearest multiple of
                      1024.

                      If a wsize value is not specified, or if the specified
                      wsize value is larger than the maximum that either
                      client or server can support, the client and server
                      negotiate the largest wsize value that they can both
                      support.

                      The wsize mount option as specified on the mount(8)
                      command line appears in the /etc/mtab file. However, the
                      effective wsize value negotiated by the client and
                      server is reported in the /proc/mounts file.

       ac / noac      Selects whether the client may cache file attributes. If
                      neither option is specified (or if ac is specified), the
                      client caches file attributes.

                      To improve performance, NFS clients cache file
                      attributes. Every few seconds, an NFS client checks the
                      server's version of each file's attributes for updates.
                      Changes that occur on the server in those small
                      intervals remain undetected until the client checks the
                      server again. The noac option prevents clients from
                      caching file attributes so that applications can more
                      quickly detect file changes on the server.

                      In addition to preventing the client from caching file
                      attributes, the noac option forces application writes to
                      become synchronous so that local changes to a file
                      become visible on the server immediately.  That way,
                      other clients can quickly detect recent writes when they
                      check the file's attributes.

                      Using the noac option provides greater cache coherence
                      among NFS clients accessing the same files, but it
                      extracts a significant performance penalty.  As such,
                      judicious use of file locking is encouraged instead.
                      The DATA AND METADATA COHERENCE section contains a
                      detailed discussion of these trade-offs.

       acregmin=n     The minimum time (in seconds) that the NFS client caches
                      attributes of a regular file before it requests fresh
                      attribute information from a server.  If this option is
                      not specified, the NFS client uses a 3-second minimum.
                      See the DATA AND METADATA COHERENCE section for a full
                      discussion of attribute caching.

       acregmax=n     The maximum time (in seconds) that the NFS client caches
                      attributes of a regular file before it requests fresh
                      attribute information from a server.  If this option is
                      not specified, the NFS client uses a 60-second maximum.
                      See the DATA AND METADATA COHERENCE section for a full
                      discussion of attribute caching.

       acdirmin=n     The minimum time (in seconds) that the NFS client caches
                      attributes of a directory before it requests fresh
                      attribute information from a server.  If this option is
                      not specified, the NFS client uses a 30-second minimum.
                      See the DATA AND METADATA COHERENCE section for a full
                      discussion of attribute caching.

       acdirmax=n     The maximum time (in seconds) that the NFS client caches
                      attributes of a directory before it requests fresh
                      attribute information from a server.  If this option is
                      not specified, the NFS client uses a 60-second maximum.
                      See the DATA AND METADATA COHERENCE section for a full
                      discussion of attribute caching.

       actimeo=n      Using actimeo sets all of acregmin, acregmax, acdirmin,
                      and acdirmax to the same value.  If this option is not
                      specified, the NFS client uses the defaults for each of
                      these options listed above.

       bg / fg        Determines how the mount(8) command behaves if an
                      attempt to mount an export fails.  The fg option causes
                      mount(8) to exit with an error status if any part of the
                      mount request times out or fails outright.  This is
                      called a "foreground" mount, and is the default behavior
                      if neither the fg nor bg mount option is specified.

                      If the bg option is specified, a timeout or failure
                      causes the mount(8) command to fork a child which
                      continues to attempt to mount the export.  The parent
                      immediately returns with a zero exit code.  This is
                      known as a "background" mount.

                      If the local mount point directory is missing, the
                      mount(8) command acts as if the mount request timed out.
                      This permits nested NFS mounts specified in /etc/fstab
                      to proceed in any order during system initialization,
                      even if some NFS servers are not yet available.
                      Alternatively these issues can be addressed using an
                      automounter (refer to automount(8) for details).

       rdirplus / nordirplus
                      Selects whether to use NFS v3 or v4 READDIRPLUS
                      requests.  If this option is not specified, the NFS
                      client uses READDIRPLUS requests on NFS v3 or v4 mounts
                      to read small directories.  Some applications perform
                      better if the client uses only READDIR requests for all
                      directories.

       retry=n        The number of minutes that the mount(8) command retries
                      an NFS mount operation in the foreground or background
                      before giving up.  If this option is not specified, the
                      default value for foreground mounts is 2 minutes, and
                      the default value for background mounts is 10000 minutes
                      (80 minutes shy of one week).  If a value of zero is
                      specified, the mount(8) command exits immediately after
                      the first failure.

                      Note that this only affects how many retries are made
                      and doesn't affect the delay caused by each retry.  For
                      UDP each retry takes the time determined by the timeo
                      and retrans options, which by default will be about 7
                      seconds.  For TCP the default is 3 minutes, but system
                      TCP connection timeouts will sometimes limit the timeout
                      of each retransmission to around 2 minutes.

       sec=flavors    A colon-separated list of one or more security flavors
                      to use for accessing files on the mounted export. If the
                      server does not support any of these flavors, the mount
                      operation fails.  If sec= is not specified, the client
                      attempts to find a security flavor that both the client
                      and the server supports.  Valid flavors are none, sys,
                      krb5, krb5i, and krb5p.  Refer to the SECURITY
                      CONSIDERATIONS section for details.

       sharecache / nosharecache
                      Determines how the client's data cache and attribute
                      cache are shared when mounting the same export more than
                      once concurrently.  Using the same cache reduces memory
                      requirements on the client and presents identical file
                      contents to applications when the same remote file is
                      accessed via different mount points.

                      If neither option is specified, or if the sharecache
                      option is specified, then a single cache is used for all
                      mount points that access the same export.  If the
                      nosharecache option is specified, then that mount point
                      gets a unique cache.  Note that when data and attribute
                      caches are shared, the mount options from the first
                      mount point take effect for subsequent concurrent mounts
                      of the same export.

                      As of kernel 2.6.18, the behavior specified by
                      nosharecache is legacy caching behavior. This is
                      considered a data risk since multiple cached copies of
                      the same file on the same client can become out of sync
                      following a local update of one of the copies.

       resvport / noresvport
                      Specifies whether the NFS client should use a privileged
                      source port when communicating with an NFS server for
                      this mount point.  If this option is not specified, or
                      the resvport option is specified, the NFS client uses a
                      privileged source port.  If the noresvport option is
                      specified, the NFS client uses a non-privileged source
                      port.  This option is supported in kernels 2.6.28 and
                      later.

                      Using non-privileged source ports helps increase the
                      maximum number of NFS mount points allowed on a client,
                      but NFS servers must be configured to allow clients to
                      connect via non-privileged source ports.

                      Refer to the SECURITY CONSIDERATIONS section for
                      important details.

       lookupcache=mode
                      Specifies how the kernel manages its cache of directory
                      entries for a given mount point.  mode can be one of
                      all, none, pos, or positive.  This option is supported
                      in kernels 2.6.28 and later.

                      The Linux NFS client caches the result of all NFS LOOKUP
                      requests.  If the requested directory entry exists on
                      the server, the result is referred to as positive.  If
                      the requested directory entry does not exist on the
                      server, the result is referred to as negative.

                      If this option is not specified, or if all is specified,
                      the client assumes both types of directory cache entries
                      are valid until their parent directory's cached
                      attributes expire.

                      If pos or positive is specified, the client assumes
                      positive entries are valid until their parent
                      directory's cached attributes expire, but always
                      revalidates negative entires before an application can
                      use them.

                      If none is specified, the client revalidates both types
                      of directory cache entries before an application can use
                      them.  This permits quick detection of files that were
                      created or removed by other clients, but can impact
                      application and server performance.

                      The DATA AND METADATA COHERENCE section contains a
                      detailed discussion of these trade-offs.

       fsc / nofsc    Enable/Disables the cache of (read-only) data pages to
                      the local disk using the FS-Cache facility. See
                      cachefilesd(8) and
                      <kernel_soruce>/Documentation/filesystems/caching for
                      detail on how to configure the FS-Cache facility.
                      Default value is nofsc.

   Options for NFS versions 2 and 3 only
       Use these options, along with the options in the above subsection, for
       NFS versions 2 and 3 only.

       proto=netid    The netid determines the transport that is used to
                      communicate with the NFS server.  Available options are
                      udp, udp6, tcp, tcp6, and rdma.  Those which end in 6
                      use IPv6 addresses and are only available if support for
                      TI-RPC is built in. Others use IPv4 addresses.

                      Each transport protocol uses different default retrans
                      and timeo settings.  Refer to the description of these
                      two mount options for details.

                      In addition to controlling how the NFS client transmits
                      requests to the server, this mount option also controls
                      how the mount(8) command communicates with the server's
                      rpcbind and mountd services.  Specifying a netid that
                      uses TCP forces all traffic from the mount(8) command
                      and the NFS client to use TCP.  Specifying a netid that
                      uses UDP forces all traffic types to use UDP.

                      Before using NFS over UDP, refer to the TRANSPORT
                      METHODS section.

                      If the proto mount option is not specified, the mount(8)
                      command discovers which protocols the server supports
                      and chooses an appropriate transport for each service.
                      Refer to the TRANSPORT METHODS section for more details.

       udp            The udp option is an alternative to specifying
                      proto=udp.  It is included for compatibility with other
                      operating systems.

                      Before using NFS over UDP, refer to the TRANSPORT
                      METHODS section.

       tcp            The tcp option is an alternative to specifying
                      proto=tcp.  It is included for compatibility with other
                      operating systems.

       rdma           The rdma option is an alternative to specifying
                      proto=rdma.

       port=n         The numeric value of the server's NFS service port.  If
                      the server's NFS service is not available on the
                      specified port, the mount request fails.

                      If this option is not specified, or if the specified
                      port value is 0, then the NFS client uses the NFS
                      service port number advertised by the server's rpcbind
                      service.  The mount request fails if the server's
                      rpcbind service is not available, the server's NFS
                      service is not registered with its rpcbind service, or
                      the server's NFS service is not available on the
                      advertised port.

       mountport=n    The numeric value of the server's mountd port.  If the
                      server's mountd service is not available on the
                      specified port, the mount request fails.

                      If this option is not specified, or if the specified
                      port value is 0, then the mount(8) command uses the
                      mountd service port number advertised by the server's
                      rpcbind service.  The mount request fails if the
                      server's rpcbind service is not available, the server's
                      mountd service is not registered with its rpcbind
                      service, or the server's mountd service is not available
                      on the advertised port.

                      This option can be used when mounting an NFS server
                      through a firewall that blocks the rpcbind protocol.

       mountproto=netid
                      The transport the NFS client uses to transmit requests
                      to the NFS server's mountd service when performing this
                      mount request, and when later unmounting this mount
                      point.

                      netid may be one of udp, and tcp which use IPv4 address
                      or, if TI-RPC is built into the mount.nfs command, udp6,
                      and tcp6 which use IPv6 addresses.

                      This option can be used when mounting an NFS server
                      through a firewall that blocks a particular transport.
                      When used in combination with the proto option,
                      different transports for mountd requests and NFS
                      requests can be specified.  If the server's mountd
                      service is not available via the specified transport,
                      the mount request fails.

                      Refer to the TRANSPORT METHODS section for more on how
                      the mountproto mount option interacts with the proto
                      mount option.

       mounthost=name The hostname of the host running mountd.  If this option
                      is not specified, the mount(8) command assumes that the
                      mountd service runs on the same host as the NFS service.

       mountvers=n    The RPC version number used to contact the server's
                      mountd.  If this option is not specified, the client
                      uses a version number appropriate to the requested NFS
                      version.  This option is useful when multiple NFS
                      services are running on the same remote server host.

       namlen=n       The maximum length of a pathname component on this
                      mount.  If this option is not specified, the maximum
                      length is negotiated with the server. In most cases,
                      this maximum length is 255 characters.

                      Some early versions of NFS did not support this
                      negotiation.  Using this option ensures that pathconf(3)
                      reports the proper maximum component length to
                      applications in such cases.

       lock / nolock  Selects whether to use the NLM sideband protocol to lock
                      files on the server.  If neither option is specified (or
                      if lock is specified), NLM locking is used for this
                      mount point.  When using the nolock option, applications
                      can lock files, but such locks provide exclusion only
                      against other applications running on the same client.
                      Remote applications are not affected by these locks.

                      NLM locking must be disabled with the nolock option when
                      using NFS to mount /var because /var contains files used
                      by the NLM implementation on Linux.  Using the nolock
                      option is also required when mounting exports on NFS
                      servers that do not support the NLM protocol.

       cto / nocto    Selects whether to use close-to-open cache coherence
                      semantics.  If neither option is specified (or if cto is
                      specified), the client uses close-to-open cache
                      coherence semantics. If the nocto option is specified,
                      the client uses a non-standard heuristic to determine
                      when files on the server have changed.

                      Using the nocto option may improve performance for read-
                      only mounts, but should be used only if the data on the
                      server changes only occasionally.  The DATA AND METADATA
                      COHERENCE section discusses the behavior of this option
                      in more detail.

       acl / noacl    Selects whether to use the NFSACL sideband protocol on
                      this mount point.  The NFSACL sideband protocol is a
                      proprietary protocol implemented in Solaris that manages
                      Access Control Lists. NFSACL was never made a standard
                      part of the NFS protocol specification.

                      If neither acl nor noacl option is specified, the NFS
                      client negotiates with the server to see if the NFSACL
                      protocol is supported, and uses it if the server
                      supports it.  Disabling the NFSACL sideband protocol may
                      be necessary if the negotiation causes problems on the
                      client or server.  Refer to the SECURITY CONSIDERATIONS
                      section for more details.

       local_lock=mechanism
                      Specifies whether to use local locking for any or both
                      of the flock and the POSIX locking mechanisms.
                      mechanism can be one of all, flock, posix, or none.
                      This option is supported in kernels 2.6.37 and later.

                      The Linux NFS client provides a way to make locks local.
                      This means, the applications can lock files, but such
                      locks provide exclusion only against other applications
                      running on the same client. Remote applications are not
                      affected by these locks.

                      If this option is not specified, or if none is
                      specified, the client assumes that the locks are not
                      local.

                      If all is specified, the client assumes that both flock
                      and POSIX locks are local.

                      If flock is specified, the client assumes that only
                      flock locks are local and uses NLM sideband protocol to
                      lock files when POSIX locks are used.

                      If posix is specified, the client assumes that POSIX
                      locks are local and uses NLM sideband protocol to lock
                      files when flock locks are used.

                      To support legacy flock behavior similar to that of NFS
                      clients < 2.6.12, use 'local_lock=flock'. This option is
                      required when exporting NFS mounts via Samba as Samba
                      maps Windows share mode locks as flock. Since NFS
                      clients > 2.6.12 implement flock by emulating POSIX
                      locks, this will result in conflicting locks.

                      NOTE: When used together, the 'local_lock' mount option
                      will be overridden by 'nolock'/'lock' mount option.

   Options for NFS version 4 only
       Use these options, along with the options in the first subsection
       above, for NFS version 4.0 and newer.

       proto=netid    The netid determines the transport that is used to
                      communicate with the NFS server.  Supported options are
                      tcp, tcp6, and rdma.  tcp6 use IPv6 addresses and is
                      only available if support for TI-RPC is built in. Both
                      others use IPv4 addresses.

                      All NFS version 4 servers are required to support TCP,
                      so if this mount option is not specified, the NFS
                      version 4 client uses the TCP protocol.  Refer to the
                      TRANSPORT METHODS section for more details.

       minorversion=n Specifies the protocol minor version number.  NFSv4
                      introduces "minor versioning," where NFS protocol
                      enhancements can be introduced without bumping the NFS
                      protocol version number.  Before kernel 2.6.38, the
                      minor version is always zero, and this option is not
                      recognized.  After this kernel, specifying
                      "minorversion=1" enables a number of advanced features,
                      such as NFSv4 sessions.

                      Recent kernels allow the minor version to be specified
                      using the vers= option.  For example, specifying
                      vers=4.1 is the same as specifying
                      vers=4,minorversion=1.

       port=n         The numeric value of the server's NFS service port.  If
                      the server's NFS service is not available on the
                      specified port, the mount request fails.

                      If this mount option is not specified, the NFS client
                      uses the standard NFS port number of 2049 without first
                      checking the server's rpcbind service.  This allows an
                      NFS version 4 client to contact an NFS version 4 server
                      through a firewall that may block rpcbind requests.

                      If the specified port value is 0, then the NFS client
                      uses the NFS service port number advertised by the
                      server's rpcbind service.  The mount request fails if
                      the server's rpcbind service is not available, the
                      server's NFS service is not registered with its rpcbind
                      service, or the server's NFS service is not available on
                      the advertised port.

       cto / nocto    Selects whether to use close-to-open cache coherence
                      semantics for NFS directories on this mount point.  If
                      neither cto nor nocto is specified, the default is to
                      use close-to-open cache coherence semantics for
                      directories.

                      File data caching behavior is not affected by this
                      option.  The DATA AND METADATA COHERENCE section
                      discusses the behavior of this option in more detail.

       clientaddr=n.n.n.n

       clientaddr=n:n:...:n
                      Specifies a single IPv4 address (in dotted-quad form),
                      or a non-link-local IPv6 address, that the NFS client
                      advertises to allow servers to perform NFS version 4.0
                      callback requests against files on this mount point. If
                      the  server is unable to establish callback connections
                      to clients, performance may degrade, or accesses to
                      files may temporarily hang.  Can specify a value of
                      IPv4_ANY (0.0.0.0) or equivalent IPv6 any address which
                      will signal to the NFS server that this NFS client does
                      not want delegations.

                      If this option is not specified, the mount(8) command
                      attempts to discover an appropriate callback address
                      automatically.  The automatic discovery process is not
                      perfect, however.  In the presence of multiple client
                      network interfaces, special routing policies, or
                      atypical network topologies, the exact address to use
                      for callbacks may be nontrivial to determine.

                      NFS protocol versions 4.1 and 4.2 use the client-
                      established TCP connection for callback requests, so do
                      not require the server to connect to the client.  This
                      option is therefore only affect NFS version 4.0 mounts.

       migration / nomigration
                      Selects whether the client uses an identification string
                      that is compatible with NFSv4 Transparent State
                      Migration (TSM).  If the mounted server supports NFSv4
                      migration with TSM, specify the migration option.

                      Some server features misbehave in the face of a
                      migration-compatible identification string.  The
                      nomigration option retains the use of a traditional
                      client indentification string which is compatible with
                      legacy NFS servers.  This is also the behavior if
                      neither option is specified.  A client's open and lock
                      state cannot be migrated transparently when it
                      identifies itself via a traditional identification
                      string.

                      This mount option has no effect with NFSv4 minor
                      versions newer than zero, which always use TSM-
                      compatible client identification strings.

nfs4 FILE SYSTEM TYPE
       The nfs4 file system type is an old syntax for specifying NFSv4 usage.
       It can still be used with all NFSv4-specific and common options,
       excepted the nfsvers mount option.

MOUNT CONFIGURATION FILE
       If the mount command is configured to do so, all of the mount options
       described in the previous section can also be configured in the
       /etc/nfsmount.conf file. See nfsmount.conf(5) for details.

EXAMPLES
       To mount an export using NFS version 2, use the nfs file system type
       and specify the nfsvers=2 mount option.  To mount using NFS version 3,
       use the nfs file system type and specify the nfsvers=3 mount option.
       To mount using NFS version 4, use either the nfs file system type, with
       the nfsvers=4 mount option, or the nfs4 file system type.

       The following example from an /etc/fstab file causes the mount command
       to negotiate reasonable defaults for NFS behavior.

               server:/export  /mnt  nfs   defaults                      0 0

       Here is an example from an /etc/fstab file for an NFS version 2 mount
       over UDP.

               server:/export  /mnt  nfs   nfsvers=2,proto=udp           0 0

       This example shows how to mount using NFS version 4 over TCP with
       Kerberos 5 mutual authentication.

               server:/export  /mnt  nfs4  sec=krb5                      0 0

       This example shows how to mount using NFS version 4 over TCP with
       Kerberos 5 privacy or data integrity mode.

               server:/export  /mnt  nfs4  sec=krb5p:krb5i               0 0

       This example can be used to mount /usr over NFS.

               server:/export  /usr  nfs   ro,nolock,nocto,actimeo=3600  0 0

       This example shows how to mount an NFS server using a raw IPv6 link-
       local address.

               [fe80::215:c5ff:fb3e:e2b1%eth0]:/export /mnt nfs defaults 0 0

TRANSPORT METHODS
       NFS clients send requests to NFS servers via Remote Procedure Calls, or
       RPCs.  The RPC client discovers remote service endpoints automatically,
       handles per-request authentication, adjusts request parameters for
       different byte endianness on client and server, and retransmits
       requests that may have been lost by the network or server.  RPC
       requests and replies flow over a network transport.

       In most cases, the mount(8) command, NFS client, and NFS server can
       automatically negotiate proper transport and data transfer size
       settings for a mount point.  In some cases, however, it pays to specify
       these settings explicitly using mount options.

       Traditionally, NFS clients used the UDP transport exclusively for
       transmitting requests to servers.  Though its implementation is simple,
       NFS over UDP has many limitations that prevent smooth operation and
       good performance in some common deployment environments.  Even an
       insignificant packet loss rate results in the loss of whole NFS
       requests; as such, retransmit timeouts are usually in the subsecond
       range to allow clients to recover quickly from dropped requests, but
       this can result in extraneous network traffic and server load.

       However, UDP can be quite effective in specialized settings where the
       networks MTU is large relative to NFSs data transfer size (such as
       network environments that enable jumbo Ethernet frames).  In such
       environments, trimming the rsize and wsize settings so that each NFS
       read or write request fits in just a few network frames (or even in  a
       single  frame) is advised.  This reduces the probability that the loss
       of a single MTU-sized network frame results in the loss of an entire
       large read or write request.

       TCP is the default transport protocol used for all modern NFS
       implementations.  It performs well in almost every conceivable network
       environment and provides excellent guarantees against data corruption
       caused by network unreliability.  TCP is often a requirement for
       mounting a server through a network firewall.

       Under normal circumstances, networks drop packets much more frequently
       than NFS servers drop requests.  As such, an aggressive retransmit
       timeout  setting for NFS over TCP is unnecessary. Typical timeout
       settings for NFS over TCP are between one and ten minutes.  After  the
       client exhausts its retransmits (the value of the retrans mount
       option), it assumes a network partition has occurred, and attempts to
       reconnect to the server on a fresh socket. Since TCP itself makes
       network data transfer reliable, rsize and wsize can safely be allowed
       to default to the largest values supported by both client and server,
       independent of the network's MTU size.

   Using the mountproto mount option
       This section applies only to NFS version 2 and version 3 mounts since
       NFS version 4 does not use a separate protocol for mount requests.

       The Linux NFS client can use a different transport for contacting an
       NFS server's rpcbind service, its mountd service, its Network Lock
       Manager (NLM) service, and its NFS service.  The exact transports
       employed by the Linux NFS client for each mount point depends on the
       settings of the transport mount options, which include proto,
       mountproto, udp, and tcp.

       The client sends Network Status Manager (NSM) notifications via UDP no
       matter what transport options are specified, but listens for server NSM
       notifications on both UDP and TCP.  The NFS Access Control List
       (NFSACL) protocol shares the same transport as the main NFS service.

       If no transport options are specified, the Linux NFS client uses UDP to
       contact the server's mountd service, and TCP to contact its NLM and NFS
       services by default.

       If the server does not support these transports for these services, the
       mount(8) command attempts to discover what the server supports, and
       then retries the mount request once using the discovered transports.
       If the server does not advertise any transport supported by the client
       or is misconfigured, the mount request fails.  If the bg option is in
       effect, the mount command backgrounds itself and continues to attempt
       the specified mount request.

       When the proto option, the udp option, or the tcp option is specified
       but the mountproto option is not, the specified transport is used to
       contact both the server's mountd service and for the NLM and NFS
       services.

       If the mountproto option is specified but none of the proto, udp or tcp
       options are specified, then the specified transport is used for the
       initial mountd request, but the mount command attempts to discover what
       the server supports for the NFS protocol, preferring TCP if both
       transports are supported.

       If both the mountproto and proto (or udp or tcp) options are specified,
       then the transport specified by the mountproto option is used for the
       initial mountd request, and the transport specified by the proto option
       (or the udp or tcp options) is used for NFS, no matter what order these
       options appear.  No automatic service discovery is performed if these
       options are specified.

       If any of the proto, udp, tcp, or mountproto options are specified more
       than once on the same mount command line, then the value of the
       rightmost instance of each of these options takes effect.

   Using NFS over UDP on high-speed links
       Using NFS over UDP on high-speed links such as Gigabit can cause silent
       data corruption.

       The problem can be triggered at high loads, and is caused by problems
       in IP fragment reassembly. NFS read and writes typically transmit UDP
       packets of 4 Kilobytes or more, which have to be broken up into several
       fragments in order to be sent over the Ethernet link, which limits
       packets to 1500 bytes by default. This process happens at the IP
       network layer and is called fragmentation.

       In order to identify fragments that belong together, IP assigns a 16bit
       IP ID value to each packet; fragments generated from the same UDP
       packet will have the same IP ID. The receiving system will collect
       these fragments and combine them to form the original UDP packet. This
       process is called reassembly. The default timeout for packet reassembly
       is 30 seconds; if the network stack does not receive all fragments of a
       given packet within this interval, it assumes the missing fragment(s)
       got lost and discards those it already received.

       The problem this creates over high-speed links is that it is possible
       to send more than 65536 packets within 30 seconds. In fact, with heavy
       NFS traffic one can observe that the IP IDs repeat after about 5
       seconds.

       This has serious effects on reassembly: if one fragment gets lost,
       another fragment from a different packet but with the same IP ID will
       arrive within the 30 second timeout, and the network stack will combine
       these fragments to form a new packet. Most of the time, network layers
       above IP will detect this mismatched reassembly - in the case of UDP,
       the UDP checksum, which is a 16 bit checksum over the entire packet
       payload, will usually not match, and UDP will discard the bad packet.

       However, the UDP checksum is 16 bit only, so there is a chance of 1 in
       65536 that it will match even if the packet payload is completely
       random (which very often isn't the case). If that is the case, silent
       data corruption will occur.

       This potential should be taken seriously, at least on Gigabit Ethernet.
       Network speeds of 100Mbit/s should be considered less problematic,
       because with most traffic patterns IP ID wrap around will take much
       longer than 30 seconds.

       It is therefore strongly recommended to use NFS over TCP where
       possible, since TCP does not perform fragmentation.

       If you absolutely have to use NFS over UDP over Gigabit Ethernet, some
       steps can be taken to mitigate the problem and reduce the probability
       of corruption:

       Jumbo frames:  Many Gigabit network cards are capable of transmitting
                      frames bigger than the 1500 byte limit of traditional
                      Ethernet, typically 9000 bytes. Using jumbo frames of
                      9000 bytes will allow you to run NFS over UDP at a page
                      size of 8K without fragmentation. Of course, this is
                      only feasible if all involved stations support jumbo
                      frames.

                      To enable a machine to send jumbo frames on cards that
                      support it, it is sufficient to configure the interface
                      for a MTU value of 9000.

       Lower reassembly timeout:
                      By lowering this timeout below the time it takes the IP
                      ID counter to wrap around, incorrect reassembly of
                      fragments can be prevented as well. To do so, simply
                      write the new timeout value (in seconds) to the file
                      /proc/sys/net/ipv4/ipfrag_time.

                      A value of 2 seconds will greatly reduce the probability
                      of IPID clashes on a single Gigabit link, while still
                      allowing for a reasonable timeout when receiving
                      fragmented traffic from distant peers.

DATA AND METADATA COHERENCE
       Some modern cluster file systems provide perfect cache coherence among
       their clients.  Perfect cache coherence among disparate NFS clients is
       expensive to achieve, especially on wide area networks.  As such, NFS
       settles for weaker cache coherence that satisfies the requirements of
       most file sharing types.

   Close-to-open cache consistency
       Typically file sharing is completely sequential.  First client A opens
       a file, writes something to it, then closes it.  Then client B opens
       the same file, and reads the changes.

       When an application opens a file stored on an NFS version 3 server, the
       NFS client checks that the file exists on the server and is permitted
       to the opener by sending a GETATTR or ACCESS request.  The NFS client
       sends these requests regardless of the freshness of the file's cached
       attributes.

       When the application closes the file, the NFS client writes back any
       pending changes to the file so that the next opener can view the
       changes.  This also gives the NFS client an opportunity to report write
       errors to the application via the return code from close(2).

       The behavior of checking at open time and flushing at close time is
       referred to as close-to-open cache consistency, or CTO.  It can be
       disabled for an entire mount point using the nocto mount option.

   Weak cache consistency
       There are still opportunities for a client's data cache to contain
       stale data.  The NFS version 3 protocol introduced "weak cache
       consistency" (also known as WCC) which provides a way of efficiently
       checking a file's attributes before and after a single request.  This
       allows a client to help identify changes that could have been made by
       other clients.

       When a client is using many concurrent operations that update the same
       file at the same time (for example, during asynchronous write behind),
       it is still difficult to tell whether it was that client's updates or
       some other client's updates that altered the file.

   Attribute caching
       Use the noac mount option to achieve attribute cache coherence among
       multiple clients.  Almost every file system operation checks file
       attribute information.  The client keeps this information cached for a
       period of time to reduce network and server load.  When noac is in
       effect, a client's file attribute cache is disabled, so each operation
       that needs to check a file's attributes is forced to go back to the
       server.  This permits a client to see changes to a file very quickly,
       at the cost of many extra network operations.

       Be careful not to confuse the noac option with "no data caching."  The
       noac mount option prevents the client from caching file metadata, but
       there are still races that may result in data cache incoherence between
       client and server.

       The NFS protocol is not designed to support true cluster file system
       cache coherence without some type of application serialization.  If
       absolute cache coherence among clients is required, applications should
       use file locking. Alternatively, applications can also open their files
       with the O_DIRECT flag to disable data caching entirely.

   File timestamp maintainence
       NFS servers are responsible for managing file and directory timestamps
       (atime, ctime, and mtime).  When a file is accessed or updated on an
       NFS server, the file's timestamps are updated just like they would be
       on a filesystem local to an application.

       NFS clients cache file attributes, including timestamps.  A file's
       timestamps are updated on NFS clients when its attributes are retrieved
       from the NFS server.  Thus there may be some delay before timestamp
       updates on an NFS server appear to applications on NFS clients.

       To comply with the POSIX filesystem standard, the Linux NFS client
       relies on NFS servers to keep a file's mtime and ctime timestamps
       properly up to date.  It does this by flushing local data changes to
       the server before reporting mtime to applications via system calls such
       as stat(2).

       The Linux client handles atime updates more loosely, however.  NFS
       clients maintain good performance by caching data, but that means that
       application reads, which normally update atime, are not reflected to
       the server where a file's atime is actually maintained.

       Because of this caching behavior, the Linux NFS client does not support
       generic atime-related mount options.  See mount(8) for details on these
       options.

       In particular, the atime/noatime, diratime/nodiratime,
       relatime/norelatime, and strictatime/nostrictatime mount options have
       no effect on NFS mounts.

       /proc/mounts may report that the relatime mount option is set on NFS
       mounts, but in fact the atime semantics are always as described here,
       and are not like relatime semantics.

   Directory entry caching
       The Linux NFS client caches the result of all NFS LOOKUP requests.  If
       the requested directory entry exists on the server, the result is
       referred to as a positive lookup result.  If the requested directory
       entry does not exist on the server (that is, the server returned
       ENOENT), the result is referred to as negative lookup result.

       To detect when directory entries have been added or removed on the
       server, the Linux NFS client watches a directory's mtime.  If the
       client detects a change in a directory's mtime, the client drops all
       cached LOOKUP results for that directory.  Since the directory's mtime
       is a cached attribute, it may take some time before a client notices it
       has changed.  See the descriptions of the acdirmin, acdirmax, and noac
       mount options for more information about how long a directory's mtime
       is cached.

       Caching directory entries improves the performance of applications that
       do not share files with applications on other clients.  Using cached
       information about directories can interfere with applications that run
       concurrently on multiple clients and need to detect the creation or
       removal of files quickly, however.  The lookupcache mount option allows
       some tuning of directory entry caching behavior.

       Before kernel release 2.6.28, the Linux NFS client tracked only
       positive lookup results.  This permitted applications to detect new
       directory entries created by other clients quickly while still
       providing some of the performance benefits of caching.  If an
       application depends on the previous lookup caching behavior of the
       Linux NFS client, you can use lookupcache=positive.

       If the client ignores its cache and validates every application lookup
       request with the server, that client can immediately detect when a new
       directory entry has been either created or removed by another client.
       You can specify this behavior using lookupcache=none.  The extra NFS
       requests needed if the client does not cache directory entries can
       exact a performance penalty.  Disabling lookup caching should result in
       less of a performance penalty than using noac, and has no effect on how
       the NFS client caches the attributes of files.

   The sync mount option
       The NFS client treats the sync mount option differently than some other
       file systems (refer to mount(8) for a description of the generic sync
       and async mount options).  If neither sync nor async is specified (or
       if the async option is specified), the NFS client delays sending
       application writes to the server until any of these events occur:

              Memory pressure forces reclamation of system memory resources.

              An application flushes file data explicitly with sync(2),
              msync(2), or fsync(3).

              An application closes a file with close(2).

              The file is locked/unlocked via fcntl(2).

       In other words, under normal circumstances, data written by an
       application may not immediately appear on the server that hosts the
       file.

       If the sync option is specified on a mount point, any system call that
       writes data to files on that mount point causes that data to be flushed
       to the server before the system call returns control to user space.
       This provides greater data cache coherence among clients, but at a
       significant performance cost.

       Applications can use the O_SYNC open flag to force application writes
       to individual files to go to the server immediately without the use of
       the sync mount option.

   Using file locks with NFS
       The Network Lock Manager protocol is a separate sideband protocol used
       to manage file locks in NFS version 2 and version 3.  To support lock
       recovery after a client or server reboot, a second sideband protocol --
       known as the Network Status Manager protocol -- is also required.  In
       NFS version 4, file locking is supported directly in the main NFS
       protocol, and the NLM and NSM sideband protocols are not used.

       In most cases, NLM and NSM services are started automatically, and no
       extra configuration is required.  Configure all NFS clients with fully-
       qualified domain names to ensure that NFS servers can find clients to
       notify them of server reboots.

       NLM supports advisory file locks only.  To lock NFS files, use fcntl(2)
       with the F_GETLK and F_SETLK commands.  The NFS client converts file
       locks obtained via flock(2) to advisory locks.

       When mounting servers that do not support the NLM protocol, or when
       mounting an NFS server through a firewall that blocks the NLM service
       port, specify the nolock mount option. NLM locking must be disabled
       with the nolock option when using NFS to mount /var because /var
       contains files used by the NLM implementation on Linux.

       Specifying the nolock option may also be advised to improve the
       performance of a proprietary application which runs on a single client
       and uses file locks extensively.

   NFS version 4 caching features
       The data and metadata caching behavior of NFS version 4 clients is
       similar to that of earlier versions.  However, NFS version 4 adds two
       features that improve cache behavior: change attributes and file
       delegation.

       The change attribute is a new part of NFS file and directory metadata
       which tracks data changes.  It replaces the use of a file's
       modification and change time stamps as a way for clients to validate
       the content of their caches.  Change attributes are independent of the
       time stamp resolution on either the server or client, however.

       A file delegation is a contract between an NFS version 4 client and
       server that allows the client to treat a file temporarily as if no
       other client is accessing it.  The server promises to notify the client
       (via a callback request) if another client attempts to access that
       file.  Once a file has been delegated to a client, the client can cache
       that file's data and metadata aggressively without contacting the
       server.

       File delegations come in two flavors: read and write.  A read
       delegation means that the server notifies the client about any other
       clients that want to write to the file.  A write delegation means that
       the client gets notified about either read or write accessors.

       Servers grant file delegations when a file is opened, and can recall
       delegations at any time when another client wants access to the file
       that conflicts with any delegations already granted.  Delegations on
       directories are not supported.

       In order to support delegation callback, the server checks the network
       return path to the client during the client's initial contact with the
       server.  If contact with the client cannot be established, the server
       simply does not grant any delegations to that client.

SECURITY CONSIDERATIONS
       NFS servers control access to file data, but they depend on their RPC
       implementation to provide authentication of NFS requests.  Traditional
       NFS access control mimics the standard mode bit access control provided
       in local file systems.  Traditional RPC authentication uses a number to
       represent each user (usually the user's own uid), a number to represent
       the user's group (the user's gid), and a set of up to 16 auxiliary
       group numbers to represent other groups of which the user may be a
       member.

       Typically, file data and user ID values appear unencrypted (i.e. "in
       the clear") on the network.  Moreover, NFS versions 2 and 3 use
       separate sideband protocols for mounting, locking and unlocking files,
       and reporting system status of clients and servers.  These auxiliary
       protocols use no authentication.

       In addition to combining these sideband protocols with the main NFS
       protocol, NFS version 4 introduces more advanced forms of access
       control, authentication, and in-transit data protection.  The NFS
       version 4 specification mandates support for strong authentication and
       security flavors that provide per-RPC integrity checking and
       encryption.  Because NFS version 4 combines the function of the
       sideband protocols into the main NFS protocol, the new security
       features apply to all NFS version 4 operations including mounting, file
       locking, and so on.  RPCGSS authentication can also be used with NFS
       versions 2 and 3, but it does not protect their sideband protocols.

       The sec mount option specifies the security flavor used for operations
       on behalf of users on that NFS mount point.  Specifying sec=krb5
       provides cryptographic proof of a user's identity in each RPC request.
       This provides strong verification of the identity of users accessing
       data on the server.  Note that additional configuration besides adding
       this mount option is required in order to enable Kerberos security.
       Refer to the rpc.gssd(8) man page for details.

       Two additional flavors of Kerberos security are supported: krb5i and
       krb5p.  The krb5i security flavor provides a cryptographically strong
       guarantee that the data in each RPC request has not been tampered with.
       The krb5p security flavor encrypts every RPC request to prevent data
       exposure during network transit; however, expect some performance
       impact when using integrity checking or encryption.  Similar support
       for other forms of cryptographic security is also available.

   NFS version 4 filesystem crossing
       The NFS version 4 protocol allows a client to renegotiate the security
       flavor when the client crosses into a new filesystem on the server.
       The newly negotiated flavor effects only accesses of the new
       filesystem.

       Such negotiation typically occurs when a client crosses from a server's
       pseudo-fs into one of the server's exported physical filesystems, which
       often have more restrictive security settings than the pseudo-fs.

   NFS version 4 Leases
       In NFS version 4, a lease is a period of time during which a server
       irrevocably grants a file lock to a client.  If the lease expires, the
       server is allowed to revoke that lock.  Clients periodically renew
       their leases to prevent lock revocation.

       After an NFS version 4 server reboots, each client tells the server
       about all file open and lock state under its lease before operation can
       continue.  If the client reboots, the server frees all open and lock
       state associated with that client's lease.

       As part of establishing a lease, therefore, a client must identify
       itself to a server.  A fixed string is used to distinguish that client
       from others, and a changeable verifier is used to indicate when the
       client has rebooted.

       A client uses a particular security flavor and principal when
       performing the operations to establish a lease.  If two clients happen
       to present the same identity string, a server can use their principals
       to detect that they are different clients, and prevent one client from
       interfering with the other's lease.

       The Linux NFS client establishes one lease for each server.  Lease
       management operations, such as lease renewal, are not done on behalf of
       a particular file, lock, user, or mount point, but on behalf of the
       whole client that owns that lease.  These operations must use the same
       security flavor and principal that was used when the lease was
       established, even across client reboots.

       When Kerberos is configured on a Linux NFS client (i.e., there is a
       /etc/krb5.keytab on that client), the client attempts to use a Kerberos
       security flavor for its lease management operations.  This provides
       strong authentication of the client to each server it contacts.  By
       default, the client uses the host/ or nfs/ service principal in its
       /etc/krb5.keytab for this purpose.

       If the client has Kerberos configured, but the server does not, or if
       the client does not have a keytab or the requisite service principals,
       the client uses AUTH_SYS and UID 0 for lease management.

   Using non-privileged source ports
       NFS clients usually communicate with NFS servers via network sockets.
       Each end of a socket is assigned a port value, which is simply a number
       between 1 and 65535 that distinguishes socket endpoints at the same IP
       address.  A socket is uniquely defined by a tuple that includes the
       transport protocol (TCP or UDP) and the port values and IP addresses of
       both endpoints.

       The NFS client can choose any source port value for its sockets, but
       usually chooses a privileged port.  A privileged port is a port value
       less than 1024.  Only a process with root privileges may create a
       socket with a privileged source port.

       The exact range of privileged source ports that can be chosen is set by
       a pair of sysctls to avoid choosing a well-known port, such as the port
       used by ssh.  This means the number of source ports available for the
       NFS client, and therefore the number of socket connections that can be
       used at the same time, is practically limited to only a few hundred.

       As described above, the traditional default NFS authentication scheme,
       known as AUTH_SYS, relies on sending local UID and GID numbers to
       identify users making NFS requests.  An NFS server assumes that if a
       connection comes from a privileged port, the UID and GID numbers in the
       NFS requests on this connection have been verified by the client's
       kernel or some other local authority.  This is an easy system to spoof,
       but on a trusted physical network between trusted hosts, it is entirely
       adequate.

       Roughly speaking, one socket is used for each NFS mount point.  If a
       client could use non-privileged source ports as well, the number of
       sockets allowed, and thus the maximum number of concurrent mount
       points, would be much larger.

       Using non-privileged source ports may compromise server security
       somewhat, since any user on AUTH_SYS mount points can now pretend to be
       any other when making NFS requests.  Thus NFS servers do not support
       this by default.  They explicitly allow it usually via an export
       option.

       To retain good security while allowing as many mount points as
       possible, it is best to allow non-privileged client connections only if
       the server and client both require strong authentication, such as
       Kerberos.

   Mounting through a firewall
       A firewall may reside between an NFS client and server, or the client
       or server may block some of its own ports via IP filter rules.  It is
       still possible to mount an NFS server through a firewall, though some
       of the mount(8) command's automatic service endpoint discovery
       mechanisms may not work; this requires you to provide specific endpoint
       details via NFS mount options.

       NFS servers normally run a portmapper or rpcbind daemon to advertise
       their service endpoints to clients. Clients use the rpcbind daemon to
       determine:

              What network port each RPC-based service is using

              What transport protocols each RPC-based service supports

       The rpcbind daemon uses a well-known port number (111) to help clients
       find a service endpoint.  Although NFS often uses a standard port
       number (2049), auxiliary services such as the NLM service can choose
       any unused port number at random.

       Common firewall configurations block the well-known rpcbind port.  In
       the absense of an rpcbind service, the server administrator fixes the
       port number of NFS-related services so that the firewall can allow
       access to specific NFS service ports.  Client administrators then
       specify the port number for the mountd service via the mount(8)
       command's mountport option.  It may also be necessary to enforce the
       use of TCP or UDP if the firewall blocks one of those transports.

   NFS Access Control Lists
       Solaris allows NFS version 3 clients direct access to POSIX Access
       Control Lists stored in its local file systems.  This proprietary
       sideband protocol, known as NFSACL, provides richer access control than
       mode bits.  Linux implements this protocol for compatibility with the
       Solaris NFS implementation.  The NFSACL protocol never became a
       standard part of the NFS version 3 specification, however.

       The NFS version 4 specification mandates a new version of Access
       Control Lists that are semantically richer than POSIX ACLs.  NFS
       version 4 ACLs are not fully compatible with POSIX ACLs; as such, some
       translation between the two is required in an environment that mixes
       POSIX ACLs and NFS version 4.

THE REMOUNT OPTION
       Generic mount options such as rw and sync can be modified on NFS mount
       points using the remount option.  See mount(8) for more information on
       generic mount options.

       With few exceptions, NFS-specific options are not able to be modified
       during a remount.  The underlying transport or NFS version cannot be
       changed by a remount, for example.

       Performing a remount on an NFS file system mounted with the noac option
       may have unintended consequences.  The noac option is a combination of
       the generic option sync, and the NFS-specific option actimeo=0.

   Unmounting after a remount
       For mount points that use NFS versions 2 or 3, the NFS umount
       subcommand depends on knowing the original set of mount options used to
       perform the MNT operation.  These options are stored on disk by the NFS
       mount subcommand, and can be erased by a remount.

       To ensure that the saved mount options are not erased during a remount,
       specify either the local mount directory, or the server hostname and
       export pathname, but not both, during a remount.  For example,

               mount -o remount,ro /mnt

       merges the mount option ro with the mount options already saved on disk
       for the NFS server mounted at /mnt.

FILES
       /etc/fstab     file system table

       /etc/nfsmount.conf
                      Configuration file for NFS mounts

NOTES
       Before 2.4.7, the Linux NFS client did not support NFS over TCP.

       Before 2.4.20, the Linux NFS client used a heuristic to determine
       whether cached file data was still valid rather than using the standard
       close-to-open cache coherency method described above.

       Starting with 2.4.22, the Linux NFS client employs a Van Jacobsen-based
       RTT estimator to determine retransmit timeout values when using NFS
       over UDP.

       Before 2.6.0, the Linux NFS client did not support NFS version 4.

       Before 2.6.8, the Linux NFS client used only synchronous reads and
       writes when the rsize and wsize settings were smaller than the system's
       page size.

       The Linux client's support for protocol versions depend on whether the
       kernel was built with options CONFIG_NFS_V2, CONFIG_NFS_V3,
       CONFIG_NFS_V4, CONFIG_NFS_V4_1, and CONFIG_NFS_V4_2.

SEE ALSO
       fstab(5), mount(8), umount(8), mount.nfs(5), umount.nfs(5), exports(5),
       nfsmount.conf(5), netconfig(5), ipv6(7), nfsd(8), sm-notify(8),
       rpc.statd(8), rpc.idmapd(8), rpc.gssd(8), rpc.svcgssd(8), kerberos(1)

       RFC 768 for the UDP specification.
       RFC 793 for the TCP specification.
       RFC 1094 for the NFS version 2 specification.
       RFC 1813 for the NFS version 3 specification.
       RFC 1832 for the XDR specification.
       RFC 1833 for the RPC bind specification.
       RFC 2203 for the RPCSEC GSS API protocol specification.
       RFC 7530 for the NFS version 4.0 specification.
       RFC 5661 for the NFS version 4.1 specification.
       RFC 7862 for the NFS version 4.2 specification.



                                9 October 2012                          NFS(5)