Switch to SpeakEasy.net DSL

The Modular Manual Browser

Home Page
Manual: (NetBSD-2.0)
Apropos / Subsearch:
optional field

MOUNT_NFS(8)              BSD System Manager's Manual             MOUNT_NFS(8)

     mount_nfs -- mount NFS file systems

     mount_nfs [-23bcCdiKlpPqsTUX] [-D deadthresh] [-I readdirsize]
               [-L leaseterm] [-R retrycnt] [-a maxreadahead] [-g maxgroups]
               [-m realm] [-o options] [-r readsize] [-t timeout]
               [-w writesize] [-x retrans] rhost:path node

     The mount_nfs command calls the mount(2) system call to prepare and graft
     a remote NFS file system (rhost:path) on to the file system tree at the
     mount point node.  This command is normally executed by mount(8).  It
     implements the mount protocol as described in RFC 1094, Appendix A and
     NFS: Network File System Version 3 Protocol Specification, Appendix I.

     The options are:

     -2      Use the NFS Version 2 protocol.

     -3      Use the NFS Version 3 protocol.  The default is to try version 3
             first, and fall back to version 2 if the mount fails.

     -D      Used with NQNFS to set the ``dead server threshold'' to the spec-
             ified number of round trip timeout intervals.  After a ``dead
             server threshold'' of retransmit timeouts, cached data for the
             unresponsive server is assumed to still be valid.  Values may be
             set in the range of 1 - 9, with 9 referring to an ``infinite dead
             threshold'' (i.e. never assume cached data still valid).  This
             option is not generally recommended and is really an experimental

     -I      Set the readdir read size to the specified value.  The value
             should normally be a multiple of DIRBLKSIZ that is <= the read
             size for the mount.

     -K      Pass Kerberos authenticators to the server for client-to-server
             user-credential mapping.  This requires that the NetBSD kernel be
             built with the NFSKERB option(4).  Refer to RFC 2695,
             Authentication Mechanisms for ONC RPC, for more information.

     -L      Used with NQNFS to set the lease term to the specified number of
             seconds.  Only use this argument for mounts with a large round
             trip delay.  Values are normally in the 10-30 second range.

     -P      Use a reserved socket port number.  This is the default, and
             available for backwards compatibility purposes only.

     -R      Set the retry count for doing the mount to the specified value.

     -T      Use TCP transport instead of UDP.  This is recommended for
             servers that are not on the same physical network as the client.
             Not all NFS servers, especially not old ones, support this.

     -U      Force the mount protocol to use UDP transport, even for TCP NFS
             mounts.  This is necessary for some old BSD servers.

     -X      Perform 32 <-> 64 bit directory cookie translation for version 3
             mounts.  This may be need in the case of a server using the upper
             32 bits of version 3 directory cookies, and when you are running
             emulated binaries that access such a filesystem.  Native NetBSD
             binaries will never need this option.  This option introduces
             some overhead.

     -a      Set the read-ahead count to the specified value.  This may be in
             the range of 0 - 4, and determines how many blocks will be read
             ahead when a large file is being read sequentially.  Trying a
             value greater than 1 for this is suggested for mounts with a
             large bandwidth * delay product.

     -b      If an initial attempt to contact the server fails, fork off a
             child to keep trying the mount in the background.  Useful for
             fstab(5), where the filesystem mount is not critical to multiuser

     -c      For UDP mount points, do not do a connect(2).  This flag is dep-
             recated and connectionless UDP mounts are the default.

     -C      For UDP mount points, do a connect(2).  Although this flag
             increases the efficiency of UDP mounts it cannot be used for
             servers that do not reply to requests from the standard NFS port
             number 2049, or for servers with multiple network interfaces.  In
             these cases if the socket is connected and the server replies
             from a different port number or a different network interface the
             client will get ICMP port unreachable and the mount will hang.

     -d      Turn off the dynamic retransmit timeout estimator.  This may be
             useful for UDP mounts that exhibit high retry rates, since it is
             possible that the dynamically estimated timeout interval is too

     -g      Set the maximum size of the group list for the credentials to the
             specified value.  This should be used for mounts on old servers
             that cannot handle a group list size of 16, as specified in RFC
             1057.  Try 8, if users in a lot of groups cannot get response
             from the mount point.

     -i      Make the mount interruptible, which implies that file system
             calls that are delayed due to an unresponsive server will fail
             with EINTR when a termination signal is posted for the process.

     -l      Used with NQNFS and NFS Version 3 to specify that the
             ReaddirPlus() RPC should be used.  This option reduces RPC traf-
             fic for cases such as ls -l, but tends to flood the attribute and
             name caches with prefetched entries.  Try this option and see
             whether performance improves or degrades.  Probably most useful
             for client to server network interconnects with a large bandwidth
             times delay product.

     -m      Set the Kerberos realm to the string argument.  Used with the -K
             option for mounts to other realms.

     -o      Options are specified with a -o flag followed by a comma sepa-
             rated string of options.  See the mount(8) man page for possible
             options and their meanings.

     -p      Do not use a reserved port number for RPCs.  This option is pro-
             vided only to be able to mimic the old default behavior of not
             using a reserved port, and should rarely be useful.

     -q      Use the leasing extensions to the NFS Version 3 protocol to main-
             tain cache consistency.  This protocol version 2 revision to Not
             Quite NFS (NQNFS) is only supported by this updated release of
             NFS code.  It is not backwards compatible with the version 1
             NQNFS protocol that was part of the first release of 4.4BSD-Lite.
             To interoperate with a first release 4.4BSD-Lite NFS system you
             will have to avoid this option until you have had an opportunity
             to upgrade the NFS code to release 2 of 4.4BSD-Lite on all your

     -r      Set the read data size to the specified value in bytes.  It
             should normally be a power of 2 greater than or equal to 1024.

             This should be used for UDP mounts when the ``fragments dropped
             after timeout'' value is getting large while actively using a
             mount point.  Use netstat(1) with the -s option to see what the
             ``fragments dropped after timeout'' value is.  See the mount_nfs
             -w option also.

     -s      A soft mount, which implies that file system calls will fail
             after retrycnt round trip timeout intervals.

     -t      Set the initial retransmit timeout to the specified value.  May
             be useful for fine tuning UDP mounts over internetworks with high
             packet loss rates or an overloaded server.  Try increasing the
             interval if nfsstat(1) shows high retransmit rates while the file
             system is active or reducing the value if there is a low retrans-
             mit rate but long response delay observed.  Normally, the -d
             option should be specified when using this option to manually
             tune the timeout interval.

     -w      Set the write data size to the specified value in bytes.

             The same logic applies for use of this option as with the
             mount_nfs -r option, but using the ``fragments dropped after
             timeout'' value on the NFS server instead of the client.  Note
             that both the -r and -w options should only be used as a last
             ditch effort at improving performance when mounting servers that
             do not support TCP mounts.

     -x      Set the retransmit timeout count for soft mounts to the specified

     The simplest way to invoke mount_nfs is with a command like:

           mount remotehost:/filesystem /localmountpoint
           mount -t nfs remotehost:/filesystem /localmountpoint

     It is also possible to automatically mount filesystems at boot from your
     /etc/fstab by using a line like:

           remotehost:/home /home nfs rw 0 0

     As can be derived from the comments accompanying the options, performance
     tuning of NFS can be a non-trivial task.  Here are some common points to

           o   Increasing the read and write size with the -r and -w options
               respectively will increase throughput if the network interface
               can handle the larger packet sizes.

               The default size for NFS version 2 is 8K when using UDP, 64K
               when using TCP.

               The default size for NFS version 3 is platform dependent: on
               NetBSD/i386, the default is 32K, for other platforms it is 8K.
               Values over 32K are only supported for TCP, where 64K is the

               Any value over 32K is unlikely to get you more performance,
               unless you have a very fast network.

           o   If the network interface cannot handle larger packet sizes or a
               long train of back to back packets, you may see low performance
               figures or even temporary hangups during NFS activity.

               This can especially happen with older Ethernet network inter-
               faces.  What happens is that either the receive buffer on the
               network interface on the client side is overflowing, or that
               similar events occur on the server, leading to a lot of dropped

               In this case, decreasing the read and write size, using TCP, or
               a combination of both will usually lead to better throughput.
               Should you need to decrease the read and write size for all
               your NFS mounts because of a slow Ethernet network interface
               (e.g. a USB 1.1 to 10/100 Ethernet network interface), you can

               options NFS_RSIZE=value
               options NFS_WSIZE=value

               in your kernel config(8) file to avoid having do specify the
               sizes for all mounts.

           o   For connections that are not on the same LAN, and/or may expe-
               rience packet loss, using TCP is strongly recommended.

     Some common problems with mount_nfs can be difficult for first time users
     to understand.

           mount_nfs: can't access /foo: Permission denied

     This message means that the remote host, is either not exporting the
     filesystem you requested, or is not exporting it to your host.  If you
     believe the remote host is indeed exporting a filesystem to you, make
     sure the exports(5) file is exporting the proper directories.

     A common mistake is that mountd(8) will not export a filesystem with the
     -alldirs option, unless it is a mount point on the exporting host.  It is
     not possible to remotely mount a subdirectory of an exported mount,
     unless it is exported with the -alldirs option.

     The following error:

           NFS Portmap: RPC: Program not registered

     means that the remote host is not running mountd(8).  The program
     rpcinfo(8) can be used to determine if the remote host is running nfsd,
     and mountd by issuing the command:

           rpcinfo -p remotehostname

     If the remote host is running nfsd, and mountd, it would display:

           100005    3   udp    719  mountd
           100005    1   tcp    720  mountd
           100005    3   tcp    720  mountd
           100003    2   udp   2049  nfs
           100003    3   udp   2049  nfs
           100003    2   tcp   2049  nfs
           100003    3   tcp   2049  nfs

     The error:

           mount_nfs: can't get net id for host

     indicates that mount_nfs cannot resolve the name of the remote host.

     nfsstat(1), mount(2), unmount(2), options(4), exports(5), fstab(5),
     mount(8), mountd(8), rpcinfo(8)

     NFS: Network File System Protocol specification, RFC 1094, March 1989.

     NFS Version 2 and Version 3 Security Issues and the NFS Protocol's Use of
     RPCSEC_GCC and Kerberos V5, RFC 2623, June 1999.

     NFS Version 4 Design Considerations, RFC 2624, June 1999.

     Authentication Mechanisms for ONC RPC, RFC 2695, September 1999.

BSD                               May 4, 2004                              BSD