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


     mount_nfs - mount NFS file systems


     mount_nfs [-23bcdilsTU] [-a maxreadahead] [-g maxgroups] [-I readdirsize]
               [-o options] [-R retrycnt] [-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
     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 as follows:

     -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.

     -a maxreadahead
             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 file system mount is not critical to
             multiuser operation.

     -c      For UDP mount points, do not do a connect(2).  This must be used
             for servers that do not reply to requests from the standard NFS
             port number 2049.  It may also be required for servers with more
             than one IP address (only necessary if replies come from an
             address other than the one specified in the mount request).

     -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 maxgroups
             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 a response
             from the mount point.

     -I readdirsize
             Set the readdir read size to the specified value.  The value
             should normally be a multiple of DIRBLKSIZ that is less than or
             equal to the read size for the mount.

     -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 NFSV3 to specify that the "readdir plus" RPC should be
             used.  This option reduces RPC traffic 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-delay product.

     -o options
             Options are specified with a -o flag followed by a comma
             separated string of options.  The prefix "no" may be added to
             invert the behavior of default options that do not take
             arguments.  See the mount(8) man page for possible options and
             their meanings.

             The following NFS specific options are also available:

             ac               Enable attribute caching for both files and
                              directories (default).

             acdirmax=num     Cache directory attributes for no more than num
                              seconds.  The default is 60 seconds.

             acdirmin=num     Cache directory attributes for at least num
                              seconds.  The default is 5 seconds.

             acregmax=num     Cache file attributes for no more than num
                              seconds.  The default is 60 seconds.

             acregmin=num     Cache file attributes for at least num seconds.
                              The default is 5 seconds.

             port=portnumber  Use the specified port number for NFS requests.
                              The default is to query the portmapper for the
                              NFS port.

     -R retrycnt
             Set the retry count for doing the mount to the specified value.
             The default is 10000.

     -r readsize
             Set the read data size to the specified value.  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 this value
             is.)  See the -w option as well.

     -s      A soft mount, which implies that file system calls will fail
             after retrans round trip timeout intervals have been reached (see

     -T      Use TCP instead of UDP.  Note that TCP may not be supported by
             some very old NFS servers.

     -t timeout
             Set the initial retransmit timeout to the specified value in
             milliseconds.  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 retransmit rate but long response delay
             observed.  (Normally, the -d option should be specified when
             using this option to manually tune the timeout interval.)

     -U      Force the mount protocol to use UDP, even for TCP NFS mounts.
             (Necessary for some old BSD servers.)

     -w writesize
             Set the write data size to the specified value.  Ditto the
             comments w.r.t. the -r option, but using the "fragments dropped
             after timeout" value on the 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 retrans
             Set the retransmit timeout count for soft mounts to the specified
             value.  Defaults to 10.

     In versions prior to OpenBSD 2.7, nfsiod daemons were running to improve
     performance of client NFS I/O.  This is no longer done this way.  Use
     sysctl(8) or modify sysctl.conf(5) to adjust the vfs.nfs.iothreads value,
     which is the number of kernel threads created to serve asynchronous NFS
     I/O requests.


     nfsstat(1), mount(2), tcp(4), udp(4), fstab(5), mount(8), nfsd(8),
     sysctl(8), umount(8)


     The -P flag historically informed the kernel to use a reserved port when
     communicating with clients.  In OpenBSD, a reserved port is always used.


     Due to the way that Sun RPC is implemented on top of UDP (unreliable
     datagram), tuning such mounts is really a black art that can only be
     expected to have limited success.

OpenBSD 6.2                      June 6, 2009                      OpenBSD 6.2

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