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

NAME
     mount_nfs - mount NFS file systems

SYNOPSIS
     mount_nfs [-23bcdilsTU] [-a maxreadahead] [-g maxgroups] [-I readdirsize]
               [-o options] [-R retrycnt] [-r readsize] [-t timeout]
               [-w writesize] [-x retrans] rhost:path node

DESCRIPTION
     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 imple-
     ments 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 multius-
             er 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 ad-
             dress 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
             short.

     -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 separat-
             ed 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 di-
                              rectories (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 sec-
                              onds.  The default is 5 seconds.

             acregmax=num     Cache file attributes for no more than num sec-
                              onds.  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 normal-
             ly 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 af-
             ter retrans round trip timeout intervals have been reached (see
             -x).

     -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 mil-
             liseconds.  May be useful for fine tuning UDP mounts over inter-
             networks 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 op-
             tion 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 com-
             ments 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.

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

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

BUGS
     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 ex-
     pected to have limited success.

OpenBSD 4.7                      June 6, 2009                                3
