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.
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
-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
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.
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.
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
The following NFS specific options are also available:
ac Enable attribute caching for both files and
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
Set the retry count for doing the mount to the specified value.
The default is 10000.
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.
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.)
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.
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
nfsstat(1), mount(2), tcp(4), udp(4), fstab(5), mount(8), nfsd(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 5.9 June 6, 2009 OpenBSD 5.9
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