mount_nfs — mount
NFS file systems
-x retrans] rhost:path
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:
- Use the NFS Version 2 protocol.
- 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.
- 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.
- 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).
- 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.
- 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.
- 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.
- 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 their meanings.
The following NFS specific options are also available:
- Enable attribute caching for both files and directories
- Cache directory attributes for no more than num
seconds. The default is 60 seconds.
- Cache directory attributes for at least num
seconds. The default is 5 seconds.
- Cache file attributes for no more than num
seconds. The default is 60 seconds.
- Cache file attributes for at least num seconds.
The default is 5 seconds.
- Use the specified port number for NFS requests. The default is to
query the portmapper for the NFS port.
- 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.
- A soft mount, which implies that file system calls will fail after
retrans round trip timeout intervals have been
- 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.)
- 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.
-r option, but using the “fragments
dropped after timeout” value on the server instead of the client.
Note that both the
-w options should only be used as a last ditch
effort at improving performance when mounting servers that do not support
- 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.
-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.