sbin/mount_nfs/mount_nfs.8

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.\"	@(#)mount_nfs.8	8.3 (Berkeley) 3/29/95
.\"
.Dd January 24, 2021
.Dt MOUNT_NFS 8
.Os
.Sh NAME
.Nm mount_nfs
.Nd mount NFS file systems
.Sh SYNOPSIS
.Nm
.Op Fl 23AbCcdilPpqsTUuX
.Op Fl a Ar maxreadahead
.Op Fl D Ar deadthresh
.Op Fl g Ar maxgroups
.Op Fl I Ar readdirsize
.Op Fl L Ar leaseterm
.Op Fl o Ar options
.Op Fl R Ar retrycnt
.Op Fl r Ar readsize
.Op Fl t Ar timeout
.Op Fl w Ar writesize
.Op Fl x Ar retrans
.Ar rhost:path node
.Sh DESCRIPTION
The
.Nm
command calls the
.Xr 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
.Ar node .
The directory specified by
.Ar node
is converted to an absolute path before use.
This command is normally executed by
.Xr mount 8 .
It implements the mount protocol as described in RFC 1094, Appendix A and
.%T "NFS: Network File System Version 3 Protocol Specification" ,
Appendix I.
.Pp
The options are:
.Bl -tag -width indent
.It Fl 2
Use the NFS Version 2 protocol.
.It Fl 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.
.It Fl A
Disable the attribute cache in the client, making it consult the server
for the current version of a given file's attributes every time the file
is accessed.
.Pp
By default the attribute cache is enabled, making the client only consult
the server every handful of seconds.
This improves performance, however at the cost of the risk of a
mid-air collision when changes are made simultaneously by different
NFS clients and/or locally on the NFS server that affect the same file.
In that case changes made elsewhere may be missed, leading to
consistency issues.
.It Fl a Ar 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.
.It Fl b
If an initial attempt to contact the server fails, fork off a child to keep
trying the mount in the background.
Useful for
.Xr fstab 5 ,
where the filesystem mount is not critical to multiuser operation.
.It Fl C
For UDP mount points, do a
.Xr 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.
.It Fl c
For UDP mount points, do not do a
.Xr connect 2 .
This flag is deprecated and connectionless UDP mounts are the default.
.It Fl D Ar deadthresh
Set the
.Dq "dead server threshold"
to the specified number of round trip timeout intervals.
After a
.Dq "dead server threshold"
of retransmit timeouts,
.Dq "not responding"
message is printed to a tty.
.It Fl 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.
.It Fl g Ar 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 response from the mount
point.
.It Fl I Ar readdirsize
Set the readdir read size to the specified value.
The value should normally
be a multiple of
.Dv DIRBLKSIZ
that is \*[Le] the read size for the mount.
.It Fl i
Make the mount interruptible, which implies that file system calls that
are delayed due to an unresponsive server will fail with
.Er EINTR
when a
termination signal is posted for the process.
.It Fl L Ar leaseterm
Ignored.
It used to be NQNFS lease term.
.It Fl l
Used with NFS Version 3 to specify that the
.Fn ReaddirPlus
RPC should be used.
This option reduces RPC traffic for cases such as
.Ic "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.
.It Fl o Ar options
Options are specified with a
.Fl o
flag followed by a comma separated string of options.
See the
.Xr mount 8
man page for possible options and their meanings.
.Pp
The following NFS specific options are also available:
.Bl -tag -width indent
.It Cm bg
Same as
.Fl b .
.It Cm conn
Same as
.Fl C .
.It Cm deadthresh Ns = Ns Aq Ar deadthresh
Same as
.Fl D Ar deadthresh .
.It Cm dumbtimer
Same as
.Fl d .
.It Cm intr
Same as
.Fl i .
.It Cm leaseterm Ns = Ns Aq Ar leaseterm
Same as
.Fl L Ar leaseterm .
.It Cm maxgrps Ns = Ns Aq Ar maxgroups
Same as
.Fl g Ar maxgroups .
.It Cm mntudp
Same as
.Fl U .
.It Cm nfsv2
Same as
.Fl 2 .
.It Cm nfsv3
Same as
.Fl 3 .
.It Cm noac
Same as
.Fl A .
.It Cm noresport
Same as
.Fl p .
.It Cm nqnfs
Same as
.Fl q .
.It Cm port Ns = Ns Aq Ar portnumber
Use the specified port number for NFS requests.
The default is to query the portmapper for the NFS port.
.It Cm rdirplus
Same as
.Fl l .
.It Cm readahead Ns = Ns Aq Ar maxreadahead
Same as
.Fl a Ar maxreadahead .
.It Cm rsize Ns = Ns Aq Ar readsize
Same as
.Fl r Ar readsize .
.It Cm soft
Same as
.Fl s .
.It Cm tcp
Same as
.Fl T .
.It Cm udp
Same as
.Fl u .
.It Cm timeo Ns = Ns Aq Ar timeout
Same as
.Fl t Ar timeout .
.It Cm wsize Ns = Ns Aq Ar writesize
Same as
.Fl w Ar writesize .
.El
.It Fl P
Use a reserved socket port number.
This is the default, and available
for backwards compatibility purposes only.
.It Fl p
Do not use a reserved port number for RPCs.
This option is provided only to be able to mimic the old
default behavior of not using a reserved port, and should rarely be useful.
.It Fl q
A synonym of
.Fl 3 .
It used to specify NQNFS.
.It Fl R Ar retrycnt
Set the retry count for doing the mount to the specified value.
The default is 10000.
.It Fl r Ar readsize
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.
.Pp
This should be used for UDP mounts when the
.Dq "fragments dropped after timeout"
value is getting large while actively using a mount point.
Use
.Xr netstat 1
with the
.Fl s
option to see what the
.Dq "fragments dropped after timeout"
value is.
See the
.Nm
.Fl w
option also.
.It Fl s
A soft mount, which implies that file system calls will fail
after
.Ar retrans
round trip timeout intervals.
.It Fl T
Use TCP transport instead of UDP.
This is the default;
the flag is maintained for backwards compatibility.
.It Fl t Ar timeout
Set the initial retransmit timeout to the specified value in 0.1 seconds.
May be useful for fine tuning UDP mounts over internetworks
with high packet loss rates or an overloaded server.
Try increasing the interval if
.Xr 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.
The default is 3 seconds.
.It Fl U
Force the mount protocol to use UDP transport, even for TCP NFS mounts.
This is necessary for some old
.Bx
servers.
.It Fl u
Use UDP transport instead of TCP.
This may be necessary for some very old servers.
.It Fl w Ar writesize
Set the write data size to the specified value in bytes.
.Pp
The same logic applies for use of this option as with the
.Nm
.Fl r
option, but using the
.Dq "fragments dropped after timeout"
value on the NFS server instead of the client.
Note that both the
.Fl r
and
.Fl w
options should only be used as a last ditch effort at improving performance
when mounting servers that do not support TCP mounts.
.It Fl X
Perform 32 <-> 64 bit directory cookie translation for version 3 mounts.
This may be needed 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
.Nx
binaries will never need this option.
This option introduces some overhead.
.It Fl x Ar retrans
Set the retransmit timeout count for soft mounts to the specified value.
The default is 10.
.El
.Sh EXAMPLES
The simplest way to invoke
.Nm
is with a command like:
.Pp
.Dl "mount -t nfs remotehost:/filesystem /localmountpoint"
.Pp
It is also possible to automatically mount filesystems at boot from your
.Pa /etc/fstab
by using a line like:
.Pp
.Dl "remotehost:/home /home nfs rw 0 0"
.Sh PERFORMANCE
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 watch:
.Bl -bullet -offset indent
.It
Increasing the read and write size with the
.Fl r
and
.Fl w
options respectively will increase throughput if the network
interface can handle the larger packet sizes.
.Pp
The default size for NFS version 2 is 8K when using UDP,
64K when using TCP.
.Pp
The default size for NFS version 3 is platform dependent:
on
.Nx Ns /aarch64 ,
.Nx Ns /amd64 ,
and
.Nx Ns /i386 ,
the default is 32K, for other platforms it is 8K.
Values over 32K are only supported for TCP,
where 64K is the maximum.
.Pp
Any value over 32K is unlikely to get you more performance, unless
you have a very fast network.
.It
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.
.Pp
This can especially happen with older Ethernet network interfaces.
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 packets.
.Pp
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
.Pq e.g. a USB 1.1 to 10/100 Ethernet network interface ,
you can use
.Pp
.Bl -ohang -compact
.It Cd options NFS_RSIZE=value
.It Cd options NFS_WSIZE=value
.El
.Pp
in your kernel
.Xr config 1
file to avoid having do specify the sizes for all mounts.
.It
For connections that are not on the same LAN,
and/or may experience packet loss, using TCP is strongly recommended.
.El
.Sh ERRORS
Some common problems with
.Nm
can be difficult for first time users to understand.
.Pp
.Dl "mount_nfs: can't access /foo: Permission denied"
.Pp
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
.Xr exports 5
file is exporting the proper directories.
.Pp
A common mistake is that
.Xr mountd 8
will not export a filesystem with the
.Fl 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
.Fl alldirs
option.
.Pp
The following error:
.Pp
.Dl "NFS Portmap: RPC: Program not registered"
.Pp
means that the remote host is not running
.Xr mountd 8 .
The program
.Xr rpcinfo 8
can be used to determine if the remote host is running nfsd, and mountd by issuing
the command:
.Pp
.Dl rpcinfo -p remotehostname
.Pp
If the remote host is running nfsd, and mountd, it would display:
.Pp
.Dl "100005    3   udp    719  mountd"
.Dl "100005    1   tcp    720  mountd"
.Dl "100005    3   tcp    720  mountd"
.Dl "100003    2   udp   2049  nfs"
.Dl "100003    3   udp   2049  nfs"
.Dl "100003    2   tcp   2049  nfs"
.Dl "100003    3   tcp   2049  nfs"
.Pp
The error:
.Pp
.Dl "mount_nfs: can't get net id for host"
.Pp
indicates that
.Nm
cannot resolve the name of the remote host.
.Sh SEE ALSO
.Xr nfsstat 1 ,
.Xr mount 2 ,
.Xr unmount 2 ,
.Xr options 4 ,
.Xr exports 5 ,
.Xr fstab 5 ,
.Xr mount 8 ,
.Xr mountd 8 ,
.Xr rpcinfo 8
.Rs
.%R RFC 1094
.%D March 1989
.%T "NFS: Network File System Protocol specification"
.Re
.Rs
.%R RFC 2623
.%D June 1999
.%T "NFS Version 2 and Version 3 Security Issues and the NFS Protocol's Use of RPCSEC_GCC and Kerberos V5"
.Re
.Rs
.%R RFC 2624
.%D June 1999
.%T "NFS Version 4 Design Considerations"
.Re
.Rs
.%R RFC 2695
.%D September 1999
.%T "Authentication Mechanisms for ONC RPC"
.Re
.Sh HISTORY
A version of the
.Nm
utility appeared in
.Bx 4.4 .
.Sh CAVEATS
An NFS server should not mount its own exported file systems
.Pq loopback fashion
because doing so is fundamentally prone to deadlock.