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add support for modifying attribute cache parameters
inspired by PR2567, the included diff was wrong tho and not used here
manpage from millert@
ok millert@ on code and jmc@ on manpage

.\"	$OpenBSD: mount_nfs.8,v 1.32 2003/10/07 18:35:57 henning Exp $
.\"	$NetBSD: mount_nfs.8,v 1.3 1996/02/18 11:59:10 fvdl Exp $
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.\"	@(#)mount_nfs.8	8.3 (Berkeley) 3/29/95
.\"
.Dd March 29, 1995
.Dt MOUNT_NFS 8
.Os
.Sh NAME
.Nm mount_nfs
.Nd mount NFS file systems
.Sh SYNOPSIS
.Nm mount_nfs
.Bk -words
.Op Fl 23PTUbcdilqs
.Op Fl D Ar deadthresh
.Op Fl I Ar readdirsize
.Op Fl L Ar leaseterm
.Op Fl R Ar retrycnt
.Op Fl a Ar maxreadahead
.Op Fl g Ar maxgroups
.Op Fl o Ar options
.Op Fl r Ar readsize
.Op Fl t Ar timeout
.Op Fl w Ar writesize
.Op Fl x Ar retrans
.Ar rhost:path node
.Ek
.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 point
.Ar node .
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 as follows:
.Bl -tag -width Ds
.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 I Ar readdirsize
Set the readdir read size to the specified value.
The value should normally
be a multiple of DIRBLKSIZ that is <= the read size for the mount.
.It Fl P
The kernel always uses a reserved port number to communicate with
clients.
This option is ignored, and exists solely for compatibility
with older systems.
.It Fl R Ar retrycnt
Set the retry count for doing the mount to the specified value.
The default is 10000.
.It Fl T
Use TCP transport instead of UDP.
This is recommended for servers that are not on the same LAN cable as
the client.
(NB: This is NOT supported by most non-BSD servers.)
.It Fl U
Force the mount protocol to use UDP transport, even for TCP NFS mounts.
(Necessary for some old BSD servers.)
.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 not do a
.Xr 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).
.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 a response from the mount
point.
.It Fl 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.
.It Fl l
Used with NFSV3 to specify that the \fBReaddirPlus\fR RPC should
be used.
This option reduces RPC traffic for cases such as
.Dq "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.
The following NFS specific options are also available:
.Bl -tag -width indent
.It acregmax=num
Cache file attributes for no more than
.Ar num
seconds.
The default is 60 seconds.
.It acregmin=num
Cache file attributes for at least
.Ar num
seconds.
The default is 5 seconds.
.It acdirmax=num
Cache directory attributes for no more than
.Ar num
seconds.
The default is 60 seconds.
.It acdirmin=num
Cache directory attributes for at least
.Ar num
seconds.
The default is 5 seconds.
.It noac
Disable attribute caching for both files and directories.
.It port=portnumber
Use specified port number for NFS requests.
The default is to query the portmapper for the NFS port.
.El
.It Fl r Ar 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
.Dq "fragments dropped due to 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 due to timeout"
value is.)
See the
.Fl w
option as well.
.It Fl s
A soft mount, which implies that file system calls will fail
after \fBRetry\fR round trip timeout intervals.
.It Fl t Ar timeout
Set the initial retransmit timeout to the specified value.
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
.Fl d
option should be specified when using this option to manually
tune the timeout
interval.)
.It Fl w Ar writesize
Set the write data size to the specified value.
Ditto the comments w.r.t. the
.Fl r
option, but using the
.Dq "fragments dropped due to timeout"
value on the 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 Ar retrans
Set the retransmit timeout count for soft mounts to the specified value.
.El
.Pp
In versions prior to
.Ox 2.7 ,
.Nm nfsiod
daemons were running to improve performance of client NFS I/O.
This is no longer done this way.
Use
.Xr sysctl 8
or modify
.Xr sysctl.conf 5
to adjust the
.Nm vfs.nfs.iothreads
value, which is the number of kernel threads created
to serve asynchronous NFS I/O requests.
.Sh SEE ALSO
.Xr mount 2 ,
.Xr fstab 5 ,
.Xr mount 8 ,
.Xr nfsd 8 ,
.Xr sysctl 8 ,
.Xr umount 8
.Sh BUGS
Due to the way that Sun RPC is implemented on top of UDP (unreliable datagram)
transport, tuning such mounts is really a black art that can only be expected
to have limited success.
For clients mounting servers that are not on the same
LAN cable or that tend to be overloaded,
TCP transport is strongly recommended,
but unfortunately this is restricted to mostly
.Bx 4.4
servers.