MOUNT_NFS(8) BSD System Manager's Manual MOUNT_NFS(8)
mount_nfs - mount NFS filesystems
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 filesystem (rhost:path) on to the filesystem 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
Filesystem 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 filesystem mount is not critical to multiuser
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 filesystem 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 in-
terconnects 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 directories (de-
fault).
acregmax=num
Cache file attributes for no more than num seconds. The de-
fault is 60 seconds.
acregmin=num
Cache file attributes for at least num seconds. The default
is 5 seconds.
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 de-
fault 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 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 filesystem calls will fail after
retrans round trip timeout intervals have been reached (see -x).
-T Use TCP instead of UDP. This is recommended for servers that are
not on the same LAN cable as the client. (NB: This is NOT sup-
ported by most non-BSD servers.)
-t 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 in-
terval if nfsstat(1) shows high retransmit rates while the
filesystem is active or reducing the value if there is a low re-
transmit 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.)
-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.
nfsstat(1), mount(2), tcp(4), udp(4), fstab(5), mount(8), nfsd(8),
sysctl(8), umount(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.
Upon the first NFS mount, a number of kernel threads is started, which
currently, due to unknown bugs, can cause the system to hang/crash upon
shutdown, leaving an open RAID dirty.
Due to the way that Sun RPC is implemented on top of UDP (unreliable da-
tagram), tuning such mounts is really a black art that can only be ex-
pected to have limited success. For clients mounting servers that are not
on the same LAN cable or that tend to be overloaded, TCP is strongly
recommended, but unfortunately this is restricted to mostly 4.4BSD
servers.
MirBSD #10-current March 29, 1995 2