MOUNT_NFS(8) BSD System Manager's Manual MOUNT_NFS(8)NAMEmount_nfs — mount NFS file systems
SYNOPSISmount_nfs [-o options] server:/path directory
DESCRIPTION
The mount_nfs command calls the mount(2) system call to prepare and graft
a remote NFS file system ( server:/path ) on to the file system tree at
the point directory.
This command is expected to be executed by the mount(8) command. Direct
use of mount_nfs to mount NFS file systems is strongly discouraged
because there is little practical benefit of using it instead of
mount(8).
For NFS versions that use a separate mount protocol, mount_nfs implements
the mount protocol as described in RFC 1094, Appendix A and NFS: Network
File System Version 3 Protocol Specification, RFC 1813, Appendix I.
By default, mount_nfs will attempt the mount twice before exiting with an
error. If the -o bg option is given, it will attempt the mount once and
then background itself to continue trying another 10,000 times (pausing
for one minute between attempts). The option -o retrycnt=⟨num⟩ can be
used if a different retry behavior is desired for a mount.
If the server becomes unresponsive while an NFS file system is mounted,
any new or outstanding file operations on that file system will hang
uninterruptibly until the server comes back (or that NFS file system is
forcibly unmounted). To modify this default behaviour, see the -o intr
and -o soft mount options.
Mount options are specified with a -o flag followed by a comma separated
string of options. See the mount(8) man page for possible options and
their meanings. The following NFS-specific options are also available:
bg Retry mount in background. If an initial attempt to contact the
server fails, fork off a child to keep trying the mount in the
background. Useful for startup scripts where the file system
mount is not critical to multiuser operation.
retrycnt=⟨num⟩
Set the retry count for doing the mount to the specified value.
The default is 1 for foreground mounts and 10,000 for background
mounts.
udp Use UDP transport protocol.
tcp Use the TCP transport protocol instead of UDP. The default is to
try TCP first, then fall back to UDP if the server doesn't sup‐
port TCP.
inet Use only IPv4 addresses.
inet6 Use only IPv6 addresses.
proto=⟨netid⟩
Use the transport protocol and address family as specified by the
given ONC RPC Netid (RFC 5665). Valid netid values are: tcp (TCP
over IPv4), udp (UDP over IPv4), tcp6 (TCP over IPv6), and udp6
(UDP over IPv6). Note that this option differs from the separate
tcp and udp options described above in that each netid value
specifies both a transport protocol and address family (IP ver‐
sion).
mntudp Force the mount protocol to use UDP transport, even for TCP NFS
mounts. (Necessary for some old BSD servers.)
mountport=⟨port-number⟩
Connect to the NFS server's mount daemon using the given port
number.
port=⟨port-number⟩
Connect to an NFS server at the given port number.
noconn Do not connect UDP sockets. 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 if replies
come from an address other than the one specified in the
requests.
resvport
Use a reserved socket port number. This is useful for mounting
servers that require clients to use a reserved port number on the
mistaken belief that this makes NFS more secure. (For the rare
case where the client has a trusted root account but untrustwor‐
thy users and the network cables are in secure areas this does
help, but for normal desktop clients this does not apply.)
intr 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.
soft Make the mount soft, which means that file system calls will fail
after retrans round trip timeout intervals. Note: mounts which
are both soft and read-only will also have the locallocks mount
option enabled by default - unless explicitly overridden with a
lock option (for example, nolocks or nolocallocks ).
vers=⟨num⟩
nfsvers=⟨num⟩
Set the NFS protocol version number - 2 for NFSv2, 3 for NFSv3
and 4 for NFSv4. The default is to try version 3 first, and fall
back to version 2 if the mount fails.
nfsv2
nfsv3
nfsv4 Deprecated. Use -o vers=⟨num⟩ to specify NFS protocol version.
sec=⟨mechanism⟩
Force a specific security mechanism to be used for the mount,
where mechanism is one of: krb5p, krb5i, krb5, or sys. When this
option is not given the security mechanism will be negotiated
transparently with the remote server.
realm=⟨realm⟩
Use the default credential for realm or security domain. For Ker‐
beros realms are usually uppercase. If the realm specified does
not begin with an “@”, an “@” sign will be prepended to it. Note
specifying the realm is typically used for automounter maps when
clients may have multiple credential caches, and tells the client
what cache to use on the mount.
principal=⟨principal⟩
Use the specified principal for acquiring credentials for the
mount. That principal will be used for all accesses by the
mounting credential on the mounted file system. Note specifying a
principal is useful for user initiated command line mounts, where
the user knows the particular credential to use.
sprincipal=⟨server-principal⟩
Use the specified server-principal for establishing credentials
for the mount. That server principal will be use for all mount
access. If no server principal is specified, then the
GSS_C_NT_HOSTBASED_SERVICE nfs@server is used, where server is
taken from the mount argument server:/path. Note its rare to use
this option.
rsize=⟨readsize⟩
Set the read data size to the specified value. The default is
8192 for UDP mounts and 32768 for TCP mounts. It should normally
be a power of 2 greater than or equal to 1024. Values greater
than 4096 should be multiples of 4096. It may need to be lowered
for UDP mounts when the “fragments dropped due to timeout” value
is getting large while actively using a mount point. (Use
netstat(1) with the -s option to see what the “fragments dropped
due to timeout” value is.)
wsize=⟨writesize⟩
Set the write data size to the specified value. Ditto the com‐
ments w.r.t. the rsize option, but using the “fragments dropped
due to timeout” value on the server instead of the client. Note
that both the rsize and wsize options should only be used as a
last ditch effort at improving performance when mounting servers
that do not support TCP mounts.
rwsize=⟨size⟩
Set both the read data size and write data size to the specified
value.
dsize=⟨readdirsize⟩
Set the directory read size to the specified value. The value
should normally be a multiple of DIRBLKSIZ that is <= the read
size for the mount. The default is 8192 for UDP mounts and 32768
for TCP mounts.
readahead=⟨maxreadahead⟩
Set the maximum read-ahead count to the specified value. The
default is 16. This may be in the range of 0 - 128, and deter‐
mines how many blocks will be read ahead when a large file is
being read sequentially. Trying larger values for this is sug‐
gested for mounts with a large bandwidth * delay product.
rdirplus
Used with NFS v3/v4 to specify that directory read operations
should retrieve additional information about each entry (e.g. use
the NFSv3 ReaddirPlus RPC). This option typically reduces RPC
traffic for cases such as directory listings that use or display
basic attributes (e.g. “ls -F” and “find . -type f” ). Note
that the long directory listing format case (i.e. “ls -l” ) may
not be helped much when the file system does not natively support
extended attributes. Older implementations tended to flood the
vnode and name caches with prefetched entries which may not be
referenced. The current implementation avoids creating those
entries until they are referenced. 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.
acregmin=⟨seconds⟩
acregmax=⟨seconds⟩
acdirmin=⟨seconds⟩
acdirmax=⟨seconds⟩
These options set the minimum and maximum attribute cache time‐
outs for directories and "regular" (non-directory) files. The
default minimum is 5 seconds and the default maximum is 60 sec‐
onds. Setting both the minimum and maximum to zero will disable
attribute caching. The algorithm to calculate the timeout is
based on the age of the file or directory. The older it is, the
longer the attribute cache is considered valid, subject to the
limits above. Note that the effectiveness of this algorithm
depends on how well the clocks on the client and server are syn‐
chronized.
actimeo=⟨seconds⟩
Set all attribute cache timeouts to the same value.
noac Disable attribute caching. Equivalent to setting actimeo to 0.
nonegnamecache
Disable negative name caching.
locallocks
Perform all file locking operations locally on the NFS client (in
the VFS layer) instead of on the NFS server. This option can
provide file locking support on an NFS file system for which the
server does not support file locking. However, because the file
locking is only performed on the client, the NFS server and other
NFS clients will have no knowledge of the locks. Note: mounts
which are both soft and read-only will also have the locallocks
mount option enabled by default - unless explicitly overridden
with a lock option (for example, nolocks or nolocallocks ).
nolocks
nolockd
nolock
nonlm Do not support NFS file locking operations. Any attempt to per‐
form file locking operations on this mount will return the error
ENOTSUP regardless of whether or not the NFS server supports NFS
file locking.
noquota
Do not support file system quota operations that would normally
be serviced by using the RQUOTA protocol. Any attempt to perform
quota operations on this mount will return the error ENOTSUP
regardless of whether or not the NFS server supports the RQUOTA
service.
maxgroups=⟨num⟩
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.
dumbtimer
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.
timeo=⟨timeout⟩
Set the initial retransmit timeout to the specified value. The
default is 1 second. 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 dumbtimer option should be specified
when using this option to manually tune the timeout interval.)
retrans=⟨count⟩
Set the retransmit timeout count for soft mounts to the specified
value. The default value is 10.
deadtimeout=⟨timeout⟩
If the mount is still unresponsive timeout seconds after it is
initially reported unresponsive, then mark the mount as dead so
that it will be forcibly unmounted. Note: mounts which are both
soft and read-only will also have the deadtimeout mount option
set to 60 seconds. This can be explicitly overridden by setting
deadtimeout=0.
mutejukebox
When NFS requests repeatedly get jukebox errors (NFS3ERR_JUKEBOX,
NFS4ERR_DELAY) from the server the NFS file system is reported as
being unresponsive. Use of this option will prevent the file
system from being included in the list of unresponsive file sys‐
tems that would be included in a dialog presented to the user.
This option may be useful when a file system is expected to get
such errors during normal operation. For example, when it's
backed by a hierarchical storage management system.
async Assume that unstable write requests have actually been committed
to stable storage on the server, and thus will not require
resending in the event that the server crashes. Use of this
option may improve performance but only at the risk of data loss
if the server crashes. Note: this mount option will only be hon‐
ored if the nfs.client.allow_async option in nfs.conf(5) is also
enabled.
sync Perform I/O requests (specifically, write requests) syn‐
chronously. The operation will not return until a response is
received from the server. (The default, nosync, behavior is to
return once the I/O has been queued up.)
nocallback
For NFSv4 mounts, don't support callback requests from the
server. This should effectively disable features that require
callback requests such as delegations.
nonamedattr
For NFSv4 mounts, don't support named attributes even if the
server does. By default, if the server appears to support named
attributes, they will be used to store extended attributes and
named streams (e.g. FinderInfo and resource forks).
noacl For NFSv4 mounts, don't support ACLs even if the server does.
ACLs are currently disabled by default to avoid issues with the
way ACLs and modes are handled differently on other operating
systems. This may be overriden by specifying the acl option.
aclonly
For NFSv4 mounts, only support ACLs; do not support the mode
attribute. (Any mode attribute values returned will have all
permission bits set - regardless of the value of any ACL or
access mode stored in the file system.) This option overrides
the noacl option.
nfc Convert name strings to Unicode Normalization Form C (NFC) when
sending them to the NFS server. This option may be used to
improve interoperability with NFS clients and servers that typi‐
cally use names in the NFC form.
nfs.conf(5) can be used to configure some NFS client options. In partic‐
ular, nfs.client.mount.options can be used to specify default mount
options. This can be useful in situations where it is not easy to con‐
figure the command-line options. Some NFS client options in nfs.conf(5)
correspond to kernel configuration values which will get set by mount_nfs
when performing a mount. To update these values without performing a
mount, use the command: mount_nfs configupdate.
COMPATIBILITY
The following mount_nfs command line flags have equivalent -o option
forms (shown in parentheses) and their use is strongly discouraged.
These command line flags are deprecated and the -o option forms should be
used instead.
-2 (vers=2), -3 (vers=3), -4 (vers=4), -L (nolocks), -P (resvport), -T
(tcp), -U (mntudp), -b (bg), -c (noconn), -d (dumbtimer), -i (intr), -l
(rdirplus), -s (soft), -I readdirsize (dsize=#), -R retrycnt
(retrycnt=#), -a maxreadahead (readahead=#), -g maxgroups (maxgroups=#),
-r readsize (rsize=#), -t timeout (timeo=#), -w writesize (wsize=#), -x
retrans (retrans=#).
EXAMPLES
The simplest way to invoke mount_nfs is with a command like:
mount remotehost:/filesystem /localmountpoint
or:
mount -t nfs remotehost:/filesystem /localmountpoint
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:
· Use of the sync option will probably have a detrimental affect on
performance. Its use is discouraged as it provides little bene‐
fit.
· Use of the async option may improve performance, but only at the
risk of losing data if the server crashes because the client will
not be making sure that all data is committed to stable storage
on the server.
· Increasing the read and write size with the rsize and wsize
options respectively will increase throughput if the network
interface can handle the larger packet sizes.
The default read and write sizes are 8K when using UDP, and 32K
when using TCP. Values over 16K are only supported for TCP,
where 64K is the maximum.
Any value over 32K is unlikely to get you more performance,
unless you have a very fast network.
· 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.
This can especially happen with lossy network connections (e.g.
wireless networks) which can lead to a lot of dropped packets.
In this case, decreasing the read and write size, using TCP, or a
combination of both will usually lead to better throughput.
· For connections that are not on the same LAN, and/or may experi‐
ence packet loss, using TCP is strongly recommended.
ERRORS
Some common problems with mount_nfs can be difficult for first time users
to understand.
mount_nfs: can't access /foo: Permission denied
This message means that the remote host is either not exporting the file
system you requested or is not exporting it to your host. If you believe
the remote host is indeed exporting a file system to you, make sure the
exports(5) file is exporting the proper directories. The program
showmount(8) can be used to see a server's exports list. The command
“showmount -e remotehostname” will display what file systems the remote
host is exporting.
A common mistake is that mountd(8) will not export a file system with the
-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 -alldirs option.
The following error:
NFS Portmap: RPC: Program not registered
means that the remote host is not running nfsd(8). or mountd(8). The
program rpcinfo(8) can be used to determine if the remote host is running
nfsd and mountd by issuing the command:
rpcinfo -p remotehostname
If the remote host is running nfsd, mountd, rpc.statd, and rpc.lockd it
would display:
program vers proto port
100000 2 tcp 111 portmapper
100000 2 udp 111 portmapper
100005 1 udp 950 mountd
100005 3 udp 950 mountd
100005 1 tcp 884 mountd
100005 3 tcp 884 mountd
100003 2 udp 2049 nfs
100003 3 udp 2049 nfs
100003 2 tcp 2049 nfs
100003 3 tcp 2049 nfs
100024 1 udp 644 status
100024 1 tcp 918 status
100021 0 udp 630 nlockmgr
100021 1 udp 630 nlockmgr
100021 3 udp 630 nlockmgr
100021 4 udp 630 nlockmgr
100021 0 tcp 917 nlockmgr
100021 1 tcp 917 nlockmgr
100021 3 tcp 917 nlockmgr
100021 4 tcp 917 nlockmgr
The error:
mount_nfs: can't resolve host
indicates that mount_nfs could not resolve the name of the remote host.
SEE ALSOmount(2), unmount(2), mount(8), umount(8), nfsstat(1), netstat(1),
rpcinfo(8), showmount(8), automount(8), nfs.conf(5)CAVEATS
An NFS server shouldn't loopback-mount its own exported file systems
because it's fundamentally prone to deadlock.
BSD February 28, 2010 BSD
