nfs(5): Replace the term "netid" in mount option descriptions

[nfs-utils.git] / utils / mount / nfs.man

.\"@(#)nfs.5"
.TH NFS 5 "2 November 2007"
.SH NAME
nfs \- fstab format and options for the
.B nfs
and
.B nfs4
file systems
.SH SYNOPSIS
.I /etc/fstab
.SH DESCRIPTION
NFS is an Internet Standard protocol
created by Sun Microsystems in 1984. NFS was developed
to allow file sharing between systems residing
on a local area network.
The Linux NFS client supports three versions
of the NFS protocol:
NFS version 2 [RFC1094],
NFS version 3 [RFC1813],
and NFS version 4 [RFC3530].
.P
The
.BR mount (8)
command attaches a file system to the system's
name space hierarchy at a given mount point.
The
.I /etc/fstab
file describes how
.BR mount (8)
should assemble a system's file name hierarchy
from various independent file systems 
(including file systems exported by NFS servers).
Each line in the
.I /etc/fstab
file describes a single file system, its mount point,
and a set of default mount options for that mount point.
.P
For NFS file system mounts, a line in the
.I /etc/fstab
file specifies the server name,
the path name of the exported server directory to mount,
the local directory that is the mount point,
the type of file system that is being mounted,
and a list of mount options that control
the way the filesystem is mounted and
how the NFS client behaves when accessing
files on this mount point.
The fifth and sixth fields on each line are not used
by NFS, thus conventionally each contain the digit zero. For example:
.P
.SP
.NF
.TA 2.5i +0.75i +0.75i +1.0i
        server:path     /mountpoint     fstype  option,option,...       0 0
.FI
.P
The server's hostname and export pathname
are separated by a colon, while
the mount options are separated by commas. The remaining fields 
are separated by blanks or tabs.
The server's hostname can be an unqualified hostname,
a fully qualified domain name,
or a dotted quad IPv4 address.
The
.I fstype
field contains either "nfs" (for version 2 or version 3 NFS mounts)
or "nfs4" (for NFS version 4 mounts).
The
.B nfs
and
.B nfs4
file system types share similar mount options,
which are described below. 
.SH "MOUNT OPTIONS"
Refer to 
.BR mount (8)
for a description of generic mount options
available for all file systems. If you do not need to 
specify any mount options, use the generic option 
.B defaults
in
.IR /etc/fstab .
. 
.DT
.SS "Valid options for either the nfs or nfs4 file system type"
These options are valid to use when mounting either
.B nfs
or
.B nfs4
file system types.
They imply the same behavior
and have the same default for both file system types.
.TP 1.5i
.BR soft " / " hard
Determines the recovery behavior of the NFS client
after an NFS request times out.
If neither option is specified (or if the
.B hard
option is specified), NFS requests are retried indefinitely.
If the
.B soft
option is specified, then the NFS client fails an NFS request
after 
.B retrans
retransmissions have been sent,
causing the NFS client to return an error
to the calling application.
.IP
.I NB:
A so-called "soft" timeout can cause
silent data corruption in certain cases. As such, use the
.B soft
option only when client responsiveness
is more important than data integrity.
Using NFS over TCP or increasing the value of the
.B retrans
option may mitigate some of the risks of using the
.B soft
option.
.TP 1.5i
.BI timeo= n
The time (in tenths of a second) the NFS client waits for a 
response before it retries an NFS request. If this 
option is not specified, requests are retried every
60 seconds for NFS over TCP.
The NFS client does not perform any kind of timeout backoff
for NFS over TCP.
.IP
However, for NFS over UDP, the client uses an adaptive
algorithm to estimate an appropriate timeout value for frequently used
request types (such as READ and WRITE requests), but uses the 
.B timeo
setting for infrequently used request types (such as FSINFO requests).
If the
.B timeo
option is not specified,
infrequently used request types are retried after 1.1 seconds.
After each retransmission, the NFS client doubles the timeout for
that request,
up to a maximum timeout length of 60 seconds.
.TP 1.5i
.BI retrans= n
The number of times the NFS client retries a request before
it attempts further recovery action. If the 
.B retrans
option is not specified, the NFS client tries each request 
three times.
.IP
The NFS client generates a "server not responding" message
after 
.B retrans
retries, then attempts further recovery (depending on whether the
.B hard
mount option is in effect).
.TP 1.5i
.BI rsize= n
The maximum number of bytes in each network READ request
that the NFS client can receive when reading data from a file
on an NFS server.
The actual data payload size of each NFS READ request is equal to
or smaller than the
.B rsize
setting. The largest read payload supported by the Linux NFS client
is 1,048,576 bytes (one megabyte).
.IP
The
.B rsize
value is a positive integral multiple of 1024.
Specified 
.B rsize
values lower than 1024 are replaced with 4096; values larger than
1048576 are replaced with 1048576. If a specified value is within the supported
range but not a multiple of 1024, it is rounded down to the nearest 
multiple of 1024.
.IP
If an
.B rsize
value is not specified, or if the specified 
.B rsize 
value is larger than the maximum that either client or server can support,
the client and server negotiate the largest
.B rsize
value that they can both support. 
.IP
The
.B rsize
mount option as specified on the
.BR mount (8)
command line appears in the
.I /etc/mtab
file. However, the effective
.B rsize
value negotiated by the client and server is reported in the
.I /proc/mounts
file.
.TP 1.5i
.BI wsize= n
The maximum number of bytes per network WRITE request
that the NFS client can send when writing data to a file
on an NFS server. The actual data payload size of each 
NFS WRITE request is equal to
or smaller than the
.B wsize
setting. The largest write payload supported by the Linux NFS client
is 1,048,576 bytes (one megabyte).
.IP
Similar to
.B rsize
, the
.B wsize 
value is a positive integral multiple of 1024.
Specified 
.B wsize
values lower than 1024 are replaced with 4096; values larger than
1048576 are replaced with 1048576. If a specified value is within the supported
range but not a multiple of 1024, it is rounded down to the nearest 
multiple of 1024.
.IP
If a
.B wsize
value is not specified, or if the specified 
.B wsize 
value is larger than the maximum that either client or server can support,
the client and server negotiate the largest
.B wsize
value that they can both support.
.IP
The
.B wsize
mount option as specified on the
.BR mount (8)
command line appears in the
.I /etc/mtab
file. However, the effective
.B wsize
value negotiated by the client and server is reported in the
.I /proc/mounts
file. 
.TP 1.5i
.BR ac " / " noac
Selects whether the client may cache file attributes. If neither  
option is specified (or if 
.B ac
is specified), the client caches file  
attributes.  
.IP
To improve performance, NFS clients cache file  
attributes. Every few seconds, an NFS client checks the server's version of each  
file's attributes for updates.  Changes that occur on the server in  
those small intervals remain undetected until the client checks the  
server again. The 
.B noac
option prevents clients from caching file  
attributes so that applications can more quickly detect file changes  
on the server.
.IP
In addition to preventing the client from caching file attributes,  
the 
.B noac
option forces application writes to become synchronous so  
that local changes to a file become visible on the server  
immediately.  That way, other clients can quickly detect recent  
writes when they check the file's attributes.
.IP
Using the
.B noac
option provides greater cache coherence among NFS clients
accessing the same files,
but it extracts a significant performance penalty.
As such, judicious use of file locking is encouraged instead.
The DATA AND METADATA COHERENCE section contains a detailed discussion
of these trade-offs.
.TP 1.5i
.BI acregmin= n
The minimum time (in seconds) that the NFS client caches
attributes of a regular file before it requests
fresh attribute information from a server.
If this option is not specified, the NFS client uses
a 3-second minimum.
.TP 1.5i
.BI acregmax= n
The maximum time (in seconds) that the NFS client caches
attributes of a regular file before it requests
fresh attribute information from a server.
If this option is not specified, the NFS client uses
a 60-second maximum.
.TP 1.5i
.BI acdirmin= n
The minimum time (in seconds) that the NFS client caches
attributes of a directory before it requests
fresh attribute information from a server.
If this option is not specified, the NFS client uses
a 30-second minimum.
.TP 1.5i
.BI acdirmax= n
The maximum time (in seconds) that the NFS client caches
attributes of a directory before it requests
fresh attribute information from a server.
If this option is not specified, the NFS client uses
a 60-second maximum.
.TP 1.5i
.BI actimeo= n
Using
.B actimeo
sets all of
.BR acregmin ,
.BR acregmax ,
.BR acdirmin ,
and
.B acdirmax
to the same value.
If this option is not specified, the NFS client uses
the defaults for each of these options listed above.
.TP 1.5i
.BR bg " / " fg
Determines how the
.BR mount (8)
command behaves if an attempt to mount an export fails.
The
.B fg
option causes
.BR mount (8)
to exit with an error status if any part of the mount request
times out or fails outright.
This is called a "foreground" mount,
and is the default behavior if neither the
.B fg
nor
.B bg
mount option is specified.
.IP
If the
.B bg
option is specified, a timeout or failure causes the
.BR mount (8)
command to fork a child which continues to attempt
to mount the export.
The parent immediately returns with a zero exit code.
This is known as a "background" mount.
.IP
If the local mount point directory is missing, the
.BR mount (8)
command acts as if the mount request timed out.
This permits nested NFS mounts specified in
.I /etc/fstab
to proceed in any order during system initialization,
even if some NFS servers are not yet available.
Alternatively these issues can be addressed
using an automounter (refer to
.BR automount (8)
for details).
.TP 1.5i
.BI retry= n
The number of minutes that the
.BR mount (8)
command retries an NFS mount operation
in the foreground or background before giving up.
If this option is not specified, the default value for foreground mounts
is 2 minutes, and the default value for background mounts is 10000 minutes (80 minutes shy of one week).
.TP 1.5i
.BI sec= mode
The RPCGSS security flavor to use for accessing files on this mount point.
If the
.B sec
option is not specified, or if
.B sec=sys
is specified, the NFS client uses the AUTH_SYS security flavor
for all NFS requests on this mount point.
Valid security flavors are
.BR none ,
.BR sys ,
.BR krb5 ,
.BR krb5i ,
.BR krb5p ,
.BR lkey ,
.BR lkeyi ,
.BR lkeyp ,
.BR spkm ,
.BR spkmi ,
and
.BR spkmp .
Refer to the SECURITY CONSIDERATIONS section for details.
.TP 1.5i
.BR sharecache " / " nosharecache
Determines how the client's data cache and attribute cache are shared
when mounting the same export more than once concurrently.  Using the  
same cache reduces memory requirements on the client and presents  
identical file contents to applications when the same remote file is  
accessed via different mount points.
.IP
If neither option is specified, or if the 
.B sharecache
option is  
specified, then a single cache is used for all mount points that  
access the same export.  If the 
.B nosharecache
option is specified,  
then that mount point gets a unique cache.  Note that when data and  
attribute caches are shared, the mount options from the first mount  
point take effect for subsequent concurrent mounts of the same export.
.IP
As of kernel 2.6.18, the behavior specified by
.B nosharecache
is legacy caching behavior. This
is considered a data risk since multiple cached copies
of the same file on the same client can become out of sync
following a local update of one of the copies.
.SS "Valid options for the nfs file system type"
Use these options, along with the options in the above subsection,
for mounting the
.B nfs
file system type.
.TP 1.5i
.BI proto= transport
The transport the NFS client uses
to transmit requests to the NFS server for this mount point.
.I transport
can be either
.B udp
or
.BR tcp .
Each transport uses different default
.B retrans
and
.B timeo
settings; refer to the description of these two mount options for details.
.IP
In addition to controlling how the NFS client transmits requests to
the server, this mount option also controls how the
.BR mount (8)
command communicates with the server's rpcbind and mountd services.
Specifying 
.B proto=tcp
forces all traffic from the 
.BR mount (8)
command and the NFS client to use TCP.
Specifying
.B proto=udp
forces all traffic types to use UDP.
.IP
If the
.B proto
mount option is not specified, the
.BR mount (8)
command discovers which protocols the server supports
and chooses an appropriate transport for each service.
Refer to the TRANSPORT METHODS section for more details.
.TP 1.5i
.B udp
The
.B udp
option is an alternative to specifying
.BR proto=udp.
It is included for compatibility with other operating systems.
.TP 1.5i
.B tcp
The
.B tcp
option is an alternative to specifying
.BR proto=tcp.
It is included for compatibility with other operating systems.
.TP 1.5i
.BI port= n
The numeric value of the server's NFS service port.
If the server's NFS service is not available on the specified port,
the mount request fails.
.IP
If this option is not specified, or if the specified port value is 0,
then the NFS client uses the NFS service port number
advertised by the server's rpcbind service.
The mount request fails if the server's rpcbind service is not available,
the server's NFS service is not registered with its rpcbind service,
or the server's NFS service is not available on the advertised port.
.TP 1.5i
.BI mountport= n
The numeric value of the server's mountd port.
If the server's mountd service is not available on the specified port,
the mount request fails.
.IP
If this option is not specified,
or if the specified port value is 0, then the
.BR mount (8)
command uses the mountd service port number
advertised by the server's rpcbind service.
The mount request fails if the server's rpcbind service is not available,
the server's mountd service is not registered with its rpcbind service,
or the server's mountd service is not available on the advertised port.
.IP
This option can be used when mounting an NFS server
through a firewall that blocks the rpcbind protocol.
.TP 1.5i
.BI mountproto= transport
The transport the NFS client uses
to transmit requests to the NFS server's mountd service when performing
this mount request, and when later unmounting this mount point.
.I transport
can be either
.B udp
or
.BR tcp .
.IP
This option can be used when mounting an NFS server
through a firewall that blocks a particular transport.
When used in combination with the
.B proto
option, different transports for mountd requests and NFS requests
can be specified.
If the server's mountd service is not available via the specified
transport, the mount request fails.
.TP 1.5i
.BI mounthost= name
The hostname of the host running mountd.
If this option is not specified, the
.BR mount (8)
command assumes that the mountd service runs
on the same host as the NFS service.
.TP 1.5i
.BI mountvers= n
The RPC version number used to contact the server's mountd.
If this option is not specified, the client uses a version number
appropriate to the requested NFS version.
This option is useful when multiple NFS services
are running on the same remote server host.
.TP 1.5i
.BI namlen= n
The maximum length of a pathname component on this mount.
If this option is not specified, the maximum length is negotiated
with the server. In most cases, this maximum length is 255 characters.
.IP
Some early versions of NFS did not support this negotiation.
Using this option ensures that
.BR pathconf (3)
reports the proper maximum component length to applications
in such cases.
.TP 1.5i
.BI nfsvers= n
The NFS protocol version number used to contact the server's NFS service.
The Linux client supports version 2 and version 3 of the NFS protocol
when using the file system type
.BR nfs .
If the server does not support the requested version,
the mount request fails.
If this option is not specified, the client attempts to use version 3,
but negotiates the NFS version with the server if version 3 support
is not available.
.TP 1.5i
.BI vers= n
This option is an alternative to the
.B nfsvers
option.
It is included for compatibility with other operating systems.
.TP 1.5i
.BR lock " / " nolock
Selects whether to use the NLM sideband protocol to lock files on the server.
If neither option is specified (or if 
.B lock 
is specified), NLM locking is used for this mount point. 
When using the
.B nolock
option, applications can lock files,
but such locks provide exclusion only against other applications
running on the same client.
Remote applications are not affected by these locks.
.IP
NLM locking must be disabled with the
.B nolock
option when using NFS to mount
.I /var
because
.I /var
contains files used by the NLM implementation on Linux.
Using the
.B nolock
option is also required when mounting exports on NFS servers
that do not support the NLM protocol.
.TP 1.5i
.BR intr " / " nointr
Selects whether to allow signals to interrupt file operations
on this mount point. If neither option 
is specified (or if 
.B nointr
is specified),
signals do not interrupt NFS file operations. If
.B intr 
is specified, system calls return EINTR if an in-progress NFS operation is interrupted by 
a signal.
.IP
Using the
.B intr
option is preferred to using the
.B soft
option because it is significantly less likely to result in data corruption.
.TP 1.5i
.BR cto " / " nocto
Selects whether to use close-to-open cache coherence semantics.
If neither option is specified (or if 
.B cto
is specified), the client uses close-to-open
cache coherence semantics. If the 
.B nocto 
option is specified, the client uses a non-standard heuristic to determine when
files on the server have changed. 
.IP
Using the
.B nocto
option may improve performance for read-only mounts,
but should be used only if the data on the server changes only occasionally.
The DATA AND METADATA COHERENCE section discusses the behavior
of this option in more detail.
.TP 1.5i
.BR acl " / " noacl
Selects whether to use the NFSACL sideband protocol on this mount point.
The NFSACL sideband protocol is a proprietary protocol
implemented in Solaris that manages Access Control Lists. NFSACL was never 
made a standard part of the NFS protocol specification.
.IP
If neither 
.B acl
nor 
.B noacl 
option is specified,
the NFS client negotiates with the server
to see if the NFSACL protocol is supported,
and uses it if the server supports it.
Disabling the NFSACL sideband protocol may be necessary
if the negotiation causes problems on the client or server.
Refer to the SECURITY CONSIDERATIONS section for more details.
.TP 1.5i
.BR rdirplus " / " nordirplus
Selects whether to use NFS version 3 READDIRPLUS requests.
If this option is not specified, the NFS client uses READDIRPLUS requests
on NFS version 3 mounts to read small directories.
Some applications perform better if the client uses only READDIR requests
for all directories.  
.SS "Valid options for the nfs4 file system type"
Use these options, along with the options in the first subsection above,
for mounting the
.B nfs4
file system type.
.TP 1.5i
.BI proto= transport
The transport the NFS client uses
to transmit requests to the NFS server for this mount point.
.I transport
can be either
.B udp
or
.BR tcp .
All NFS version 4 servers are required to support TCP,
so if this mount option is not specified, the NFS version 4 client 
uses the TCP transport protocol. 
Refer to the TRANSPORT METHODS section for more details.
.TP 1.5i
.BI port= n
The numeric value of the server's NFS service port.
If the server's NFS service is not available on the specified port,
the mount request fails.
.IP
If this mount option is not specified,
the NFS client uses the standard NFS port number of 2049
without first checking the server's rpcbind service.
This allows an NFS version 4 client to contact an NFS version 4
server through a firewall that may block rpcbind requests.
.IP
If the specified port value is 0,
then the NFS client uses the NFS service port number
advertised by the server's rpcbind service.
The mount request fails if the server's rpcbind service is not available,
the server's NFS service is not registered with its rpcbind service,
or the server's NFS service is not available on the advertised port.
.TP 1.5i
.BR intr " / " nointr
Selects whether to allow signals to interrupt file operations
on this mount point. If neither option is specified (or if 
.B intr 
is specified), system calls return EINTR if an in-progress NFS operation  
is interrupted by a signal.  If 
.B nointr
is specified, signals do not  
interrupt NFS operations.
.IP
Using the
.B intr
option is preferred to using the
.B soft
option because it is significantly less likely to result in data corruption.
.TP 1.5i
.BR cto " / " nocto
Selects whether to use close-to-open cache coherence semantics
for NFS directories on this mount point.
If neither
.B cto
nor
.B nocto 
is specified,
the default is to use close-to-open cache coherence
semantics for directories.
.IP
File data caching behavior is not affected by this option.
The DATA AND METADATA COHERENCE section discusses
the behavior of this option in more detail.
.TP 1.5i
.BI clientaddr= n.n.n.n
Specifies  a  single  IPv4  address  (in dotted-quad form) 
that the NFS client advertises to allow servers 
to perform NFS version 4 callback requests against 
files on this mount point. If  the  server is unable to 
establish callback connections to clients, performance 
may degrade, or accesses to files may temporarily hang.
.IP
If this option is not specified, the
.BR mount (8)
command attempts to discover an appropriate callback address automatically.
The automatic discovery process is not perfect, however.
In the presence of multiple client network interfaces,
special routing policies,
or atypical network topologies,
the exact address to use for callbacks may be nontrivial to determine. 
.SH EXAMPLES
To mount an export using NFS version 2,
use the
.B nfs
file system type and specify the
.B nfsvers=2
mount option.
To mount using NFS version 3,
use the
.B nfs
file system type and specify the
.B nfsvers=3
mount option.
To mount using NFS version 4,
use the
.B nfs4
file system type.
The
.B nfsvers
mount option is not supported for the
.B nfs4
file system type.
.P
The following example from an
.I /etc/fstab
file causes the mount command to negotiate
reasonable defaults for NFS behavior.
.P
.NF
.TA 2.5i +0.7i +0.7i +.7i
        server:/export  /mnt    nfs     defaults        0 0
.FI
.P
Here is an example from an /etc/fstab file for an NFS version 2 mount over UDP.
.P
.NF
.TA 2.5i +0.7i +0.7i +.7i
        server:/export  /mnt    nfs     nfsvers=2,proto=udp     0 0
.FI
.P
Try this example to mount using NFS version 4 over TCP
with Kerberos 5 mutual authentication.
.P
.NF
.TA 2.5i +0.7i +0.7i +.7i
        server:/export  /mnt    nfs4    sec=krb5        0 0
.FI
.P
This example can be used to mount /usr over NFS.
.P
.NF
.TA 2.5i +0.7i +0.7i +.7i
        server:/export  /usr    nfs     ro,nolock,nocto,actimeo=3600    0 0
.FI
.SH "TRANSPORT METHODS"
NFS clients send requests to NFS servers via
Remote Procedure Calls, or
.IR RPCs .
The RPC client discovers remote service endpoints automatically,
handles per-request authentication,
adjusts request parameters for different byte endianness on client and server,
and retransmits requests that may have been lost by the network or server.
RPC requests and replies flow over a network transport.
.P
In most cases, the
.BR mount (8)
command, NFS client, and NFS server
can automatically negotiate proper transport
and data transfer size settings for a mount point.
In some cases, however, it pays to specify
these settings explicitly using mount options.
.P
Traditionally, NFS clients used the UDP transport exclusively for  
transmitting requests to servers.  Though its implementation is  
simple, NFS over UDP has many limitations that prevent smooth  
operation and good performance in some common deployment  
environments.  Even an insignificant packet loss rate results in the  
loss of whole NFS requests; as such, retransmit timeouts are usually  
in the subsecond range to allow clients to recover quickly from  
dropped requests, but this can result in extraneous network traffic  
and server load.
.P
However, UDP can be quite effective in specialized settings where  
the network’s MTU is large relative to NFS’s data transfer size (such  
as network environments that enable jumbo Ethernet frames).  In such  
environments, trimming the 
.B rsize 
and 
.B wsize 
settings so that each  
NFS read or write request fits in just a few network frames (or even  
in  a single  frame) is advised.  This reduces the probability that  
the loss of a single MTU-sized network frame results in the loss of  
an entire large read or write request.
.P
TCP is the default transport protocol used for all modern NFS 
implementations.  It performs well in almost every conceivable
network environment and provides excellent guarantees against data 
corruption caused by network unreliability.  TCP is often a  
requirement for mounting a server through a network firewall.
.P
Under normal circumstances, networks drop packets much more
frequently than NFS servers drop requests.  As such, an aggressive
retransmit timeout  setting for NFS over TCP is unnecessary. Typical
timeout settings for NFS over TCP are between one and ten minutes.
After  the client exhausts its retransmits (the value of the
.B retrans
mount option), it assumes a network partition has occurred,
and attempts to reconnect to the server on a fresh socket. Since
TCP itself makes network data transfer reliable, 
.B rsize
and 
.B wsize
can safely be allowed to default to the largest values supported by  
both client and server, independent of the network's MTU size.
.SH "DATA AND METADATA COHERENCE"
Some modern cluster file systems provide
perfect cache coherence among their clients.
Perfect cache coherence among disparate NFS clients
is expensive to achieve, especially on wide area networks.
As such, NFS settles for weaker cache coherence that 
satisfies the requirements of most file sharing types. Normally, 
file sharing is completely sequential:
first client A opens a file, writes something to it, then closes it;
then client B opens the same file, and reads the changes.
.DT
.SS "Close-to-open cache consistency"
When an application opens a file stored on an NFS server,
the NFS client checks that it still exists on the server
and is permitted to the opener by sending a GETATTR or ACCESS request.
When the application closes the file,
the NFS client writes back any pending changes
to the file so that the next opener can view the changes.
This also gives the NFS client an opportunity to report
any server write errors to the application
via the return code from
.BR close (2).
The behavior of checking at open time and flushing at close time
is referred to as close-to-open cache consistency.
.SS "Weak cache consistency"
There are still opportunities for a client's data cache
to contain stale data.
The NFS version 3 protocol introduced "weak cache consistency"
(also known as WCC) which provides a way of efficiently checking
a file's attributes before and after a single request.
This allows a client to help identify changes
that could have been made by other clients.
.P
When a client is using many concurrent operations
that update the same file at the same time
(for example, during asynchronous write behind),
it is still difficult to tell whether it was
that client's updates or some other client's updates
that altered the file.
.SS "Attribute caching"
Use the 
.B noac
mount option to achieve attribute cache coherence
among multiple clients.
Almost every file system operation checks
file attribute information.
The client keeps this information cached
for a period of time to reduce network and server load.
When
.B noac
is in effect, a client's file attribute cache is disabled,
so each operation that needs to check a file's attributes
is forced to go back to the server.
This permits a client to see changes to a file very quickly,
at the cost of many extra network operations.
.P
Be careful not to confuse the
.B noac
option with "no data caching."
The
.B noac
mount option prevents the client from caching file metadata,
but there are still races that may result in data cache incoherence
between client and server.
.P
The NFS protocol is not designed to support
true cluster file system cache coherence
without some type of application serialization.
If absolute cache coherence among clients is required,
applications should use file locking. Alternatively, applications 
can also open their files with the O_DIRECT flag
to disable data caching entirely.
.SS "The sync mount option"
The NFS client treats the
.B sync
mount option differently than some other file systems
(refer to 
.BR mount (8)
for a description of the generic
.B sync
and
.B async
mount options).
If neither
.B sync
nor
.B async
is specified (or if the 
.B async 
option is specified),
the NFS client delays sending application
writes to the server
until any of these events occur: 
.IP
Memory pressure forces reclamation of system memory resources.
.IP
An application flushes file data explicitly with 
.BR sync (2),
.BR msync (2),
or
.BR fsync (3).
.IP
An application closes a file with
.BR close (2).
.IP
The file is locked/unlocked via
.BR fcntl (2).
.P
In other words, under normal circumstances,
data written by an application may not immediately appear
on the server that hosts the file.
.P
If the
.B sync
option is specified on a mount point,
any system call that writes data to files on that mount point
causes that data to be flushed to the server
before the system call returns control to user space.
This provides greater data cache coherence among clients,
but at a significant performance cost.
.P
Applications can use the O_SYNC open flag to force application
writes to individual files to go to the server immediately without
the use of the
.B sync 
mount option.
.SS "Using file locks with NFS"
The Network Lock Manager protocol is a separate sideband protocol
used to manage file locks in NFS version 2 and version 3.
To support lock recovery after a client or server reboot,
a second sideband protocol --
known as the Network Status Manager protocol --
is also required.
In NFS version 4,
file locking is supported directly in the main NFS protocol,
and the NLM and NSM sideband protocols are not used.
.P
In most cases, NLM and NSM services are started automatically,
and no extra configuration is required.
Configure all NFS clients with fully-qualified domain names
to ensure that NFS servers can find clients to notify them of server reboots.
.P
NLM supports advisory file locks only.
To lock NFS files, use
.BR fcntl (2)
with the F_GETLK and F_SETLK commands.
The NFS client converts file locks obtained via
.BR flock (2)
to advisory locks.
.P
When mounting servers that do not support the NLM protocol,
or when mounting an NFS server through a firewall
that blocks the NLM service port,
specify the
.B nolock
mount option. NLM locking must be disabled with the
.B nolock
option when using NFS to mount
.I /var
because 
.I /var 
contains files used by the NLM implementation on Linux.
.P
Specifying the
.B nolock
option may also be advised to improve the performance
of a proprietary application which runs on a single client
and uses file locks extensively. 
.SS "NFS version 4 caching features"
The data and metadata caching behavior of NFS version 4
clients is similar to that of earlier versions.
However, NFS version 4 adds two features that improve
cache behavior:
.I change attributes
and
.IR "file delegation" .
.P
The
.I change attribute
is a new part of NFS file and directory metadata
which tracks data changes.
It replaces the use of a file's modification
and change time stamps
as a way for clients to validate the content
of their caches.
Change attributes are independent of the time stamp
resolution on either the server or client, however.
.P
A
.I file delegation
is a contract between an NFS version 4 client
and server that allows the client to treat a file temporarily
as if no other client is accessing it.
The server promises to notify the client (via a callback request) if another client
attempts to access that file.
Once a file has been delegated to a client, the client can
cache that file's data and metadata aggressively without
contacting the server.
.P
File delegations come in two flavors:
.I read
and
.IR write .
A
.I read
delegation means that the server notifies the client
about any other clients that want to write to the file.
A
.I write
delegation means that the client gets notified about
either read or write accessors.
.P
Servers grant file delegations when a file is opened,
and can recall delegations at any time when another
client wants access to the file that conflicts with
any delegations already granted.
Delegations on directories are not supported.
.P
In order to support delegation callback, the server
checks the network return path to the client during
the client's initial contact with the server.
If contact with the client cannot be established,
the server simply does not grant any delegations to
that client.
.SH "SECURITY CONSIDERATIONS"
NFS servers control access to file data,
but they depend on their RPC implementation
to provide authentication of NFS requests.
Traditional NFS access control mimics
the standard mode bit access control provided in local file systems.
Traditional RPC authentication uses a number
to represent each user
(usually the user's own uid),
a number to represent the user's group (the user's gid),
and a set of up to 16 auxiliary group numbers
to represent other groups of which the user may be a member.
.P
Typically, file data and user ID values appear unencrypted
(i.e. "in the clear") on the network.
Moreover, NFS versions 2 and 3 use
separate sideband protocols for mounting,
locking and unlocking files,
and reporting system status of clients and servers.
These auxiliary protocols use no authentication.
.P
In addition to combining these sideband protocols with the main NFS protocol,
NFS version 4 introduces more advanced forms of access control,
authentication, and in-transit data protection.
The NFS version 4 specification mandates NFSv4 ACLs,
RPCGSS authentication, and RPCGSS security flavors
that provide per-RPC integrity checking and encryption.
Because NFS version 4 combines the  
function of the sideband protocols into the main NFS protocol,
the new security features apply to all NFS version 4 operations
including mounting, file locking, and so on.
RPCGSS authentication can also be used with NFS versions 2 and 3,
but does not protect their sideband protocols.
.P
The
.B sec
mount option specifies the RPCGSS security mode
that is in effect on a given NFS mount point.
Specifying
.B sec=krb5
provides cryptographic proof of a user's identity in each RPC request.
This provides strong verification of the identity of users 
accessing data on the server.
Note that additional configuration besides adding this mount option
is required in order to enable Kerberos security.
Refer to the 
.BR rpc.gssd (8)
man page for details.
.P
Two additional flavors of Kerberos security are supported:
.B krb5i
and
.BR krb5p .
The
.B krb5i
security flavor provides a cryptographically strong guarantee
that the data in each RPC request has not been tampered with.
The
.B krb5p
security flavor encrypts every RPC request
to prevent data exposure during network transit; however,
expect some performance impact
when using integrity checking or encryption.
Similar support for other forms of cryptographic security (such as lipkey and SPKM3)
is also available.
.P
The NFS version 4 protocol allows
clients and servers to negotiate among multiple security flavors
during mount processing.
However, Linux does not yet implement such negotiation.
The Linux client specifies a single security flavor at mount time
which remains in effect for the lifetime of the mount.
If the server does not support this flavor,
the initial mount request is rejected by the server.
.SS "Mounting through a firewall"
A firewall may reside between an NFS client and server,
or the client or server may block some of its own ports via IP
filter rules.
It is still possible to mount an NFS server through a firewall,
though some of the
.BR mount (8)
command's automatic service endpoint discovery mechanisms may not work; this 
requires you to provide specific endpoint details via NFS mount options.
.P
NFS servers normally run a portmapper or rpcbind daemon to advertise
their service endpoints to clients. Clients use the rpcbind daemon to determine: 
.IP
What network port each RPC-based service is using
.IP
What transport protocols each RPC-based service supports 
.P
The rpcbind daemon uses a well-known port number (111) to help clients find a service endpoint.
Although NFS often uses a standard port number (2049),
auxiliary services such as the NLM service can choose
any unused port number at random.
.P
Common firewall configurations block the well-known rpcbind port.
In the absense of an rpcbind service,
the server administrator fixes the port number
of NFS-related services so that the firewall
can allow access to specific NFS service ports.
Client administrators then specify the port number
for the mountd service via the
.BR mount (8)
command's
.B mountport
option.
It may also be necessary to enforce the use of TCP or UDP
if the firewall blocks one of those transports.
.SS "NFS Access Control Lists"
Solaris allows NFS version 3 clients direct access
to POSIX Access Control Lists stored in its local file systems.
This proprietary sideband protocol, known as NFSACL,
provides richer access control than mode bits.
Linux implements this protocol
for compatibility with the Solaris NFS implementation.
The NFSACL protocol never became a standard part
of the NFS version 3 specification, however.
.P
The NFS version 4 specification mandates a new version
of Access Control Lists that are semantically richer than POSIX ACLs.
NFS version 4 ACLs are not fully compatible with POSIX ACLs; as such, 
some translation between the two is required
in an environment that mixes POSIX ACLs and NFS version 4. 
.SH FILES
.TP 1.5i
.I /etc/fstab
file system table
.SH BUGS
The generic
.B remount
option is not fully supported.
Generic options, such as
.BR rw " and " ro
can be modified using the
.B remount
option,
but NFS-specific options are not all supported.
The underlying transport or NFS version
cannot be changed by a remount, for example.
Performing a remount on an NFS file system mounted with the
.B noac
option may have unintended consequences.
The
.B noac
option is a mixture of a generic option,
.BR sync ,
and an NFS-specific option
.BR actimeo=0 .
.P
Before 2.4.7, the Linux NFS client did not support NFS over TCP.
.P
Before 2.4.20, the Linux NFS client used a heuristic
to determine whether cached file data was still valid
rather than using the standard close-to-open cache coherency method
described above.
.P
Starting with 2.4.22, the Linux NFS client employs
a Van Jacobsen-based RTT estimator to determine retransmit
timeout values when using NFS over UDP.
.P
Before 2.6.0, the Linux NFS client did not support NFS version 4.
.P
Before 2.6.8, the Linux NFS client used only synchronous reads and writes
when the
.BR rsize " and " wsize
settings were smaller than the system's page size.
.P
The Linux NFS client does not yet support
certain optional features of the NFS version 4 protocol,
such as security negotiation, server referrals, and named attributes.
.SH "SEE ALSO"
.BR fstab (5),
.BR mount (8),
.BR umount (8),
.BR mount.nfs (5),
.BR umount.nfs (5),
.BR exports (5),
.BR nfsd (8),
.BR rpc.idmapd (8),
.BR rpc.gssd (8),
.BR rpc.svcgssd (8),
.BR kerberos (1)
.sp
RFC 768 for the UDP specification.
.br
RFC 793 for the TCP specification.
.br
RFC 1094 for the NFS version 2 specification.
.br
RFC 1813 for the NFS version 3 specification.
.br
RFC 1832 for the XDR specification.
.br
RFC 1833 for the RPC bind specification.
.br
RFC 2203 for the RPCSEC GSS API protocol specification.
.br
RFC 3530 for the NFS version 4 specification.