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NFS(5) Linux Programmer's Manual NFS(5)
NAME
nfs - nfs fstab format and options
SYNOPSIS
/etc/fstab
DESCRIPTION
The fstab file contains information about which filesystems to mount
where and with what options. For NFS mounts, it contains the server
name and exported server directory to mount from, the local directory
that is the mount point, and the NFS specific options that control the
way the filesystem is mounted.
Here is an example from an /etc/fstab file from an NFS mount.
server:/usr/local/pub /pub nfs rsize=8192,wsize=8192,timeo=14,intr
Options
rsize=n The number of bytes NFS uses when reading files from an
NFS server. The default value is dependent on the ker-
nel, currently 1024 bytes. (However, throughput is
improved greatly by asking for rsize=8192.)
wsize=n The number of bytes NFS uses when writing files to an
NFS server. The default value is dependent on the ker-
nel, currently 1024 bytes. (However, throughput is
improved greatly by asking for wsize=8192.)
timeo=n The value in tenths of a second before sending the first
retransmission after an RPC timeout. The default value
is 7 tenths of a second. After the first timeout, the
timeout is doubled after each successive timeout until a
maximum timeout of 60 seconds is reached or the enough
retransmissions have occured to cause a major timeout.
Then, if the filesystem is hard mounted, each new time-
out cascade restarts at twice the initial value of the
previous cascade, again doubling at each retransmission.
The maximum timeout is always 60 seconds. Better over-
all performance may be achieved by increasing the time-
out when mounting on a busy network, to a slow server,
or through several routers or gateways.
retrans=n The number of minor timeouts and retransmissions that
must occur before a major timeout occurs. The default
is 3 timeouts. When a major timeout occurs, the file
operation is either aborted or a "server not responding"
message is printed on the console.
acregmin=n The minimum time in seconds that attributes of a regular
file should be cached before requesting fresh informa-
tion from a server. The default is 3 seconds.
acregmax=n The maximum time in seconds that attributes of a regular
file can be cached before requesting fresh information
from a server. The default is 60 seconds.
acdirmin=n The minimum time in seconds that attributes of a direc-
tory should be cached before requesting fresh informa-
tion from a server. The default is 30 seconds.
acdirmax=n The maximum time in seconds that attributes of a direc-
tory can be cached before requesting fresh information
from a server. The default is 60 seconds.
actimeo=n Using actimeo sets all of acregmin, acregmax, acdirmin,
and acdirmax to the same value. There is no default
value.
retry=n The number of minutes to retry an NFS mount operation in
the foreground or background before giving up. The
default value is 10000 minutes, which is roughly one
week.
namlen=n When an NFS server does not support version two of the
RPC mount protocol, this option can be used to specify
the maximum length of a filename that is supported on
the remote filesystem. This is used to support the
POSIX pathconf functions. The default is 255 charac-
ters.
port=n The numeric value of the port to connect to the NFS
server on. If the port number is 0 (the default) then
query the remote host's portmapper for the port number
to use. If the remote host's NFS daemon is not regis-
tered with its portmapper, the standard NFS port number
2049 is used instead.
mountport=n The numeric value of the mountd port.
mounthost=name The name of the host running mountd .
mountprog=n Use an alternate RPC program number to contact the mount
daemon on the remote host. This option is useful for
hosts that can run multiple NFS servers. The default
value is 100005 which is the standard RPC mount daemon
program number.
mountvers=n Use an alternate RPC version number to contact the mount
daemon on the remote host. This option is useful for
hosts that can run multiple NFS servers. The default
value depends on which kernel you are using.
nfsprog=n Use an alternate RPC program number to contact the NFS
daemon on the remote host. This option is useful for
hosts that can run multiple NFS servers. The default
value is 100003 which is the standard RPC NFS daemon
program number.
nfsvers=n Use an alternate RPC version number to contact the NFS
daemon on the remote host. This option is useful for
hosts that can run multiple NFS servers. The default
value is version 3.
nolock Disable NFS locking. Do not start lockd. This has to be
used with some old NFS servers that don't support lock-
ing.
bg If the first NFS mount attempt times out, retry the
mount in the background. After a mount operation is
backgrounded, all subsequent mounts on the same NFS
server will be backgrounded immediately, without first
attempting the mount. A missing mount point is treated
as a timeout, to allow for nested NFS mounts.
fg If the first NFS mount attempt times out, retry the
mount in the foreground. This is the complement of the
bg option, and also the default behavior.
soft If an NFS file operation has a major timeout then report
an I/O error to the calling program. The default is to
continue retrying NFS file operations indefinitely.
hard If an NFS file operation has a major timeout then report
"server not responding" on the console and continue
retrying indefinitely. This is the default.
intr If an NFS file operation has a major timeout and it is
hard mounted, then allow signals to interupt the file
operation and cause it to return EINTR to the calling
program. The default is to not allow file operations to
be interrupted.
posix Mount the NFS filesystem using POSIX semantics. This
allows an NFS filesystem to properly support the POSIX
pathconf command by querying the mount server for the
maximum length of a filename. To do this, the remote
host must support version two of the RPC mount protocol.
Many NFS servers support only version one.
nocto Suppress the retrieval of new attributes when creating a
file.
noac Disable all forms of attribute caching entirely. This
extracts a significant performance penalty but it allows
two different NFS clients to get reasonable results when
both clients are actively writing to a common export on
the server.
sec=mode Set the security flavor for this mount to "mode". The
default setting is sec=sys, which uses local unix uids
and gids to authenticate NFS operations (AUTH_SYS).
Other currently supported settings are: sec=krb5, which
uses Kerberos V5 instead of local unix uids and gids to
authenticate users; sec=krb5i, which uses Kerberos V5
for user authentication and performs integrity checking
of NFS operations using secure checksums to prevent data
tampering; and sec=krb5p, which uses Kerberos V5 for
user authentication and integrity checking, and encrypts
NFS traffic to prevent traffic sniffing (this is the
most secure setting). Note that there is a performance
penalty when using integrity or privacy.
tcp Mount the NFS filesystem using the TCP protocol instead
of the UDP protocol. This is the default, but in case it
fails (many NFS servers only support UDP) it will fall-
back and try UDP.
Before using NFS over UDP, please refer to the section
WARNINGS below.
udp Mount the NFS filesystem using the UDP protocol.
Before using NFS over UDP, please refer to the section
WARNINGS below.
noacl Assume no extended access control mechanisms like POSIX
ACLs are used on the NFS filesystem, and depend on the
file mode permission bits instead of using the ACCESS
remote procedure call. The default is to use the ACCESS
remote procedure call for making access decisions in NFS
version 3. (On NFS version 2 filesystems this option has
no effect.) This option also deactivates the GETACL and
SETACL remote procedure calls which are otherwise used
to manipulate ACLs.
All of the non-value options have corresponding nooption forms. For
example, nointr means don't allow file operations to be interrupted.
WARNINGS
Using NFS over UDP on high-speed links such as Gigabit can cause silent
data corruption.
The problem can be triggered at high loads, and is caused by problems
in IP fragment reassembly. NFS read and writes typically transmit UDP
packets of 4 Kilobytes or more, which have to be broken up into several
fragments in order to be sent over the Ethernet link, which limits
packets to 1500 bytes by default. This process happens at the IP net-
work layer and is called fragmentation.
In order to identify fragments that belong together, IP assigns a 16bit
IP ID value to each packet; fragments generated from the same UDP
packet will have the same IP ID. The receiving system will collect
these fragments and combine them to form the original UDP packet. This
process is called reassembly. The default timeout for packet reassembly
is 30 seconds; if the network stack does not receive all fragments of a
given packet within this interval, it assumes the missing fragment(s)
got lost and discards those it already received.
The problem this creates over high-speed links is that it is possible
to send more than 65536 packets within 30 seconds. In fact, with heavy
NFS traffic one can observe that the IP IDs repeat after about 5 sec-
onds.
This has serious effects on reassembly: if one fragment gets lost,
another fragment from a different packet but with the same IP ID will
arrive within the 30 second timeout, and the network stack will combine
these fragments to form a new packet. Most of the time, network layers
above IP will detect this mismatched reassembly - in the case of UDP,
the UDP checksum, which is a 16 bit checksum over the entire packet
payload, will usually not match, and UDP will discard the bad packet.
However, the UDP checksum is 16 bit only, so there is a chance of 1 in
65536 that it will match even if the packet payload is completely ran-
dom (which very often isn't the case). If that is the case, silent data
corruption will occur.
This potential should be taken seriously, at least on Gigabit Ethernet.
Network speeds of 100Mbit/s should be considered less problematic,
because with most traffic patterns IP ID wrap around will take much
longer than 30 seconds.
It is therefore strongly recommended to use NFS over TCP where possi
ble, since TCP does not perform fragmentation.
If you absolutely have to use NFS over UDP over Gigabit Ethernet, some
steps can be taken to mitigate the problem and reduce the probability
of corruption:
Jumbo frames: Many Gigabit network cards are capable of transmitting
frames bigger than the 1500 byte limit of traditional
Ethernet, typically 9000 bytes. Using jumbo frames of
9000 bytes will allow you to run NFS over UDP at a page
size of 8K without fragmentation. Of course, this is
only feasible if all involved stations support jumbo
frames.
To enable a machine to send jumbo frames on cards that
support it, it is sufficient to configure the interface
for a MTU value of 9000.
Lower reassembly timeout:
By lowering this timeout below the time it takes the IP
ID counter to wrap around, incorrect reassembly of frag-
ments can be prevented as well. To do so, simply write
the new timeout value (in seconds) to the file
/proc/sys/net/ipv4/ipfrag_time.
A value of 2 seconds will greatly reduce the probability
of IPID clashes on a single Gigabit link, while still
allowing for a reasonable timeout when receiving frag-
mented traffic from distant peers.
FILES
/etc/fstab
SEE ALSO
fstab(5), mount(8), umount(8), exports(5)
AUTHOR
"Rick Sladkey" <jrs@world.std.com>
BUGS
The posix, and nocto options are parsed by mount but currently are
silently ignored.
The umount command should notify the server when an NFS filesystem is
unmounted.
Linux 0.99 20 November 1993 NFS(5)
Man(1) output converted with
man2html and wrapped by fishsponge
This page was generated on Tue Feb 13 02:21:57 GMT 2007
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