MOUNT_NFS(8)

NAME
mount_nfs - mount nfs file systems

SYNOPSIS
mount_nfs [-23NPTUbcdils] [-D deadthresh] [-I readdirsize] [-R retrycnt] [-a maxreadahead] [-g maxgroups] [-o options] [-r readsize] [-t timeout] [-w writesize] [-x retrans] rhost:path node

DESCRIPTION
The mount_nfs command calls the mount(2) system call to prepare and graft a remote nfs file system (rhost:path) on to the file system 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 Appendix I.

By default, mount_nfs keeps retrying until the mount succeeds. This behaviour is intended for filesystems listed in fstab(5) that are critical to the boot process. For non-critical filesystems, the -b and -R flags provide mechanisms to prevent the boot process from hanging if the server is unavailable.

If the server becomes unresponsive while an NFS filesystem is mounted, any new or outstanding file operations on that filesystem will hang uninterruptibly until the server comes back. To modify this default behaviour, see the -i and -s flags.

The options are:

-2  Use the NFS Version 2 protocol (the default is to try version 3 first then version 2). Note that NFS version 2 has a file size limit of 2 gigabytes.

-3  Use the NFS Version 3 protocol.

-D  Used to set the “dead server threshold” to the specified number of round trip timeout intervals. After a “dead server threshold” of retransmit timeouts, cached data for the unresponsive server is assumed to still be valid. Values may be set in the range of 1 - 9, with 9 referring to an “infinite dead threshold” (i.e. never assume cached data still valid). This option is not generally recommended and is really an experimental feature.

-I  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.

-N  Do not use a reserved socket port number (see below).

-P  Use a reserved socket port number. This flag is obsolete, and only retained for compatibility reasons. Reserved port numbers are used by default now. (For the rare case where the client has a trusted root account but untrustworthy users and the network cables are in secure areas this does help, but for normal desktop clients this does not apply.)

-R  Set the mount retry count to the specified value. The default is a retry count of zero, which means to keep retrying forever. There is a 60 second delay between each attempt.

-T  Use TCP transport instead of UDP. This is recommended for servers that are not on the same LAN cable as the client. This is the default.

-U  Use UDP transport instead of TCP. This is not recommended due to the ease of which DragonFly clients can blow out available socket buffer space on the server, not to mention the impossibility of accurately calculating the proper retry interval due to disk I/O backlogs on the server. If you want to use the option anyway it is recommended that the server reserve upwards of 2 MBytes of socket buffer space to hold the received UDP packets.

-a  Set the read-ahead count to the specified value. This may be in the range of 0 - 32, 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 if the server does not reply to requests from the standard NFS port number 2049 or replies to requests using a different IP address (which can occur if the server is multi-homed). Setting the vfs.nfs.nfs_ip_paranoia sysctl to 0 will make this option the default.

-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  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 response from the mount point.

-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.

-l  Used with NFSV3 to specify that the ReaddirPlus 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 interconnects with a large bandwidth times delay product. This is the default.

-o  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 option is also available:

    port=<port_number>
        Use specified port number for NFS requests. The default is to query the portmapper for the NFS port.

    acregmin=<seconds>

    acregmax=<seconds>

    acdirmin=<seconds>

    acdirmax=<seconds>
        When attributes of files are cached, a timeout calculated to determine whether a given cache entry has expired. These four values determine the upper and lower bounds of the timeouts for ``directory'' attributes and ``regular'' (i.e.: everything else). The default values are 3 -> 60 seconds for regular files, and 30 -> 60 seconds for directories. The algorithm to calculate the timeout is based on the age of the file. The older the file, the longer the cache is considered valid, subject to the limits above.

    bg  Same as -b.

    cache
        Set cache chflags(2) flag on mount point (recursive, does not cross mounts). NFS does not support chflags(2), this is alternate method to set flag. cache flag is used by swapcache(8).

    conn
        Same as not specifying -c.

    dumbtimer
        Same as -d.

    intr
        Same as -i.

    nfsv2
        Same as -2.

    nfsv3
        Same as -3.

    rdirplus
        Same as -l.

    mntudp
        Same as -U (obsolete).

    resvport
        Same as -P.

    soft
        Same as -s.

    tcp Same as -T.

    udp Same as -U.

    retrycnt
        Same as -R.

-r  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 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.) See the -w option as well.

-s  A soft mount, which implies that file system calls will fail after Retry round trip timeout intervals.

-t  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 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 -d option should be specified when using this option to manually tune the timeout interval.)

-w  Set the write data size to the specified value. Ditto the comments w.r.t. the -r option, but using the “fragments dropped due to 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  Set the retransmit timeout count for soft mounts to the specified value.

SEE ALSO
netstat(1), nfsstat(1), mount(2), unmount(2), fstab(5), mount(8), nfsd(8), showmount(8), swapcache(8)

HISTORY
A version of the mount_nfs utility appeared in 4.4BSD.

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 4.4BSD servers.

NFS does not support chflags(2).