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131 lines
6.0 KiB
HTML
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<!--Converted with LaTeX2HTML 96.1-c (Feb 29, 1996) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds -->
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<HTML>
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<HEAD>
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<TITLE>Mounting an NFS Volume</TITLE>
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</HEAD>
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<BODY LANG="EN">
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<A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node143.html">The NFS Daemons</A>
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<B>Up:</B> <A HREF="node140.html">The Network File System</A>
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<B> Previous:</B> <A HREF="node141.html">Preparing NFS</A>
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<BR> <P>
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<H1><A NAME="SECTION0013200000">Mounting an NFS Volume</A></H1>
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<A NAME="nfsmountd"></A>
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NFS volumes<A HREF="footnode.html#5430"><IMG ALIGN=BOTTOM ALT="gif" SRC="foot_motif.gif"></A> are mounted very much the way usual file systems are mounted. You invoke
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mount using the following syntax:
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<PRE>
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# mount -t nfs nfs volume local dir options
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</PRE>
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nfs_volume is given as remote_host:remote_dir.
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Since this notation is unique to NFS file systems, you can leave out
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the -t nfs option.
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<P>
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There are a number of additional options that you may specify to
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mount upon mounting an NFS volume. These may either be given
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following the -o switch on the command line, or in the options
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field of the /etc/fstab entry for the volume. In both cases,
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multiple options are separated from each other by commas. Options
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specified on the command line always override those given in the
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fstab file.
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<P>
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A sample entry in /etc/fstab might be
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<PRE>
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# volume mount point type options
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news:/usr/spool/news /usr/spool/news nfs timeo=14,intr
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</PRE>
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This volume may then be mounted using
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<PRE>
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# mount news:/usr/spool/news
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</PRE>
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In the absence of a fstab entry, NFS mount invocations
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look a lot uglier. For instance, suppose you mount your users' home
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directories from a machine named moonshot, which uses a default
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block size of 4K for read/write operations. You might decrease
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block size to 2K to suit ' datagram size limit by issuing
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<PRE>
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# mount moonshot:/home /home -o rsize=2048,wsize=2048
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</PRE>
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The list of all valid options is described in its entirety in the
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nfs(5) manual page that comes with Rick Sladkey's NFS-aware
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mount tool which can be found in Rik Faith's util-linux
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package). The following is an incomplete list of those you would
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probably want to use:
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<DL>
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<DT>rsize=n and wsize=n<DD> These specify the datagram size used by the NFS
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clients on read and write requests, respectively. They cur-
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rently default to 1024 bytes, due to the limit on UDP datagram
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size described above.
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<DT>timeo=n<DD> This sets the time (in tenths of a second) the NFS client will
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wait for a request to complete. The default values is 0.7 sec-
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onds.
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<DT>hard<DD> Explicitly mark this volume as hard-mounted. This is on by
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default.
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<DT>soft<DD> Soft-mount the driver (as opposed to hard-mount).
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<DT>intr<DD> Allow signals to interrupt an NFS call. Useful for aborting
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when the server doesn't respond.
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</DL>
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Except for rsize and wsize, all of these options
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apply to the client's behavior if the server should become inaccessible
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temporarily. They play together in the following way: whenever the
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client sends a request to the NFS server, it expects the operation to
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have finished after a given interval (specified in the timeout
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option). If no confirmation is received within this time, a so-called
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<em>minor timeout</em> occurs, and the operation is retried with the
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timeout interval doubled. After reaching a maximum timeout of 60
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seconds, a <em>major timeout</em> occurs.
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<P>
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<A NAME="5476"></A>
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<A NAME="5477"></A>
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By default, a major timeout will cause the client to print a message to
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the console and start all over again, this time with an initial timeout
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interval twice that of the previous cascade. Potentially, this may go on
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forever. Volumes that stubbornly retry an operation until the server
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becomes available again are called <em>hard-mounted</em>. The opposite
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variety, <em>soft-mounted</em> volumes gerenates an I/O error for the
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calling process whenever a major timeout occurs. Because of the
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write-behind introduced by the buffer cache, this error condition is not
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propagated to the process itself before it calls the write(2)
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function the next time, so a program can never be sure that a write
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operation to a soft-mounted volume has succeeded at all.
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<P>
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Whether you hard- or soft-mount a volume is not simply a question of
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taste, but also has to do with what sort of information you want to
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access from this volume. For example, if you mount your X-programs by
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NFS, you certainly would not want your X-session to go berserk just
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because someone brought the network to a grinding halt by firing up
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seven copies of xv at the same time, or by pulling the Ethernet
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plug for a moment. By hard-mounting these, you make sure that your
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computer will wait until it is able to re-establish contact with your
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NFS-server. On the other hand, non-critical data such as NFS-mounted
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news partitions or FTP archives may as well be soft-mounted, so it
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doesn't hang your session in case the remote machine should be
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temporarily unreachable, or down. If your network connection to the
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server is flaky or goes through a loaded router, you may either
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increase the initial timeout using the timeo option, or
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hard-mount the volumes, but allow for signals interrupting the NFS call
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so that you may still abort any hanging file access.
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<P>
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Usually, the mountd daemon will in some way or other keep track
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of which directories have been mounted by what hosts. This information
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can be displayed using the showmount program, which is also
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included in the NFS server package. The mountd, however,
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does not do this yet.
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<P>
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<HR><A HREF="node1.html"><IMG WIDTH=65 HEIGHT=24 ALIGN=BOTTOM ALT="contents" SRC="contents_motif.gif"></A> <BR>
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<B> Next:</B> <A HREF="node143.html">The NFS Daemons</A>
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<B>Up:</B> <A HREF="node140.html">The Network File System</A>
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<B> Previous:</B> <A HREF="node141.html">Preparing NFS</A>
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<P><ADDRESS>
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<I>Andrew Anderson <BR>
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Thu Mar 7 23:22:06 EST 1996</I>
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</ADDRESS>
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