86 lines
4.4 KiB
Text
86 lines
4.4 KiB
Text
<!-- $Id: nfs.sgml,v 1.9 1997-02-22 12:58:54 peter Exp $ -->
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<!-- The FreeBSD Documentation Project -->
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<sect><heading>NFS<label id="nfs"></heading>
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<p><em>Contributed by &a.jlind;.</em>
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Certain Ethernet adapters for ISA PC systems have limitations which
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can lead to serious network problems, particularly with NFS. This
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difficulty is not specific to FreeBSD, but FreeBSD systems are affected
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by it.
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The problem nearly always occurs when (FreeBSD) PC systems are networked
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with high-performance workstations, such as those made by Silicon Graphics,
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Inc., and Sun Microsystems, Inc. The NFS mount will work fine, and some
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operations may succeed, but suddenly the server will seem to become
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unresponsive to the client, even though requests to and from other systems
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continue to be processed. This happens to the client system, whether the
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client is the FreeBSD system or the workstation. On many systems, there is
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no way to shut down the client gracefully once this problem has manifested
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itself. The only solution is often to reset the client, because the NFS
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situation cannot be resolved.
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Though the "correct" solution is to get a higher performance and capacity
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Ethernet adapter for the FreeBSD system, there is a simple workaround that
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will allow satisfactory operation. If the FreeBSD system is the SERVER,
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include the option "-w=1024" on the mount from the client. If the
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FreeBSD system is the CLIENT, then mount the NFS file system with the
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option "-r=1024". These options may be specified using the fourth
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field of the fstab entry on the client for automatic mounts, or by using
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the "-o" parameter of the mount command for manual mounts.
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It should be noted that there is a different problem,
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sometimes mistaken for this one,
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when the NFS servers and clients are on different networks.
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If that is the case, make CERTAIN that your routers are routing the
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necessary UDP information, or you will not get anywhere, no matter
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what else you are doing.
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In the following examples, "fastws" is the host (interface) name of a
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high-performance workstation, and "freebox" is the host (interface) name of
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a FreeBSD system with a lower-performance Ethernet adapter. Also,
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"/sharedfs" will be the exported NFS filesystem (see "man exports"), and
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"/project" will be the mount point on the client for the exported file
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system. In all cases, note that additional options, such as "hard" or
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"soft" and "bg" may be desirable in your application.
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Examples for the FreeBSD system ("freebox") as the client:
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in <tt>/etc/fstab</tt> on freebox:
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fastws:/sharedfs /project nfs rw,-r=1024 0 0
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as a manual mount command on freebox:
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mount -t nfs -o -r=1024 fastws:/sharedfs /project
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Examples for the FreeBSD system as the server:
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in <tt>/etc/fstab</tt> on fastws:
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freebox:/sharedfs /project nfs rw,-w=1024 0 0
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as a manual mount command on fastws:
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mount -t nfs -o -w=1024 freebox:/sharedfs /project
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Nearly any 16-bit Ethernet adapter will allow operation without the above
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restrictions on the read or write size.
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For anyone who cares, here is what happens when the failure occurs, which
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also explains why it is unrecoverable. NFS typically works with a "block"
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size of 8k (though it may do fragments of smaller sizes). Since the maximum
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Ethernet packet is around 1500 bytes, the NFS "block" gets split into
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multiple Ethernet packets, even though it is still a single unit to the
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upper-level code, and must be received, assembled, and ACKNOWLEDGED as a
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unit. The high-performance workstations can pump out the packets which
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comprise the NFS unit one right after the other, just as close together as
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the standard allows. On the smaller, lower capacity cards, the later
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packets overrun the earlier packets of the same unit before they can be
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transferred to the host and the unit as a whole cannot be reconstructed or
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acknowledged. As a result, the workstation will time out and try again,
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but it will try again with the entire 8K unit, and the process will be
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repeated, ad infinitum.
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By keeping the unit size below the Ethernet packet size limitation, we
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ensure that any complete Ethernet packet received can be acknowledged
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individually, avoiding the deadlock situation.
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Overruns may still occur when a high-performance workstations is slamming
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data out to a PC system, but with the better cards, such overruns are
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not guaranteed on NFS "units". When an overrun occurs, the units affected
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will be retransmitted, and there will be a fair chance that they will be
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received, assembled, and acknowledged.
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