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Diffstat (limited to 'contrib/FAQ/OTHER-FAQS/NFS')
| -rw-r--r-- | contrib/FAQ/OTHER-FAQS/NFS | 77 |
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diff --git a/contrib/FAQ/OTHER-FAQS/NFS b/contrib/FAQ/OTHER-FAQS/NFS new file mode 100644 index 000000000000..e6f7af8fc511 --- /dev/null +++ b/contrib/FAQ/OTHER-FAQS/NFS @@ -0,0 +1,77 @@ +FreeBSD and NFS [for a FAQ] + +Certain Ethernet adapters for ISA PC systems have limitations which +can lead to serious network problems, particularly with NFS. This +difficulty is not specific to FreeBSD, but FreeBSD systems are affected +by it. + +The problem nearly always occurs when (FreeBSD) PC systems are networked +with high-performance workstations, such as those made by Silicon Graphics, +Inc., and Sun Microsystems, Inc. The NFS mount will work fine, and some +operations may succeed, but suddenly the server will seem to become +unresponsive to the client, even though requests to and from other systems +continue to be processed. This happens to the client system, whether the +client is the FreeBSD system or the workstation. On many systems, there is +no way to shut down the client gracefully once this problem has manifested +itself. The only solution is often to reset the client, because the NFS +situation cannot be resolved. + +Though the "correct" solution is to get a higher performance and capacity +Ethernet adapter for the FreeBSD system, there is a simple workaround that +will allow satisfactory operation. If the FreeBSD system is the SERVER, +include the option "wsize=1024" on the mount from the client. If the +FreeBSD system is the CLIENT, then mount the NFS file system with the +option "rsize=1024". These options may be specified using the fourth +field of the fstab entry on the client for automatic mounts, or by using +the "-o" parameter of the mount command for manual mounts. + +In the following examples, "fastws" is the host (interface) name of a +high-performance workstation, and "freebox" is the host (interface) name of +a FreeBSD system with a lower-performance Ethernet adapter. Also, +"/sharedfs" will be the exported NFS filesystem (see "man exports"), and +"/project" will be the mount point on the client for the exported file +system. In all cases, note that additional options, such as "hard" or +"soft" and "bg" may be desireable in your application. + +Examples for the FreeBSD system ("freebox") as the client: + in /etc/fstab on freebox: +fastws:/sharedfs /project nfs rw,rsize=1024 0 0 + as a manual mount command on freebox: +mount -t nfs -o rsize=1024 fastws:/sharedfs /project + +Examples for the FreeBSD system as the server: + in /etc/fstab on fastws: +freebox:/sharedfs /project nfs rw,wsize=1024 0 0 + as a manual mount command on fastws: +mount -t nfs -o wsize=1024 freebox:/sharedfs /project + +Nearly any 16-bit Ethernet adapter will allow operation without the above +restrictions on the read or write size. + +For anyone who cares, here is what happens when the failure occurs, which +also explains why it is unrecoverable. NFS typically works with a "block" +size of 8k (though it may do fragments of smaller sizes). Since the maximum +Ethernet packet is around 1500 bytes, the NFS "block" gets split into +multiple Ethernet packets, even though it is still a single unit to the +upper-level code, and must be received, assembled, and ACKNOWLEDGED as a +unit. The high-performance workstations can pump out the packets which +comprise the NFS unit one right after the other, just as close together as +the standard allows. On the smaller, lower capacity cards, the later +packets overrun the earlier packets of the same unit before they can be +transferred to the host and the unit as a whole cannot be reconstructed or +acknowledged. As a result, the workstation will time out and try again, +but it will try again with the entire 8K unit, and the process will be +repeated, ad infinitum. + +By keeping the unit size below the Ethernet packet size limitation, we +ensure that any complete Ethernet packet received can be acknowledged +individually, avoiding the deadlock situation. + +Overruns may still occur when a high-performance workstations is slamming +data out to a PC system, but with the better cards, such overruns are +not guarranteed on NFS "units". When an overrun occurs, the units affected +will be retransmitted, and there will be a fair chance that they will be +received, assembled, and acknowledged. +-- + John Lind, Starfire Consulting Services +E-mail: john@starfire.MN.ORG USnail: PO Box 17247, Mpls MN 55417 |