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41bd9393 JH |
1 | FreeBSD and NFS [for a FAQ] |
2 | ||
3 | Certain Ethernet adapters for ISA PC systems have limitations which | |
4 | can lead to serious network problems, particularly with NFS. This | |
5 | difficulty is not specific to FreeBSD, but FreeBSD systems are affected | |
6 | by it. | |
7 | ||
8 | The problem nearly always occurs when (FreeBSD) PC systems are networked | |
9 | with high-performance workstations, such as those made by Silicon Graphics, | |
10 | Inc., and Sun Microsystems, Inc. The NFS mount will work fine, and some | |
11 | operations may succeed, but suddenly the server will seem to become | |
12 | unresponsive to the client, even though requests to and from other systems | |
13 | continue to be processed. This happens to the client system, whether the | |
14 | client is the FreeBSD system or the workstation. On many systems, there is | |
15 | no way to shut down the client gracefully once this problem has manifested | |
16 | itself. The only solution is often to reset the client, because the NFS | |
17 | situation cannot be resolved. | |
18 | ||
19 | Though the "correct" solution is to get a higher performance and capacity | |
20 | Ethernet adapter for the FreeBSD system, there is a simple workaround that | |
21 | will allow satisfactory operation. If the FreeBSD system is the SERVER, | |
22 | include the option "wsize=1024" on the mount from the client. If the | |
23 | FreeBSD system is the CLIENT, then mount the NFS file system with the | |
24 | option "rsize=1024". These options may be specified using the fourth | |
25 | field of the fstab entry on the client for automatic mounts, or by using | |
26 | the "-o" parameter of the mount command for manual mounts. | |
27 | ||
28 | In the following examples, "fastws" is the host (interface) name of a | |
29 | high-performance workstation, and "freebox" is the host (interface) name of | |
30 | a FreeBSD system with a lower-performance Ethernet adapter. Also, | |
31 | "/sharedfs" will be the exported NFS filesystem (see "man exports"), and | |
32 | "/project" will be the mount point on the client for the exported file | |
33 | system. In all cases, note that additional options, such as "hard" or | |
34 | "soft" and "bg" may be desireable in your application. | |
35 | ||
36 | Examples for the FreeBSD system ("freebox") as the client: | |
37 | in /etc/fstab on freebox: | |
38 | fastws:/sharedfs /project nfs rw,rsize=1024 0 0 | |
39 | as a manual mount command on freebox: | |
40 | mount -t nfs -o rsize=1024 fastws:/sharedfs /project | |
41 | ||
42 | Examples for the FreeBSD system as the server: | |
43 | in /etc/fstab on fastws: | |
44 | freebox:/sharedfs /project nfs rw,wsize=1024 0 0 | |
45 | as a manual mount command on fastws: | |
46 | mount -t nfs -o wsize=1024 freebox:/sharedfs /project | |
47 | ||
48 | Nearly any 16-bit Ethernet adapter will allow operation without the above | |
49 | restrictions on the read or write size. | |
50 | ||
51 | For anyone who cares, here is what happens when the failure occurs, which | |
52 | also explains why it is unrecoverable. NFS typically works with a "block" | |
53 | size of 8k (though it may do fragments of smaller sizes). Since the maximum | |
54 | Ethernet packet is around 1500 bytes, the NFS "block" gets split into | |
55 | multiple Ethernet packets, even though it is still a single unit to the | |
56 | upper-level code, and must be received, assembled, and ACKNOWLEDGED as a | |
57 | unit. The high-performance workstations can pump out the packets which | |
58 | comprise the NFS unit one right after the other, just as close together as | |
59 | the standard allows. On the smaller, lower capacity cards, the later | |
60 | packets overrun the earlier packets of the same unit before they can be | |
61 | transferred to the host and the unit as a whole cannot be reconstructed or | |
62 | acknowledged. As a result, the workstation will time out and try again, | |
63 | but it will try again with the entire 8K unit, and the process will be | |
64 | repeated, ad infinitum. | |
65 | ||
66 | By keeping the unit size below the Ethernet packet size limitation, we | |
67 | ensure that any complete Ethernet packet received can be acknowledged | |
68 | individually, avoiding the deadlock situation. | |
69 | ||
70 | Overruns may still occur when a high-performance workstations is slamming | |
71 | data out to a PC system, but with the better cards, such overruns are | |
72 | not guarranteed on NFS "units". When an overrun occurs, the units affected | |
73 | will be retransmitted, and there will be a fair chance that they will be | |
74 | received, assembled, and acknowledged. | |
75 | -- | |
76 | John Lind, Starfire Consulting Services | |
77 | E-mail: john@starfire.MN.ORG USnail: PO Box 17247, Mpls MN 55417 |