Weird NFS problems

Skylar Thompson skylar at
Thu May 26 06:18:05 PDT 2005

I'm having some problems with NFS serving on a FreeBSD 5.4-RELEASE 
machine. The FreeBSD machine is the NFS/NIS server for a group of four 
Linux clusters. The network archictecture looks like this:

234/24                                                   234/24
Cluster 1 ---                    |--------------- Cluster 3
                   | ---------------
            em0|  File server | fxp0
                   |  --------------
Cluster 2 ---                    |--------------- Cluster 4
234/24                                                    230/24

em0 and fxp0 are bridged, and em0 has a 234/24 IP address while fxp0 is 
just in promiscuous mode. 234/24 is an 802.1q VLAN on the fxp0 side of 
the server, so packets are untagged at the switch just before fxp0, and 
are forwarded to em0 through the bridge.

The problem manifests itself in large UDP NFS requests from Clusters 3 
and 4. The export can be mounted fine from both those clusters, and 
small transfers such as with ls work fine, but the moment any serious 
data transfer starts, the entire mount just hangs. Running ethereal on 
the file server shows a a lot of fragmented packets, and RPC 
retransmissions on just a single request. Reducing the read and write 
NFS buffers on the Linux clients to 1kB from the default of 4kB solves 
the issue, but kills the transfer rate. The moment I go to 2kB, the 
problem reappearss. Clusters 1 and 2 use the default of 4kB buffers, and 
have no problems communicating to em0.

Poking through the list archives, I ran across this message 
that reveals a bug in the fxp(4) driver in 4-RELEASE that incorrectly 
detects the capabilities of the NIC. Is this still an issue in 
5-RELEASE, or am I looking at a different problem? Any ideas on how I 
can get the NFS buffers up to a reasonable level?

-- Skylar Thompson (skylar at

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