Xen guests cannot serv NFS requests

This sounds weird, but I have witnessed it today, and had to work rather hard to figure the cause of the problem.

When using ” Intel Corporation 82575EB Gigabit Network Connection (rev 02)” (as lspci reports), TCP offload causes problems.

Symptoms:

• The host can communicate with the guest flawlessly (including HTTP get for larger than 2.6k files)
• Other external hosts/guests report NFS timeout during mount attempt
• Other external hosts/guests take a long while running “showmount -e” on the target guest
• Pings work flawlessly
• HTTP get from external nodes halts at about 2660 bytes, to which it reaches almost immediately
• VLAN tagged interfaces on other than the default VLAN (1) do not experience these problems (cause – unknown to me at the moment).

The solution is simple – disable the offload from the NIC, and be happy. You could do it using the following line:

ethtool -K eth0 tx off

This should do the trick. It is required only on Dom0, and was tested to work well with my own method of configuring bonds and VLAN tags, as described in this post.

I was able to find the required hint in this post, under comment by Alejandro Anadon, and from there, directly to Xen’s FAQ site and to the  solution mentioned above.

Due to ETHTOOL_OPTS parameter limitations, I have placed (in an ugly manner, I know) the relevant ethtool commands in /etc/rc.local – in contradiction to this great article which shows the correct way of doing this action. Seems to be solved since.