[Xen-users] Debian Squeeze server running 2 Ubuntu (Lucid) vms blows up under large NFS network load

[Benjamin Weaver <benjamin.weaver@xxxxxxxxxxxxxxxxxx>]

I am running Debian Squeeze with Xen 4.0. I am running a stress  test. I have 
created 2 virtual machines, each with 512 Mb of memory adn 8Gb in size. I have 
made one of the vms an NFS server, sharing out a large file (4.6 Gb). The other 
virtual machine  is an NFS client. The stress test consists of passing that big 
file back and forth via an mv command executed on the client, which moves the 
file back and forth from the nfs share directory to a local directory. The 
virtual machines are stored on a remote SAN connected to by ISCSI  and 
formatted in ocfs2.

It is true I have had better luck with some ethernet cards than others.
One of the boxes, running Intel 1000Mb cards (1 to the SAN/OCFS2, 1 to the 
outside world), runs the vms and the stress test without problems.
 
But the other box, running a 1000Mb Realtek nic to the outside
world and a 100Mb Realtek nic to the SAN, fails. The 100Mb nic was dropping 
packets to the SAN so I changed the SAN nic to the Realtek 1000Mb
 
Now I do not drop packets (aside from a handful on the 2 vif interfaces at 
startup)

And yet,

about 1 out of 2 times I attempt to mv the file from the local directory of the 
nfs client vm to the nfs share, the box running the vms reboots. It leaves no 
logs, and seldom even any messages on the screen. It just blanks out and the 
next thing I know I it is rebooting.

I have tried manipulating the size of the MTU, with out positive success.
I have noticed that all--or nearly all--the reboots occur when I attempt to mv 
the file BACK INTO the nfs shared directory.

I begun testing with tcpdump, and notice that a large number of packets go over 
with checksums correct, then, after a packet of unusually long length, all show 
checksums incorrect.  (but I am new to tcpdump and may not be interpreting the 
output correctly).

Any ideas why the host machine is rebooting, and how this could be fixed? Could 
changing the size of the ring buffer make a difference? I read about this on a 
couple of web pages.
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