[Xen-users] Slow network solved.

["IT" <it@xxxxxxxxxxxxxx>]

After about three months and dozens of hours of trawling through the mailing 
lists and google for an answer as to why I'm getting such a slow domU to domU 
network speed I've finally found the answer. I'm writing this to try to help 
anyone else who may be stuck in the same or similar situation that I was in and 
I'll try to provide as many references and details as I can.

Firstly let me describe the symptoms:
Network -> domU Good
Network -> dom0 Good
Dom0 -> domU            Good
Domu -> domU            Bad.

Mostly I didn't really mind the slow speeds and occasionally packet loss but I 
was not able to complete a backup using Backup Exec Linux Remote Agent because 
it would fail with an error e00084f8. I ran a ping against the domu while 
backing it up with Backup Exec and found that Backup Exec would cope with two 
or three lost packets in a row but any more and it would stop.

I attempted to change the kernel network parameters as described here 
http://lists.xensource.com/archives/html/xen-users/2006-03/msg00533.html but it 
made no difference.

I checked how fast the network was actually going and found using iperf that 
domU to domU is hopping between ~30Mb and 250Mb but it does sometimes stop 
completely for a few seconds.

Could not find anything unusual from tcpdump although I probably wouldn't 
really know what to look for.

I'm running a Broadcom 5703 NIC (tg3) in a HP dl380 G4 with Debian on all domUs 
and dom0 and Xen 3.0.3 with a custom kernel.

Upgrading the NIC firmware 
(http://h18023.www1.hp.com/support/files/server/us/download/24056.html?jumpid=reg_R1002_USEN)
 did not help, nor did upgrading BIOS 
(http://h18023.www1.hp.com/support/files/server/us/download/24777.html?jumpid=reg_R1002_USEN).

The solution was as simple as "ethtool -K eth0 tx off" on the domU's and 
"ethtool -K eth0 tx on" on the dom0's. After turning the tx checksumming off 
the network speed is faster and doesn't seem to be dropping packets at all 
anymore.

I hope this saves people some time and I was not having fun at all trying to 
fix this problem.

Cheers,
Jon
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