Re: [Xen-devel] vif tx drops

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

> It looks like xen0 or the bridge stuff is not getting some packets
> destined for xenU.
> 
> I traced an ssh connection from a machine running stock 2.4.25
> to a xenU domain by running tcpdump on xen0 and on the other machine.
> 
> The trace on xen0 shows a 4 second gap where no ssh packets were received
> for the xenU domain even though the other machine was sending retransmits. 
> Only the last retransmit appears in the xen0 dump.
> 
> It is unlikely the network dropped the missing packets.  This is low bandwidth
> stuff and somewhat repeatable.  I've never seen the effect on xen0, and
> in this case, there is another xenU domain on the same host
> showing no pause symptoms.
> 
> The 'netstat -i' statistics on xen0 and xenU do not show any dropped packets.

It does very much look as though the packets simply aren't getting to
the host machine. Perhaps a bad interaction with an Ethernet switch?
One thing to check is that, if you are not choosing your own MAC
addresses, that the random ones chosen by xend turn out to be unique
across the cluster --- any duplicates will simply beat each other up
in the switch forwarding caches!! This is *definitely* worth checking
out at this point, and can be fixed by picking your own MAC addresses.

 -- Keir


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