[Xen-devel] Xen & AoE problem = Ethernet padding

[Marcelo Messa <mrmessa@xxxxxxxxx>]

>
Hi there,

         This is not a "broadcast", but also, a kind of ping.

         I have an entry at  http://bugzilla.xensource.com/bugzilla/ 
show_bug.cgi?id=1140 , and maybe some of you could be interested.
          It follows:
------------------------------------
Hi,

       I made some tests about ethernet padding ( an expected  
behavior from
ethernet 10/100 on-wire, that needs at least 60 bytes of data to be  
able to
CRC. NIC chipset has the job of fullfill the packet if it is smaller  
than this
- also called "padding" ) and I notice a behavior that is not  
expected (at
least for me).


>>> Ethernet Padding Sender  /  Receiver Matrix <<<

      \RECEIVER  | Dom0_vif |FullV_DomU_2|ParaV_DomU_2|SMP_ParaV_DomU_2
SENDER/
Dom0_vif#######|  No (OK)  |  Yes (OK) |  No (NOK) | No (NOK)
FullV_DomU#####|  No (NOK) |  Yes (OK) |  No (NOK) | No (NOK)
ParaV_DomU####|  No (NOK) |  Yes (OK) |  No (NOK) | No (NOK)
SMP_ParaV_DomU|  No (NOK) |  Yes (OK) |  No (NOK) | No (NOK)

       My setup is a Centos 5.1 (2.6.18-53.1.4.el5xen) / Xen (3.0.3  
or 3.1, as I hear) in a
bridge network configuration.

       The results shows that only FullVirtualization do padding,  
and only when
receives a packet (not when send it).

       You could ask "why padding if you dont have a physical  
transport?". If
you are playing with some L2 protocol (like AoE), this  behavior do  
impact.
       A direct related problem: it's not possible to export AoE  
disks from
Dom0 and other Paravirtualized guests (AoE blade server verifies if  
the packet
has a minimal size - 60 bytes) to any local VM. I do not known if  
another
layer2 protocols has the same problem, but I think that is a expected  
feature
that a virtual NIC works as a physical one.

Best Regards
      Marcelo Messa





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