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Thanks, it helps a lot. But i want to know, why the mtu discovery can not find the appropriate mtu to send?


In http://www.chriscowley.me.uk/blog/2014/03/31/openstack-neutron-performance-problems/, it says that
 "Everything matches, however I was using GRE tunnels, which add a header to each frame. This was pushing them over 1500 on entry to br-tun causing massive network fragmentation, which basically destroyed Openvswitch every time I performed a large transfer. It showed up when deploying an image, because that is hitting the Glance API over http."


Dose it means that the pmtu is not work?





At 2014-10-27 09:51:31, "Damon Wang" <damon.devops at gmail.com> wrote:

Hi Tangqing,


This is a well-known problem, it is because GRE/VxLan 's packet header use some bytes, length of data packet should reduced.

You can get refer to bellow pages:
http://www.chriscowley.me.uk/blog/2014/03/31/openstack-neutron-performance-problems/
https://ask.openstack.org/en/question/25947/openstack-neutron-stability-problems-with-openvswitch/
https://openstack.redhat.com/forum/discussion/302/quantumneutron-and-ip-packet-mtus-questions-answers-and-suggestions/p1


Hope it helps,

Damon


2014-10-27 9:18 GMT+08:00 Li Tianqing <jazeltq at 163.com>:

Hello,
    Right now, we test neutron under havana release. We configured network_device_mtu=1450 in neutron.conf, 
   After create vm, we found the vm interface's mtu is 1500, the ping, ssh, is ok. But if we scp large file between vms
   then scp display 'stalled'. And iperf is also can not completed.
   If we configured vm's mtu to 1450, then iperf, scp all is ok.
   If we iperf with -M 1300, then the iperf is ok too.
   The vms path mtu discovery is set by default. I do not know why the vm whose mtu is 1500 can not send large file.




Thanks.





--

Best
    Li Tianqing



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