Hint: if you want to check if this answer will solve your issue, execute ifconfig eth0 mtu 1400 on the instance and check the speed.

Edit the file: /etc/quantum/dhcp_agent.ini and set:

    
      
        dnsmasq_config_file
      
      
        =
      
      
        /etc/
      
      
        neutron
      
      
        /
      
      
        dnsmasq
      
      
        -
      
      
        neutron
      
      
        .
      
      
        conf
      
    
  

Create and edit file /etc/neutron/dnsmasq-neutron.conf and put this inside:

    
      
        dhcp
      
      
        -
      
      
        option
      
      
        -
      
      
        force
      
      
        =
      
      
        26
      
      
        ,
      
      
        1400
      
    
  

Restart DHCP:

    
      
        service neutron
      
      
        -
      
      
        dhcp
      
      
        -
      
      
        agent restart
      
    
  

And reboot your instance. Verify that your MTU is 1400:

root@web:~# ifconfig eth0 Link encap:Ethernet HWaddr fa:16:3e:f0:6a:9f
inet addr:10.1.0.2 Bcast:10.1.0.255 Mask:255.255.255.0 inet6 addr: fe80::f816:3eff:fef0:6a9f/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1400 Metric:1 RX packets:934855 errors:0 dropped:0 overruns:0 frame:0 TX packets:207741 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:1210256390 (1.2 GB) TX bytes:226172592 (226.1 MB)

Test upload speed again. It should be back to normal.

親測可行。

MTU&GRE

新的數據報頭長度為20字節，GRE最小封閉字節為4字節，1500-20-4=1476，而我們的實例默認是1480，必然每個包都自動分片，導致性能下降。

ref: https://ask.openstack.org/en/question/6140/quantum-neutron-gre-slow-performance/

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openstack ovs-gre 網速慢解決方案